Industrial Data Communication

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Industrial Data

Communication

Communication protocols

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Industrial data communication

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Serial Communication

The concept of serial communication is simple. A serial port sends and receives data

one bit at a time over one wire. While it takes eight times as long to transfer each byte

of data this way, only a few wires are required.

Three Modes of Communication

Simplex Communication

A simplex system is one that is designed for sending messages in one direction

only. This is illustrated in figure 1. This is of limited interest in an industrial

communications system as feedback from the instrument is essential to confirm

the action requested has indeed occurred.


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Half Duplex Communication

Half duplex communications occurs when data flows in both directions; although

in only one direction at a time. Half duplex communications (as discussed later)

is provided by the RS-485 physical standard (to be discussed later) where only

one station can transmit at a time. A protocol (which can be thought of as the

pattern of bits and bytes) can be half duplex as well an example here is

Modbus

Full Duplex Communication

In a full duplex system, the data can flow in both directions simultaneously.

Examples of hardware standards supporting full duplex are the physical standard

EIA-232E (sometimes referred to as RS-232C).


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Serial versus Parallel communications

Most computers are equipped with serial ports and a parallel port. Although these two

types of ports are used for communicating with external devices, they work in different

ways :

Parallel ports

A parallel port sends and receives data eight bits at a time over 8 separate wires. This

allows data to be transferred very quickly. Parallel ports are typically used to connect a

PC to a printer and are rarely used for much. The cable length cannot be very long,

generally less than a few meters.

Serial Ports

A serial port sends and receives data one bit at a time over one wire. While it takes

eight times as long to transfer each byte of data this way, only a few wires are required.

In fact, two-way (full duplex) communications is possible with only three separate wires -

one to send, one to receive, and a common signal ground wire. Cables for serial

communications can be much longer than the parallel ones.

RS232 Standard

RS-232 was introduced in 1960, and is currently the most widely used communication

protocol. It is simple, inexpensive to implement, and though relatively slow. Signals are

processed by determining whether they are positive or negative when compared with a

ground. Because signals traveling this single wire are vulnerable to degradation,

RS-232 systems are recommended for communication over short distances (up to 50

feet) and at relatively slow data rates, (up to 20 kbps). However, in practice, these limits

can be exceeded. AnRS-232based system allows only two devices to communicate.

Hardware Basics

The electrical characteristics of the RS232C standard is contained in the EIA

(Electronics Industry Association). The main ones are the following :

A Space (logic 0) will be between +3 and +25 Volts.

A Mark (Logic 1) will be between -3 and -25 Volts.

The region between +3 and -3 volts is undefined.

An open circuit voltage should never exceed 25 volts. (In Reference to GND)
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A short circuit current should not exceed 500mA. The driver should be able to

handle this without damage. (Take note of this one!)

Connector Types

Serial Ports come in two sizes, There are the D -Type 25 pin connector and the D-Type

9 pin connector both of which are male on the back of the PC, thus you will require a

female connector on your device.

RS-232 Signal Descriptio ns

TxD: Transmit Data--This wire is used for sending data.

RxD: Receive Data--This line is used for receiving data.

GND: Signal Ground--This pin is the same for DTE and DCE devices, and it provides

the return path for both data and hand-shake signals.

DTR: Data Terminal Ready--Used by a DTE to signal that it is plugged in and available

to begin communication.

DSR: Data Set Ready--Sister signal to DTR, it is used by the DCE to indicate it is ready

to begin communication.

CTS: Clear to Send--Used by DCE to signal it is available to send data, and used in

response to a RTS request for data.

RTS: Request to Send--Used by a DTE to indicate that it wants to send data. Also, in a

multi-drop network, used to turn carrier on the modem on and off.

DCD: Data Carrier Detect--Used by a DCE to indicate to the DTE that it has received a

carrier signal from the modem and that real data is being transmitted.

RI: Ring Indicator--Used by DCE modem to tell the DTE that the phone is ringing and

that data will be forthcoming.

The following table gives the pin outs of the different connectors, along with the signals

involved on the serial communication port:


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Pin 6 Pin 6 DSRData Set

Ready

DSR (Data Set Ready) is the

companion to DTR in the same

way that CTS is to RTS.

Pin 7 Pin 5 SGSignal

Ground

Pin 8 Pin 1 CDCarrier

Detect

A modem uses Carrier Detect to

signal that it has made a

connection with another modem,

or has detected a carrier tone.

Pin 20 Pin 4 DTRDataTerminal

Ready

Its intended function is very

similar to the RTS line. Some

serial devices use DTR and DSR

as signals to simply confirm thata device is connected and is

turned on. The DTR and DSR

lines were originally designed to

provide an alternate method of

hardware handshaking.

Pin 22 Pin 9 RIRing

Indicator

A modem toggles the state of

this line when an incoming call

rings your phone.


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DTE devices

Communications servers

Terminals

Serial printers

Computers with native RS-232-E serial ports

DCE devicesModems and other communications equipment


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Whether DTE or DCE

Unfortunately, as more and more electronic computers and instruments outside the

telephone industry began using RS-232, it became difficult to decide if the new gadget

should be a DTE or DCE device since in reality both DTE and DCE devices transmit

and receive data.

Device Type Function DB251Pin No. DB92Pin No.

DTE Transmit 2 3

Receive 3 2

Ground 7 5

DCE Transmit 3 2

Receive 2 3

Ground 7 5


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BASIC RS-232 DATA CIRCUITS


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=


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Common RS-232 cable wirings

This document includes information how to make common wires for RS-232

connections. Pinouts for standard 25 pin connection and de-facto 9 pin connector used

in PCs are shown.

Normal DTE-DCE connection

These wirings can be used with normal Data Terminal Equipment (DTE) to Data

Communications Equipment (DCE) connections. The wiring are standard way to do

asynchronous DTE to DCE connection and they support hardware handshaking.

DTE (25 pin) DCE (25 pin)

TD 2 ------------------------> 2

RD 3 4

CTS 5 6

DCD 8 ------------------------> 8

DTR 20 ------------------------> 20

SG 7 ------------------------- 7

RI 22 2

RD 2 4

CTS 8 6

DCD 1 ------------------------> 8

DTR 4 ------------------------> 20

SG 5 ------------------------- 7

RI 9


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Two-wire DTE-DCE wiring

This wiring can be used between DTE and DCE where hardware handshaking is not

needed.

DTE (25 pin) DCE (25 pin)TD 2 ------------------------> 2

RD 3 2

RD 2


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Null modem cable

This cable can be used when you must connect two DTE equipments like computers to

each other directly without using any data communication equipment in between

computers. This wiring supports hardware handshaking.

DTE (25 pin) DTE (25 pin)

TD 2 ---------\ /------------- 2

RD 3 3

RTS 4 ---------\ /------------- 4

CTS 5 5

DSR 6 6

DCD 8 8

DTR 20 ---------/ \------------- 20

SG 7 ------------------------- 7


TD 2 ---------\ /------------- 3

RD 3 2

RTS 4 ---------\ /------------- 7

CTS 5 8

DSR 6 6

DCD 8 1

DTR 20 ---------/ \------------- 4

SG 7 ------------------------- 5


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RD 2 ---------\ /------------- 2

TD 3 3

RTS 7 ---------\ /------------- 7

CTS 8 8

DSR 6 6

DCD 1 1

DTR 4 ---------/ \------------- 4

SG 5 ------------------------- 5


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Two-wire DTE-DTE connection

This cable can be used when you must connect two DTE equipments like computers to

each other directly without using any data communication equipment between

computers. This wiring needs only two signal wires and ground between computers butdoes not support hardware handshaking.


TD 2 ---------\ /------------- 2

RD 3 3

RTS 4 ----, ,----- 4

CTS 5 5

DSR 6 6

DCD 8 8

DTR 20 ----' '----- 20

SG 7 ------------------------- 7


TD 2 ---------\ /------------- 3

RD 3 2

RTS 4 ----, ,----- 7

CTS 5 8

DSR 6 6DCD 8 1

DTR 20 ----' '----- 4

SG 7 ------------------------- 5


TD 3 ---------\ /------------- 3

RD 2 2

RTS 7 ----, ,----- 7

CTS 8 8

DSR 6 6

DCD 1 1

DTR 4 ----' '----- 4

SG 5 ------------------------- 5


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RS-422/485 SERIAL COMMUNICATION OVERVIEW

RS-422

Balanced Transmission

The RS-422 communication provides a mechanism by which serial data can be

transmitted over great distances (to 4,000 feet) and at very high speeds (to 10 Mbps).

This is accomplished by splitting each signal across two separate wires in opposite

states, one inverted and one not inverted. The difference in voltage between the two

lines is compared by the receiver to determine the logical state of the signal. This wire

configuration, called differential data transmission or balanced transmission, is well

suited to noisy environments.

WithRS-232communication, which is unbalanced transmission and uses only one wire,

signal degradation can take place if there is a difference in ground potential between the

transmitting and receiving ends of the cable.

With balanced transmission, this potential difference will affect both wires equally, and

thus not effect their inverse relationship. Twisted pairs of wire, which ensure that neither

line is permanently closer to a noise source than the other, are often used to best

equalize influences on the two lines.

Errors can be caused by high noise levels affecting one side of the receiver to a

different extent than the other. To combat this, each receiver is generally grounded.

Errors in balanced transmission systems such as RS-422 can also be caused by signal

reflections. As data transfer speeds increase and travel over longer distances, the

signal can be reflected back from the far end of the wire. To combat this, termination

resistors are placed at the far end of the cable which make the cable appear electrically

as if it is infinitely long--infinitely long lines don't have ends, and thus can't reflect from
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one end to the other. These termination resistors will differ depending on the protocol

used. For RS-422 a 100 ohm resistor is placed at the receiving device.

With RS-422 a master can use one communication line to converse with up to 10

slaves. With that many parties wanting to talk, a mechanism for controlling theconversation must be implemented. Rs-422 communication does not support

Full-Duplex.

RS-485 --The True Multidrop Network

RS-485 is an upgraded version of the RS-422 protocol that was specifically designed to

address the problem of communication between multiple devices on a single data line. It

is a balanced transmission system that is virtually identical to RS-422 with the important

addition of the ability to allow up to 32 devices to communicate using the same data

line. Thus all 32 devices can directly communicate with each other, taking on the role of

master and slave as needed. This is achieved with tristatable drivers, which are usually

controlled by a programmable handshake line to ensure that only one device acts as a

driver at any one time. Communication can be initiated from any point on the line. For

RS-485, 60 ohm resistors are placed at the two furthest points of the communication

link.


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Comparison of specs. among RS232, RS422 & RS485

RS-232 RS-422 RS-485

Mode of Operation single ended differential differential

Drivers per Line 1 1 32

Receivers per Line 1 10 32

Maximum Cable Length 50 feet 4000 feet 4000 feet

Maximum Data Rate 20 kbps 10 Mbps 10 Mbps

Driver Output Maximum Voltage 25V -0.25 to +6V -7 to +12V

Driver Output Signal Level (loaded) 5V 2V 1.5V

Driver Output Signal Level (unloaded) 15V 5V 5V

Driver Load Impedance 3kto 7k 100k 54k

Max. Driver Output Current (Power on) n/a n/a 100A

Max. Driver Output Current (Power off) VMAX/300 100A 100A

Receiver Input Voltage Range 15V -7V to +7V -7V to +12V

Receiver Input Sensitivity 3V 200mV 200mV

Receiver Input Resistance 3kto 7k 4k 12k


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The synchronization problem

Serial communication normally consists of transmitting binary data across an electrical

or optical link such as RS232 or V.35. The data, being binary, is usually represented by

two physical states. For example, +5v may represent 1 and -5v represent 0. The

accurate decoding of the data at the remote end is dependent on the sender and

receiver maintaining synchronization during decoding. The receiver must sample the

signal in phase with the sender.

If the sender and receiver were both supplied by exactly the same clock source, then

transmission could take place forever with the assurance that signal sampling at the

receiver was always in perfect synchronization with the transmitter. This is seldom the

case, so in practice the receiver is periodically brought into synch. With the transmitter.It is left to the internal clocking accuracy of the transmitter and receiver to maintain

sampling integrity between synchronization pulses.

Asynchronous Vs Synchronous Communication

Asynchronous communication

There are two approaches possible in transmitting data over a communications link. The

asynchronous approach is the more basic one used by EIA-232E which operates at a

lower speed. The higher speed Local Area Networks running at 10 Mbit/s operate usingthe more efficient synchronous communications.

An asynchronous system is one in which each character or byte is sent within a frame.

The receiver does not start detection until it receives the first bit, known as the start bit.

The start bit is in the opposite voltage state to the idle voltage and allows the receiver to

synchronize to the bits following.

In asynch. serial communication, the electrical interface is held in the mark position

between characters. The start of transmission of a character is signaled by a drop in

signal level to the space level. At this point, the receiver starts its clock. After one bit

time (the start bit) come 8 bits of true data followed by one or more stop bits at the mark


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level. The receiver tries to sample the signal in the middle of each bit time. The byte will

be read correctly if the line is still in the intended state when the last stop bit is read.

Thus the transmitter and receiver only have to have approximately the same clock

rate. A little arithmetic will show that for a 10 bit sequence, the last bit will be interpreted

correctly even if the sender and receiver clocks differ by as much as 5%.

Asynch. is relatively simple, and therefore inexpensive. However, it has a high

overhead, in that each byte carries at least two extra bits: a 25% loss of line bandwidth.

A 56kbps line can only carry 5600 bytes/second asynchronously, in ideal conditions.

Start Bit Signals the start of the frame

Data Usually 7 or 8 bits of data, but can be 5 or 6

Parity Bit Optional Error detection bit

Stop bits Usually 1, 1.5 or 2 bits.


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Synchronous Communication

In synchronous communications, data is not sent in individual bytes, but as frames of

large data blocks. Frame sizes vary from a few bytes through 1500 bytes for Ethernet or

4096 bytes for most Frame Relay systems. The clock is embedded in the data streamencoding, or provided on separate clock lines such that the sender and receiver are

always in synchronization during a frame transmission.

A synchronous system uses a string of bits to synchronize the receiver before the data

is detected. A typical synchronous system frame format is shown below in figure

Preamble This comprises one or more bytes that allow the receiving unit to

synchronies with the frame

SFD The start of frame delimiter signals the beginning of the frame

Destination The address to which the frame is sent

Source The address from which the frame is sent

Length Indicates the number of bytes in the data field

Data The actual message

FCS The Frame Check Sequence is for error detection

What is Handshaking?

The method used by RS-232 communication allows for a simple connection of three

lines: Tx, Rx, and Ground. However, for the data to be transmitted, both sides have

to be clocking the data at the same baud rate. Even though this method is sufficient

for most applications, it is limited in being able to respond to problems such as the

receiver getting overloaded. This is where serial handshaking can help.


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Following are three most forms of handshaking with RS-232:

Software Handshaking

Hardware Handshaking

Software Handshaking: This style uses actual data bytes as control characters. The

lines necessary are still the simple three line set of Tx, Rx, and ground since the control

characters are sent over the transmission line like regular data. The function SetXMode

allows the user to enable or disable the use of two control characters, XON and XOFF.

These characters are sent by the receiver of the data to pause the transmitter during

communication.

Hardware Handshaking: The second method of handshaking is to use actual

hardware lines. Like the Tx and Rx lines, the RTS/CTS and DTR/DSR lines work

together with one being the output and the other the input. The first set of lines are RTS

(Request to Send) and CTS (Clear to Send). When a receiver is ready for data, it will

assert the RTS line indicating it is ready to receive data. This is then read by the sender

at the CTS input, indicating it is clear to send the data. The next set of lines are DTR

(Data Terminal Ready) and DSR (Data Set Ready). These lines are used mainly for

modem communication. They allow the serial port and the modem to communicate their

status. For example, when the modem is ready for data to be sent from the PC, it will

assert the DTR line indicating that a connection has been made across the phone line.

This is read in through the DSR line and the PC can begin to send data. The general

rule of thumb is that the DTR/DSR lines are used to indicate that the system is ready for

communication where the RTS/CTS lines are used for individual packets of data.


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Serial Communication Protocol

A protocol is an agreement between two parties about how the two parties should

behave. A communication protocol is a protocol about how two parties should speak to

each other. Serial communication protocols assume that bits are transmitted in series

down a single channel. A serial protocolhas to address the following issues

How does the receiver know when to start looking for information?

When should the receiver look at the channel for the information bits?

What is the bit order? (MSB or LSB first)

How does the receiver know when the transmission is complete?

Open systems Model

In digital data communications, wiring together of two or more devices is one of the first

steps in establishing a network. As well as this hardware requirement, software must

also be addressed. The OSI reference Model consists of the following seven layers:

Layer 1: Physical Layer

Electrical and mechanical definition of the system

Layer 2: Data Link Layer

Framing and Error correction format of the data


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Layer 3: Network Layer

Optimum routing of messages from one network to another

Layer 4: Transport Layer

Channel for transfer of messages of one application process to another

Layer 5: Session Layer

Organization and synchronization of the data exchange

Layer 6: Presentation Layer

Data format or representation

Layer 7 Application Layer

File Transfer, message exchange

The OSI Model provides an overall framework for the vendor in which to package their

communications solutions comprising the hardware communications links and the

protocols.

In the world of instrumentation, this OSI model is often simplified to use only three

layers:Layer 1: Physical Layer

Layer 2: Data Link Layer

Layer 3: Application Layer


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Examples of how these layers are applied:

RS-232 and RS-485 are examples of the Physical Layer

The Modbus Protocol is an example of the Data Link Layer

The HART smart instrumentation protocol comprises the Physical, Data Link and

Application Layers. Foundation Fieldbus comprise the Physical, Data Link and

Application Layers.


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HART Protocol

History

The HART protocol was originated by Rosemount in the late 1980's. HART is an

acronym for "Highway Addressable Remote Transducer." The protocol was "open" for

other companies to use and a User Group formed in 1990.

In March of 1993, the group voted to create an independent, nonprofit organization to

better support the HART Protocol. In July of that year, the HART Communication

Foundation was established to provide worldwide support for application of the

technology. The Foundation would own the HART technology, manage the protocol

standards, and ensure that the technology is openly available for the benefit of the

industry.The HART Protocol - An Overview

HART- FSK Based

The HART protocol uses 1200 baud Frequency Shift Keying (FSK) based on the Bell

202 standard to superimpose digital information on the conventional 4 to 2OmA

analogue signal at a low level on top of the 4-20mA as show in Figures. The HART

protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a

host application (master) to get two or more digital updates per second from a field

device. As the average value of FSK signal is always Zero, 420 mA signal is not

affected.


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HART STRUCTURE

Specification of the HART protocol is based largely on the OSI Seven Layer

Communication Model (see Figure).

The HART protocol specifications directly address 3 layers in the OSI model: the

Physical, Data Link and Application Layers.


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The Physical Layer

The Physical Layer connects devices together and communicates a bit-stream from one

device to another. It is concerned with the mechanical and electrical properties of the

connection and the medium (the copper wire cable) connecting the devices.

Data Link Layer

While the Physical Layer transmits the bit stream, the Data Link Layer is responsible for

reliably transferring that data across the channel. It organizes the raw bit stream into

packets (framing), adds error detection codes to the data stream. The bit stream is

organized into 8-bit bytes that are further grouped into messages. A HART transaction

consists of a master command and a slave response

The Application Layer

It defines the commands, responses, data types and status reporting supported by the

Protocol.


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Backward Compatibility

Unlike other digital communication technologies, the HART Protocol provides a unique

communication solution that is backward compatible with the installed base ofinstrumentation in use today. This backward compatibility ensures that investments in

existing cabling and current control strategies will remain secure well into the future.

Two Way Communication

Designed to compliment traditional 4-20mA analog signaling, the HART Protocol

supports two way digital communications for process measurement and control devices.

and makes it possible for additional information beyond just the normal process

variable to be communicated to/from a smart field instrument.

Combination of Analog & Digital

HART Field Communications Protocol extends this 4-20mA standard to enhance

communication with smart field instruments. The HART protocol was designed

specifically for use with intelligent measurement and control instruments, which

traditionally communicate using 4-20mA analog signals. HART preserves the 4-20mA

signal and enables two-way digital communications to occur without disturbing the

integrity of the 4-20mA signal.

HARTMaster/Slave

HART is a master/slave protocol which means that a field (slave) device only speaks

when spoken to by a master. The HART protocol can be used in various modes for

communicating information to/from smart field instruments and central control or

monitoring systems. HART provides for up to two masters (primary and secondary) as

show in Figure 3.


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This allows secondary masters such as handheld communicators to be used without

interfering with communications to/from the primary master, i.e. control/monitoring

system. The most commonly employed HART communication mode is master/slave

communication of digital information simultaneous with transmission of the 4-20mA

signal as shown in Figure 4.

Network Configuration

The HART protocol permits digital communication with field devices in either point-to-

point or multidrop network configuration.

Point to Point Configuration

In point to point configuration only one slave is connected with Master

Multidrop Network Configuration

In this configuration Master is connected with several slaves (smart devices).

Considerable installation savings are possible with the multidrop networking capability

of HART, which allows multiple field devices to be connected to the same pair of wires.

In multidrop applications, communication with field devices is restricted to digital only as

the loop current is fixed at a minimum value and loses any meaning relative to the

process.


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Burst Mode

Figure 5 highlights the optional "burst" communication mode where a single slave

device can continuously broadcast a standard HART reply message. Higher update

rates are possible with this optional digital communication mode and use is normally

restricted to point-to-point topologies.

The HART Command Set

The HART Command Set is organized into following three groups and provides

read/write access to the wealth of additional information available in smart field

instruments employing this technology.Universal Commands: Must be implemented by all HART devices and provide

interoperability across the large and growing base of products from different suppliers

supporting the HART technology. Universal Commands provide access to information

that is useful in normal plant operation such as the instrument manufacturer, model, tag,

serial number, descriptor, range limits, and process variables.

Common Practice Commands: Provide access to functions, which can be carried out

by many devices though not all.

Device Specific Commands: Provide access to functions, which may be unique to a

particular device.


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Figure 7 highlights the type of information that can be obtained from these devices.

Device Description Language (DDL)

Device Description Language (DDL), a recent enhancement to the HART technology,

extends interoperability to a higher level than provided through the Universal and

common Practice Commands. As reflected in Figure 8, DDL provides a field device

(slave) product developer to create a complete description of their instrument and all

relevant characteristics, such that it can talk to any host device using the language.Universal hand-held communicators capable of configuring any HART-based instrument

through DDL are available today.


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HART DEVICE PARAMETERS

Digital Process VariableValues

Primary Variable with engineering units

Secondary Process Variables with engineering units

Loop Current (milliamps) and percent range

Status and Diagnostic Device malfunction

Primary Variable out of limits

Secondary Variable out of limits

Loop Current fixed or saturated

Configuration changed

Loop test (force loop current)

Device Identification Instrument tag and descriptor Manufacturer

Device type and revision

Final assembly number

Sensor serial number

Calibration Information Date

Range units

Upper and lower range values

Upper and lower sensor limits

Sensor min span

Damping

Message

TECHNICALINFORMATION

Communication Signals


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Data Information Data update rate:

Requst/response mode23 updates per second

Optional burst mode34 updates per second

Data byte structure:

1 start bit, 8 data bits, 1 odd parity bit, 1 stop bit

Data integrity:

Two-dimensional error checking

Status information in every reply message

Simple Command

Structure

Communication Masters Two communication masters

Variables Up to 256 variables per device

Wiring Topologies Point to point--simultaneous analog and digital

Point to point--digital only

Multidrop network--digital only (up to 15 devices)


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Cable Lengths Maximum twisted-pair length--10,000 ft (3,048 m)

Maximum multiple twisted-pair length--5,000 ft (1,524 m)

Cable length depends on characteristics of individual

products/cable; see specifications for detailed length

calculations

Intrinsically Safe With appropriate barrier/isolator

35-40 data items Standard in every HART device

Device Status & Diagnostic Alerts Process Variables & Units

Loop Current & % Range

Basic Configuration Parameters

Manufacturer & Device Tag

Standard commands provide easy access

Increases control system integrity

Get early warning of device problems

Use capability of multi-variable devices Automatically track and detect changes (mismatch) in Range or Engineering

Units

Validate PV and Loop Current values at control system against those from device

Tested and Accepted global standard

Supported by all major instrumentation manufacturers

Install and commission devices in fraction of the time

Enhanced communications and diagnostics reduce maintenance & downtime

Low or no additional cost by many suppliers Improves Plant Operation and Product Quality

Additional process variables and performance indicators

Continuous device status for early detection of warnings and errors

HART SERVER


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The HART Server is a software application that provides a method for accessing the

real time process and diagnostic information available in HART field instrumentation.

HART capable instruments can be connected to the PC serial port through commonly

available RS-232 interfaces. Using the HART Server significantly simplifies access to

HART compatible field device data.


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Field Level Networks

Overview

Types of buses

Sensor bus

Device bus

Fieldbus

Using multiple buses

Overview

What 's the r ight f ield- level bus for p rocess con trol?

Digital field networks or buses typically connect sensors, actuators, and other I/O

devices with a multi-drop wiring scheme.

Because different network technologies have different capabilities, choosing the right

bus (or buses) for your operation can help minimize project cost and maximize

operational benefits. Making the wrong choice will, at best, cost you money and it

can keep you from achieving the higher yield, better quality, and lower operating costs

your plant is capable of.

Types of Buses

Field-level buses can be grouped in three categories, depending on the device type and

Application for which they were designed:

Sensor bus

Device bus

Fieldbus


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Sensorbus

Sensor buses are common in discrete manufacturing. They're used with proximity

switches, pushbuttons, motor starters, and other simple devices where costs must be

minimized and only a few bits of information need to be transmitted.

Sensor buses are designed to handle these "bit-level" communications for simple,

transaction based control and sensing, such as turning something on or off, or indicating

an on-off state. These buses usually cover short to medium distances, using either 2 or

4 wires. They typically are not intrinsically safe. Although designed for discrete

manufacturing, some sensor buses are used in process plants.

Devicebus

Device buses are designed to meet the needs of more-complex devices, often in

fast-moving discrete operations requiring short, fast communications. Paper machines,packaging lines, and motor control centers often use this type of bus.

With message capacities from several bytes to over 200 bytes, depending on the

protocol, device buses can handle more information than sensor buses not only

discrete "on" and "off" signals, but also periodic adjustments and some ancillary analog

information.

Device buses are usually 4-wire and not intrinsically safe. They can communicate athigh speed for short distances, and slower speeds for longer distances.

Two examples of device buses DeviceNet and Profibus-DP were designed for

discrete manufacturing but have been adapted for use in process plants.

Fieldbus

The third type of field network is the most appropriate for control and diagnostics in

process operations. That's because fieldbuses provide highly reliable two-way

communications between "smart" devices and systems in time-critical applications.

They're optimized for messages containing several variables all sampled at the same

time and the status of each variable.

Fieldbuses can be a digital replacement for analog 4-20 mA communications in process


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operations. Because requirements in these operations are different from those in

discrete manufacturing, fieldbuses typically have slower transmission rates than device

or sensor buses.

Other differences include support for intrinsic safety and the ability to run on existing

field instrument wiring. In the case of FOUNDATION fieldbus, the technology also

includes standard and open function blocks that support distributed control in the field.

Using Multiple Buses

Many plants use multiple field-level networks, with different types of buses to meet

different needs.

That makes sense but the added complexity can increase implementation and

maintenance costs unless you are using a system that works with different categories of

buses without mapping or gateways.

You can minimize those added costs by limiting the number of network types at each

level of the plant hierarchy.


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Understanding Ethernet

Overview

Connecting to the business network

Connecting automation subsystems

FOUNDATION fieldbus HSE

Ethernet as a fieldbus?

Using multiple networks

Overview

What's the role of Ethernet?

Because it's widely used in office networking, Ethernet is familiar and inexpensive. But

the plant floor isn't an office, and requirements for process automation aren't the same

as for business applications.

Even so, in the right applications and with the right extensions Ethernet can

reduce costs and improve performance.

Is today's Ethernet technology appropriate for process control?

How is FOUNDATION fieldbus high-speed Ethernet different from standard Ethernet?

Connecting to the business network

Ethernet is the dominant business network technology worldwide, and it's standard

practice for automation systems to provide Ethernet connectivity for business

integration.

Connecting automation subsystems

Most automation systems are a collection of subsystems including controllers,

operator interfaces, and application processors. While some use a proprietary network

to connect these subsystems, the increasingly common approach is to use Ethernet

with proprietary extensions.

The most common method used to carry data from other protocols is tunneling.


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FOUNDATION fieldbus HSE

The FOUNDATION fieldbus HSE (high-speed Ethernet) protocol uses Ethernet in an

open, interoperable way. With support for redundancy and the FOUNDATION fieldbus

User Layer, HSE has the attributes to become a standards-based automation system

backbone.


Interest in using Ethernet to network field-level devices comes from the desire to

combine high performance connectivity and low cost. For discrete manufacturing, this

idea has merit. For process automation, the issue is more complex.

Tough requirements.A process-automation fieldbus has requirements very different

from those for an office-automation network, including

Extreme environmental conditions

Intrinsic safety

Power and signal over the same wires (for two-wire devices)

Compatibility with existing instrument wiring.

Commercial, off-the-shelf Ethernet can't meet these requirements. Industrial Ethernet

with environmentally hardened components, different memory requirements, and

greater robustness comes closer.

The down side. But the cost of adding those capabilities reduces the economic

advantage of Ethernet. And industrial Ethernet doesn't provide intrinsic safety, power

and signal over the same wires, or compatibility with standard instrument wiring.


Many plants use multiple networks, including Ethernet where appropriate. That's

reasonable, because no one bus can meet all needs.

But each added layer increases the number of tools, parts, and training as well as

overall implementation and maintenance complexity. That's why there's a trend to

simplify or flatten the overall hierarchy of networks in a plant.


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For new plants or plant expansions, using the following four types of networks offers a

realistic balance of simplicity and capability:

FOUNDATION fieldbus for basic and advanced regulatory control and for

discrete control associated with regulatory control

One type of device or sensor bus for motor control and machine control

An Ethernet-based automation-system backbone, such as FOUNDATION

fieldbus HSE

A switch or gateway to the Ethernet business network


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Introduction to FOUNDATION fieldbus

Overview

What is FOUNDATION fieldbus?

The digital bus advantage

An established standard

Interoperability

Safe and effective process control

Overview

Why sho uld I care about FOUNDATION fieldbu s?

The fact is, it can. It offers distinct advantages over traditional analog and discrete

wiring or even other digital buses at lower total installed cost and lower ongoing

costs.

FOUNDATION fieldbus can deliver these benefits because it's different from

traditional

communication technologies. That doesn't mean it's harder to learn or to use just

different.

How can FOUNDATION fieldbus carry more information than 4-20 mA wiring?

Who controls FOUNDATION fieldbus technology?

For what kind of application was FOUNDATION fieldbus originally designed?

What is FOUNDATION Fieldbus

FOUNDATION fieldbus is an all-digital, serial, two-way communications system that

serves as the base-level network in a plant or factory automation environment.


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It's ideal for applications using basic and advanced regulatory control, and for much of

the discrete control associated with those functions.

Two related implementations of FOUNDATION fieldbus have been introduced to meet

different needs within the process automation environment. These two implementations

use different physical media and communication speeds.

H1 works at 31.25 Kbit/sec and generally connects to field devices. It provides

communication and power over standard twisted-pair wiring. H1 is currently the most

common implementation.

HSE (High-speed Ethernet) works at 100 Mbit/sec and generally connects input/output

subsystems, host systems, linking devices, gateways, and field devices using standard

Ethernet cabling. It doesn't currently provide power over the cable, although work is

under way to address this.

The digital bus advantage

Conventional analog and discrete field instruments use point-to-point wiring: one wire

pair per device. They're also limited to carrying only one piece of information -- usually a

process variable or control output -- over those wires.

As a digital bus, FOUNDATION fieldbus doesn't have those limitations.


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Multidrop wiring. FOUNDATION fieldbus will support up to 32 devices on a single pair

of wires (called a segment) -- more if repeaters are used. In actual practice,

considerations such as power, process modularity, and loop execution speed make 4 to

16 devices per H1 segment more typical.

That means if you have 1000 devices -- which would require 1000 wire pairs with

traditional technology -- you only need 60 to 250 wire pairs with FOUNDATION fieldbus.

That's a lot of savings in wiring (and wiring installation).

Multivariable instruments. That same wire pair can handle multiple variables from one

field device. For example, one temperature transmitter might communicate inputs from

as many as eight sensors -- reducing both wiring and instrument costs.

Two-way communication. In addition, the information flow can now be two-way. A

valve controller can accept a control output from a host system or other source and

send back the actual valve position for more precise control. In an analog world, that

would take another pair of wires.

New types of information. Traditional analog and discrete devices have no way to tell

you if they're operating correctly, or if the process information they're sending is valid.

As a consequence, technicians spend a lot of time verifying device operation.

But FOUNDATION fieldbus devices can tell you if they're operatingcorrectly, and if the information they're sending is good, bad, or uncertain. Thiseliminates the need for most routine checks -- and helps you detect failureconditions before they cause a major process problem.

Control in the field. FOUNDATION fieldbus also offers the option of executing some or

all control algorithms in field devices rather than a central host system. Depending on

the application, control in the field may provide lower costs and better performance --

while enabling automatic control to continue even if there's a host-related failure.

An established standard

FOUNDATION fieldbus is covered by standards from three major organizations:


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ANSI/ISA 50.02

IEC 61158

CENELEC EN50170:1996/A1

The technology is managed by the independent, not-for-profit Fieldbus Foundation,

whose 150+ member companies include users as well as all major process automation

suppliers around the globe.

Safe and efficient process control

Some communication protocols that were originally designed for factory or office

automation are proving useful in specific applications in process plants. But none of

these protocols was designed with the full requirements of process control in mind. As a

result, they are less-than optimum choices for providing safe and effective process

control.

FOUNDATION fieldbus H1, on the other hand, was developed specifically to meet the

needs of the process industry.

It can withstand the harsh and hazardous environment of process plants.

It delivers power and communications over the same pair of wires.

It can use existing plant wiring.

It supports intrinsic safety.

In short, it's designed to operate where your process does.

Control you can count on. FOUNDATION fieldbus also provides deterministic process

control. If fieldbus devices lose their connection to the host system, they are capable of

maintaining safe and effective control across the bus.


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Fieldbus communications

Overview

The communications model

Physical layer

Data link and application layers

User layer

Scheduled communications

Unscheduled communications

Parameter status

Application clock

Link active scheduler

Device address assignment

Find tag service

Overview

How d oes data get wh ere it 's needed -- when i t 's n eeded?

One of the most important aspects of FOUNDATION fieldbus is its ability to collect and

deliver vast amounts of information -- not only process variables and control signals, but

other types of instrument and process data as well.

It does this consistently and reliably, while also providing interoperability between

devices from different manufacturers -- and compatibility with existing wiring. This

course describes key

The communications model

The FOUNDATION fieldbus communications model has three parts:

The physical layer

The data link and application layers

The user layer

Physical layer


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The first functional layer of the FOUNDATION fieldbus communications model is the

physical layer, which deals with translating messages into physical signals on the wire --

and vice versa.

The physical layer also provides the common electrical interface for all FOUNDATION

fieldbus devices. FOUNDATION fieldbus H1 segments require 9-32 volts DC power and

approximately 15-20 mA of current per device. They operate at a communication speed

of 31.25 kbaud.

The FOUNDATION fieldbus physical layer is defined by approved standards (IEC 1158-

2 and ANSI/ISA 50.02, part 2). It can run on existing field wiring over long distances,

supports two-wire devices, and offers intrinsic safety as an option. In short, it's an ideal

match for a typical process-automation environment.

Data link and application layers

The second part of the communication model combines several technologies that

together control transmission of data on the fieldbus. The data link and applications

layers provide a standard way of "packaging" the data, as well as managing the

schedule for communication and function-block execution.

User layer

The user layer sits on top of the communications stack, where it enables you to interact

with the other layers and with other applications.

The user layer contains resource blocks, transducer blocks, and function blocks that

describe -- and execute -- device capabilities such as control and diagnostics. Device

descriptions enable the host system to interact with and understand these blocks

without custom programming.


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Scheduled communications

All devices and function blocks on a FOUNDATION fieldbus segment execute and

communicate process control information on a regular, repeating cycle.

Timing for this type of communication is determined by a master schedule in a Link

Active Scheduler, which is a function residing in the host system. These

communications are also deterministic. This means that they always occur on a

predetermined schedule, so information is certain to be broadcast (and received)

precisely when it's needed.

The result is regular and precision execution of communication and control, which helps

reduce process variability. For fast or time-critical control loops, control on

FOUNDATION fieldbus can improve plant performance.

Unscheduled communications

FOUNDATION fieldbus supports a great deal of information beyond process loop

control data.

These other types of information include

Configuration information sent to devices or a central database

Alarm, event, and trend data

Information for operator displays

Diagnostic and status information.

This information is important, but not as time-critical as loop control information.

Flexible timing. FOUNDATION fieldbus gives this information a lower priority on the

segment than scheduled control-loop-related communications. However, a certain

amount of time in the communication cycle is reserved for these unscheduled (or

"acyclic") communications to ensure that the segment is not too loaded to carry the

information.


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Application clock

Every device on a FOUNDATION fieldbus segment shares the same time.

A system management function called the application clock periodically broadcasts

the time -- either local time or Universal Coordinated Time -- to all devices. Each device

uses an internal clock to keep time between these synchronization broadcasts.

Alarms and events are time-stamped at the device where they occur, when they occur

not later when they're received by a historian, alarm log, or other application on a host

system.

Because of this approach, FOUNDATION fieldbus provides superior time resolution and

accuracy for activities such as sequence-of-events recording and analysis.

Device address assignment

As a digital, multidrop bus, FOUNDATION fieldbus carries signals to and from several

devices over the same cable. To identify which information is associated with which

device, each device is assigned an address.

Depending on the communication protocol, addresses can be assigned in several ways,

from dip switches or off-line addressing to automatic online assignment.

Methods such as using dip switches or offline addressing carry the risk of human errors,

such as inadvertently assigning an address to more than one device. These addressing

errors can cause communication problems, or in extreme cases prevent the bus from

working. That's why FOUNDATION fieldbus doesn't allow these methods of address

assignment.


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MODBUS PROTOCOL

The MODBUS Protocol is an asynchronous protocol designed to connect directly to

computer communication ports. The protocol may be used either in a point-to-point or in

a multi-drop configuration. The protocol can be used in either half or full-duplex

operation.

Protocol overview

Master-Slave protocol

This protocol takes place at level 2 of the OSI model.

A master-slave type system has one node (the master node) that issues explicit

commands to one of the "slave" nodes and processes responses. Slave nodes will not

typically transmit data without a request from the master node, and do not communicate

with other slaves.

Layers of Modbus

At the physical level, MODBUS over Serial Line systems may use different physical

interfaces (RS485, RS232). TIA/EIA-485 (RS485) Two-Wire interface is the most

common. As an add-on option, RS485 Four-Wire interface may also be implemented. A

TIA/EIA-232- E (RS232) serial interface may also be used as an interface, when only

short point to point communication is required.

The following figure gives a general representation of MODBUS serial communication

stack compared to the 7 layers of the OSI model.


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MODBUS Data Link LayerMODBUS Master / Slaves protocol principle

The MODBUS Serial Line protocol is a Master-Slaves protocol. Only one master (at the

same time) is connected to the bus, and one or several (247 maximum number) slaves

nodes are also connected to the same serial bus. A MODBUS communication is always

initiated by the master. The slave nodes will never transmit data without receiving a

request from the master node. The slave nodes

will never communicate with each other. The master node initiates only one MODBUS

transaction at the same time.

The master node issues a MODBUS request to the slave nodes in two modes :

Unicast Mode

Broadcast Mode

Unicast Mode

In this mode, the master addresses an individual slave. After receiving and processing

the request, the slave returns a message (a 'reply') to the master.

In that mode, a MODBUS transaction consists of 2 messages : a request from the

master, and a reply from the slave. Each slave must have a unique address (from 1 to

247) so that it can be addressed independently from other nodes.

Broadcast Mode

In this mode, the master can send a request to all slaves. No response is returned to

broadcast requests sent by the master. The broadcast requests are necessarily writing

commands. All devices must accept the broadcast for writing function. The address 0 is

reserved to identify a broadcast exchange.


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MODBUS Addressing rules

The MODBUS addressing space comprises 256 different addresses.

The Address 0 is reserved as the broadcast address. All slave nodes must recognize

the broadcast address.

The MODBUS Master node has no specific address, only the slave nodes must have an

address. This address must be unique on a MODBUS serial bus.


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MODBUS frame description

The MODBUS application protocol [1] defines a simple Protocol Data Unit (PDU)

independent of the underlying communication layers:

The mapping of MODBUS protocol on a specific bus or network introduces some

additional fields on the Protocol Data Unit. The client that initiates a MODBUS

transaction builds the MODBUS PDU, and then adds fields in order to build the

appropriate communication PDU.

On MODBUS Serial Line, the Address field only contains the slave address.

As described in the previous section the valid slave nodes addresses are in the range of

0247 decimal. The individual slave devices are assigned addresses in the range of 1

247. A master addresses a slave by placing the slave address in the address field of

the message. When the slave returns its response, it places its own address in the

response address field to let the master know which slave is responding.

The function code indicates to the server what kind of action to perform. The function

code can be followed by a data field that contains request and response parameters.

Error checking field is the result of a "Redundancy Checking" calculation that is

performed on the message contents.


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2.4.1 Master State diagram

The following drawing explains the Master behavior :

Some explanations about the state diagram above :

State "Idle"= no pending request. This is the initial state after power-up. A request can

only be sent in "Idle" state. After sending a request, the Master leaves the "Idle" state,

and cannot send a second request at the same time

When a unicast request is sent to a slave, the master goes into "Waiting for reply"

state, and a Response Time-out is started.

When a reply is received, the Master checks the reply before starting the data

processing. The checking may result in an error, for example a reply from an

unexpected slave, or an error in the received frame. In case of an error detected on the

frame, a retry may be performed.The maximum number of retries depends on the

master set-up.

When a broadcast requestis sent on the serial bus, no response is returned from theslaves. Nevertheless a delay is respected by the Master in order to allow any slave to

process the current request before sending a new one. This delay is called "Turnaround

delay".


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In unicast the Response time out must be set long enough for any slave to process the

request and return the response, in broadcast the Turnaround delay must be long

enough for any slave to process only the request and be able to receive a new one.

2.4.2 Slave State Diagram

The following drawing explains the Slave behavior :

Some explanations about the above state diagram :

State "Idle" = no pending request. This is the initial state after power-up.

When a request is received, the slave checks the packet before performing the action

requested in the packet. Different errors may occur: format error in the request, invalid

action, In case of error, a reply must be sent to the master.

Once the required action has been completed, a unicast message requires that a reply

must be formatted and sent to the master.

If the slave detects an error in the received frame, no response is returned to the

master.


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Serial Transmission Modes

Two different serial transmission modes are defined.

The RTU mode

The ASCII mode.

It defines the bit contents of message fields transmitted serially on the line. It

determines how information is packed into the message fields and decoded.

RTU Transmission Mode

When devices communicate on a MODBUS serial line using the RTU (Remote Terminal

Unit) mode, each 8bit byte in a message contains two 4bit hexadecimal characters.

Each message must be transmitted in a continuous stream of characters.

The format for each byte ( 11 bits ) in RTU mode is :

Coding System: 8bit binary

Bits per Byte: 1 start bit

8 data bits, least significant bit sent first

1 bit for parity completion

1 stop bit

How Characters are Transmitted Serially :

Each character or byte is sent in this order (left to right):

Least Significant Bit (LSB) . . . Most Significant Bit (MSB)

Devices may accept by configuration either Even, Odd, or No Parity checking. If No

Parity is implemented, an additional stop bit is transmitted to fill out the character frame

to a full 11-bit asynchronous character :


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Frame description :

MODBUS Message RTU Framing

A MODBUS message is placed by the transmitting device into a frame that has a known

beginning and ending point. This allows devices that receive a new frame to begin at

the start of the message, and to know when the message is completed. Partial

messages must be detected and errors must be set as a result.

In RTU mode, message frames are separated by a silent interval of at least 3.5

character times. In the following sections, this time

interval is called t3,5.


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The entire message frame must be transmitted as a continuous stream of characters. If

a silent interval of more than 1.5 character times occurs between two characters, the

message frame is declared incomplete and

should be discarded by the receiver.

MODBUS ASCII modeWhen controllers are setup to communicate on a Modbus network using ASCII

(American Standard Code for Information Interchange) mode, each 8bit byte in amessage is sent as two ASCII characters. The main advantage of this mode is that itallows time intervals of up to one second to occur between characters

without causing an error.

This mode is used when capabilities of the device does not allow the onformance with

RTU mode requirements regarding timers management.

This mode is less efficient than RTU since each byte needs two characters.

The format for each byte in ASCII mode is:

Coding System: Hexadecimal, ASCII characters 09, AF

One hexadecimal character contained in each ASCII character of the message.

Bits per Byte: 1 start bit

7 data bits, least significant bit sent first

1 bit for even/odd parity; no bit for no parity

1 stop bit if parity is used; 2 bits if no parity

Error Check Field:Longitudinal Redundancy Check (LRC)

Example : The byte 0X5B is encoded as two characters : 0x35 and 0x42

(0x35 ="5", and 0x42 ="B" in ASCII ).


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Implementation Classes

Each device on a MODBUS Serial Line must respect all the mandatoryrequirements of

a same implementation class.

The following parameters are used to classify the MODBUS Serial Line devices :

Addressing

Broadcasting

Transmission mode

Baud rate

Character format

Electrical interface parameter

Two implementation classes are proposed, the Basic and the Regular classes.

The regular class must provide configuration capabilities.


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UHF TELEMETRY RADIO

Operational Description

The UHF Telemetry Radio Modem is typically a full duplex 9600 bits per second device,

which converts digital data into an analogue form suitable for transmission over a radio

channel. It uses specially filtered direct binary frequency modulation techniques to

achieve this. It conversely, converts the analogue signal derived from a radio channel

into a digital data signal.

The heart of the unit is the modem. This performs all waveform shaping, clock recovery,

and framing and CRC error generation and checking. These functions are performed

simultaneously, allowing full duplex operation at up to 9600bps. The user is provided

with two RS232 compatible ports, which may each be configured with a standard

interface or protocol drivers. The unit may also be configured for repeater operation. It

may be configured to use RS232 handshake lines, or XON/XOFF flow control on Port.

The host user port may be configured for baud rates of 300 to 19K2, with 7 or 8 bit

character size, 1 or 2 stop bits, and parity off/odd/even.

Configuration of the modem is fully programmable, with parameters held in non-volatile

memory. All configuration parameters are accessible with the proprietary Installation

Program.

Following are some of configuration parameters.

o XON/XOFF or RTS/CTS/DTR/DCD handshake mode.

o Default transmitter lead in delay.

o Constant specifying minimum RF RSSI for valid receive.

o Constant specifying minimum Tx power level.

o Asynchronous serial port parameters.

o User interface operating mode :

o User port interface protocol


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SPECIFICATIONSRadio Section

Rx frequency range : 923MHz to 933MHz (see note 1)

Tx frequency range : 847MHz to 857MHz (see note 2)

Channel spacing : 25kHz

Frequency stability : 1 ppm (-100C to 650C amb), [opt -300C to 700C],

Aging


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HARDWARE TECHNICAL DESCRIPTION

The unit can be divided into following section.

Radio section

Antenna diplexer section.

Modem section

Radio Section

The radio section is built on a single PCB. This section consists of the following main

blocks :

o Receiver.

o Transmitter.

o Frequency control.

ANTENNA DIPLEXER SECTION

The diplexer couples both the transmit and receive RF paths to the antenna while

providing high isolation between them.

MODEM SECTION

The modem section is a single PCB having following main blocks:

o Modem along with control circuitary

o Reset and watchdog.

o Memory (RAM & EPROM)

o Host interface.

o Radio interface.

o Transmit signal conditioning.

o Receive signal conditioning.


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SUPERVISORY CONTROL & DATA ACQUISITION SYSTEM (SCADA)

Communication Techniques adopted by SCADA

Polling method

Report by exception method

Polling

Polling communication, where a Master (normally a PLC or PC) polls the Slaves

(normally RTUs).

Report by exception

Report By Exception, where packets are sent whenever:

i) If there is a change in the STATUS of the RTU.

ii) After a user specified time out, even if there are no changes in these register

values.

Main Units of SCADA

Master Unit

RTUs

HMI Software

Master UnitNormally a PLC or PC is used as Master Unit of SCADA which polls RTUs & sends the

status of I/Os of RTUs to HMI Software for monitoring purpose.


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Typical RTU Main functions

V25

CPU/RAM/POWER

MICROPROCESSOR

SUPERVISORY Cct CIRCUIT

WATCHDOG

DECODER

ADDRESS

ADDRESS BUS

DATA BUS

1Mb

STATIC RAM EPROM

512Kb

EXTENSION

TO BUS

BOARDS

LATCHES

DATA

ENCODER

/DECODERSERIAL I/O BUS

(DIGITAL & ANALOG BOARDS

COMM11200Bd FSK

MODEM

DIGITAL & ANALOG

INPUT/OUTPUTS

BOARD

TO LINE

ISOLATION

INPUT/OUTPUTS

CONFIG & ADDRESS

SWITCHES

INPUT

POWER FILTER

+12V

REGULATOR

REGULATOR

+5V

INTERNAL POWER

CLOCK

REAL TIME

Main components of RTU

Microprocessor

RAM

EPROM

Address Decoder

Watch Dog Ckt.

FSK Modem

Addressing Modes

Configuration & address Switches

Power Supply

Buffers

Latches


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Opto couplers

A/D Convertor (ADC)

D/A Convertor

Analog Inputs

Analog Outputs

Digital Inputs

Digital Outputs

Serial Ports

Serial Input/Output bus

Analogue-to-digital converter (ADC)

The analogue signal is sampled(i.e. measured at regularly spaced instants) (Figure )

and then quantised(i.e. converted to discrete numeric values) (Figure ). The greater

the number of quantisation levels, the lesser the quantisation error. The converse

operation to the ADC is performed by a digital-to-analogue converter (DAC).

Figure: Periodic sampling of an analogue signal


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Figure: Quantisation of a sampled signal

The ADC process is governed by an important law. The Nyquist-Shannon Theorem

states that an analogue signal of bandwidth B can be completely recreated from its

sampled form provided it is sampled at a rate s equal to at least twice its bandwidth.That is:

The rate at which an ADC generates bits depends on how many bits are used in the

converter. For example, a speech signal has an approximate bandwidth of 4KHz. If this

is sampled by an 8-bit ADC at the Nyquist sampling rate, the bit rate Ris:

RTU Registers/Boards

Physical I/Os Registers/Boards

Global Communication Registers/Boards

HMI SCADA Software

HMI SCADA Software provides a user interface to monitor & control I/Os of RTUs.

Normally Master Unit of SCADA which polls slaves (RTUs) acts as a Modbus Slave &

HMI Software as Modbus Server.


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Upgradation Details of SCADA System of Dhodak

R T U 1

R T U 8

R a d io M o d e m M T U

P T U 1

P T U 2

P T U 3

P T U 4

4 8 5 B U S

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R a d io M o d e m

M T U P T U 1

P T U 2

P T U 3

P T U 4

4 8 5 B U S

AL M O S

P r o t o c o l

C o n v e r t e r

An y th i r d p a r t y p o f t w a r e

( D C S )

f o r m o n i t o r in g & c o n t r o l o f

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M O D B U S

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P l a n t S i t e

W e l l S i t e s

Di s t a n c e b e t w e e n P la n t & R e p e a t e r : 2 5 K M

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P r o p o s e d S y s t e m

W e l l S i t e s P l a n t S i t e

R S 2 3 2 / R s 4 8 5

AL M O S Pro to c o l

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R x : 9 2 8 M h z

T x : 8 5 2 M h z

R x : 9 2 8 M h z

T x : 8 5 2 M h z

R x : 9 2 8 M h z

T x : 8 5 2 M h z

R x : 9 2 8 M h z

T x : 8 5 2 M h z

R x : 9 2 8 M h z

T x : 8 5 2 M h z

R x : 9 2 8 M h z

T x : 9 2 8 M h z

R x : 8 5 2 M h z

T x : 9 2 8 M h z

R x : 8 5 2 M h z

Communication Arrangements

In the new arrangement the MTUs central role is taken over by the PC. The PC needs

at least 4 RS 232 ports to communicate to the different sub systems.

1) Connection to the Well RTU-s. This communication going through first a couple

of land line modems. The actual communication between the well RTU-s and the

central site uses a radio network including a repeater.

2) Connection to the MTU. There is an RS232 communication line between the PCand the MTU. The speed is 9600 Baud. This communication uses the MTU

CPUs RS232 port.

3) Connection to the PTU-s. A two wire 485 bus is used to communicate between

the PC and the PTU-s. The speed is 9600 Baud. This communication uses the

PTU CPU-s RS232 port.


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4) Connection to the MODBUS Master. There is an RS232 communication line

between the PC and the MODBUS Master. The speed is 9600 Baud.

PC-to-PC Communication

Communications between the Protocol converter PC and a third party SCADA package

is based on the MODBUS RTU protocol implemented on a serial line. The

communication protocol parameters can be viewed and changed used WASPED.

ALCOM-to-RTU Communication

Communications between PCs and RTU-s are based on a proprietary Almos protocol.

In the DHODAK implementation all RTU-s are connected via radio network (RTU-s),

485 bus (PTU-s) and through direct RS232. RTU-s are identified by a unique station

address, which must be unique in the entire system.

The radio network contains the HOST computer, a repeater, and the remote stations.

Because of the usage of the repeaters there are two different frequencies are used for

TX and RX.

ALCOM-to-RTU Communication

The ALCOMRTU communication protocol is a MASTER (ALCOM) SLAVE (RTU)

type communication protocol.

RTU can only talk if ALCOM has selected the RTU for talking by asking information or

by writing information into the RTU and asking for acknowledge.

Each RTU has a unique station address. Whenever ALCOM wants to communicate to

a given RTU the required communication command will contain the station address of

the given RTU.


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Field Name Explanation Default

InOutUsage N.A 0

BaudRate Communication baud rate

between the

Documents

Industrial Data Communication