Cac Loai Cong RS

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Index

o General informationo RS-232o RS-422o RS-485o Other RS standardso V.35o X.21o Current loopo Low Voltage Differential Signaling (LVDS)o Telecommunication systems backplaneso Other interfaceso Device connectivity standards

Interfaces used in telecommunication and datacommunications

General information

There are a number of implementations of the physical layer. The most common serial data exchange

interfaces are RS-232, RS-422, and RS-485 for connecting two or more devices together. All three

interfaces use data terminal equipment (DTE) and data communication equipment (DCE) terminology.The DTE is the component that wants to communicate with another component somewhere else, such

as a PC communicating with another PC. The DCE is the component actually doing the

communicating, or, performing the functions of the generator and receiver discussed in the standards. Amodem is a common example of a DCE. The interfaces between DTE and DCE can be categorized by

mechanical, electrical, functional, and procedural aspects.

o AN-216: Summary of Electrical Characteristics of Some Well Known Digital InterfaceStandards - mentions RS-232, RS-422, RS-423, RS-485, EIA/TIA-485, EIA/TIA-562,

EIA/TIA-694, EIA/TIA-612, EIA/TIA-644 (LVDS) and more, pdf document Rate this link

o Background Information: Asynchronous Data Transmission Rate this linko Cisco Equipments Pinouts - Console and Auxiliary Port Cables and Pinouts, Serial Cable

Assemblies and Pinouts, Ethernet Port Pinouts (RJ-45), T1/E1 Trunk and Digital Voice Port

Pinouts (RJ-48), Analog Voice Port Pinouts, BRI S/T Port Pinouts (CB-1D)Rate this link

o Data line isolation theory - when it comes time to protect data lines from electrical transients,

surge suppression i often the first thing that leaps to mind but sometimes data line isolation is

more effective solution Rate this link

o Dataline surge protection: stop fried boards and garbled data - surges and spikes on datalinescan fry your communications boards and garble data, this article describes the operation,

installation and selection of what is probably the most common method of data line

protection Rate this linko Fully differential amplifiers and transmission lines - Fully differential amplifiers aim at

amplifying high-frequency signals while rejecting the noise that always exists in mixed-signal
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The 25-pin RS-232 connector has also pins originally defined for clock lines. The clock signals are

only used for synchronous communications. The modem or DSU extracts the clock from the datastream and provides a steady clock signal to the DTE. Note that the transmit and receive clock signals

do not have to be the same, or even at the same baud rate. Most RS-232 implementations today do not

support syncronous communications (for example normal PC RS-232 ports do not implement

syncronous communications).

Standard RS-232 pinout on 25-pin connector (RS-232C):

Pin

No.Name Notes/Description

1 - Protective/shielded ground

2 TD Transmit Data (a.k.a TxD, Tx)

3 RD Receive Data (a.k.a RxD, Rx)

4 RTS Request To Send

5 CTS Clear To Send

6 DSR Data Set Ready

7 SGND Signal Ground

8 CD Carrier Detect (a.k.a DCD)

9 - Reserved for data set testing

10 - Reserved for data set testing

11 - Unassigned

12 SDCD Secondary Carrier Detect

13 SCTS Secondary Clear to send

14 STD Secondary Transmit Data

15 DB Transmit Clock (a.k.a TCLK, TxCLK)

16 SRD Secondary Receive Data

17 DD Receive Clock (a.k.a. RCLK)

18 LL Local Loopback

19 SRTS Secondary Request to Send

20 DTR Data Terminal Ready

21 RL/SQ Signal Quality Detector/Remote loopback

22 RI Ring Indicator (used for auto answer applications)

23 CH/CI Signal Rate selector

24 DA Auxiliary Clock (a.k.a. ACLK)

25 - Unassigned


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NOTE: Leave all pins not specified above unconnected.

De-facto PC standard to wire RS-232 to 9-pin connector (EIA/TIA 574):

Pin

No.

Name Notes/Description

1 DSR/RI Data set Ready/ring indicator

2 DCD Data Carrier Detect



5 RD Receive Data

6 TD Transmit Data

7 CTS Clear to Send

8 RTS Request to Send

RS232 on RJ45 (RS-232D) (EIA/TIA - 561):

Pin

No.Name Notes/Description

1 DSR/RI Data set Ready/ring indicator

2 DCD Data Carrier Detect



5 RD Receive Data

6 TD Transmit Data

7 CTS Clear to Send

8 RTS Request to Send

When connecting RS-232 devices keep in mind that thereare three possibilities for flow control

handshaking: no handshaking (none), hardware flow control (RTS/CTS)and Xon/Xoff (special flow

control characters).

The History of RS-232 is long. In the early 1960s, a standards committee, today known as theElectronic Industries Association (EIA), developed a common interface standard for data

communications equipment. At that time, data communications was thought to mean digital data

exchange between a centrally located mainframe computer and a remote computer terminal. These

devices were linked by telephone voice lines, and consequently required a modem at each end forsignal translation. It was thought that a standard was needed to enable the interconnection of equipmen

produced by different manufacturers, thereby fostering the benefits of mass production and

competition. From these ideas, the RS232 standard was born. It specified signal voltages, signal timingsignal function, a protocol for information exchange, and mechanical connectors.RS-232 specification


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was renamed to the "EIA232 Standard" in the early 1990's. Their standard is still maintained by

EIA/TIA. There has been also standardisation on this field in Europe.The ITU (InternationalTelecommunications Union - formerly CCITT) defines the signal properties (names) in V.24 and the

electrical properties in V.28. There is almost no practical difference between RS232 and V.24 (or

V.28).

The core of the most RS-232 port implementations is UART chip. UART is short for universal

asynchronous receiver-transmitter. The universal asynchronous receiver transmitter (UART) protocol i

commonly used to send low-speed data between devices. The term asynchronous is used because it isnot necessary to send clocking information along with the data being sent. UART links are typically

38400 baud or less and are character-based. Because the transmitter and receiver operate

asynchronously, there is no need to connect the transmit and receive clocks. Instead, the receiveroversamples the incoming data stream (usually by a factor of 8, 16 or 32) and uses some of these

samples to determine the bit value. Traditionally, the middle 3 of the 16 samples are used when UART

shift register receives incoming data on RXD pin. The value of the bit is determined by the majority of

those samples; if all do not agree, the noise indication is turned on (could be noise counter). When acomplete character has been clocked in, the contents of the receive shift register are transferred to the

receive FIFO before proceeding to the receive buffer. The UART transmit shift register sends outgoing

data on TXDx.

Two UARTs can communicate using this system if the transmitter and receiver use the same

parameters, such as the parity scheme and character length. When data is not sent, a continuous streamof ones is sent (idle condition). Because the start bit is always a zero, the receiver can detect when real

data is once again on the line. The most popular protocol that uses asynchronous characters is the RS-

232 standard, which specifies baud rates, handshaking protocols, and mechanical/electrical details.Control bits in the UART mode register define the length and format of the UART character. Bits are

received in the following order:

o 1. Start bito 2. 5 to 8 data bits (lsb first)o 3. Address/data bit (optional)o 4. Parity bit (optional)o 5. Stop bits

Besides character data sending UART specifies an all-zeros break character (line longer time on zero

condition than the lenght of one character), which is typically used to mark the ends a character transfe

sequence (some protocols can use this some some other special uses).

All standards provide handshaking signals, but some systems require only three physical lines?Tx data

Rx data, and ground. Many proprietary standards have been built over the years around the UART?s

asynchronous character frame, some of which implement a multidrop configuration where multiple

stations, each with a specific address, can be present on a network.

Practically every computer contains a UART to manage the serial ports. UART is an integrated circuit

used for serial communications, containing a transmitter (parallel-to-serial converter) and a receiver

(serial-to-parallel converter), each clocked separately. The parallel side of a UART is usually connecte

to the bus of a computer. When the computer writes a byte to the UART's transmit data register (TDR)the UART will start to transmit it on the serial line. The UART's status register contains a flag bit

which the computer can read to see if the UART is ready to transmit another byte. Another status

register bit says whether the UART has received a byte from the serial line, in which case the computeshould read it from the receive data register (RDR). If another byte is received before the previous one


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is read, the UART will signal an "overrun" error via another status. Data on the serial line is formatted

by the UART according to the setting of the UART's control register. This may also determine thetransmit and receive baud rates if the UART contains its own clock circuits or "baud rate generators". I

incorrectly formated data is received the UART may signal a "framing error" or "parity error". Often

the clock in UART will run at 16 times the baud rate (bits per second) to allow the receiver to do centr

sampling - i.e. to read each bit in the middle of its allotted time period. This makes the UART moretolerant to variations in the clock rate ("jitter") of the incoming data. An example of a late 1980s UART

was the Intel 8450. It was used in first IBM PC computers. In the 1990s, newer UARTs were develope

with on-chip buffer space for data. This allowed higher transmission speed without data loss andwithout requiring such frequent attention from the computer. For example, the Intel 16550 has a 16

byte FIFO. The UART's serial connections to outside world usually go via separate line driver and line

receiver integrated circuits which provide the power and voltages required to drive the serial line andgive some protection against noise on the line. Once 1488 transmitter and 1489 receiver were common

ICs for doing those conversion. Nowaays MAX232 and and many similar ICs that can work from

single +5V power supply (produce needed higher voltages internally from +5V) are commonly used.

RS-232 has some serious shortcomings as an electrical interface. Firstly, the interface presupposes a

common ground between the DTE and DCE. This is a reasonable assumption where a short cable

connects a DTE and DCE in the same room, but with longer lines and connections between devices tha

may be on different electrical busses, this may not be true. Large differences in the ground potentials odifferent devices can cause anything from communications errors to equipment damages and even burn

cables. If you need a reliable communications on hard environments or long distances, you need quite

often use RS-232 signal opto-isolators (available from industrial electronics suppliers).

Secondly, a signal on a single line is impossible to screen effectively for noise. By screening the entirecable one can reduce the influence of outside noise, but internally generated noise remains a problem.

As the baud rate and line length increase, the effect of capacitance between the cables introduces

serious crosstalk until a point is reached where the data itself is unreadable. Crosstalk can be reduced

by using low capacitance cable and controlling of slew rate in the signal (i.e., making the signal morerounded, rather than square, thus reduces crosstalk). The standards for RS-232 and similar interfaces

usually restrict RS-232 to 20kbps or less and line lengths of 15m (50 ft) or less. These restrictions aremostly throw backs to the days when 20kbps was considered a very high line speed, and cables werethick, with high capacitance. However, in practice, RS-232 is far more robust than the traditional

specified limits of 20kbps over a 15m line would imply. Usually you can have longer cable or higher

speed without much problems. The 15m limitation for cable length can be stretched to about 30m forordinary cable, if well screened and grounded, and about 100m if the cable is low capacitance as well.

Interestingly enough, most RS-232 ports on mainframes and midrange computers are capable of far

higher speeds than their rated 19.2kbps. Usually these "low speed" ports will run error free at 56kbps

and above. RS-232 ports on PCs are typically running at 115kbps max speed.

o Background Information: Asynchronous Data Transmission Rate this linko Do you need information about RS-232C? - enough of the details of the RS-232 standard to

allow the construction and/or debuggging of interfaces between any two RS-232 compatible

devices Rate this linko ESD Considerations for RS-232 Drivers - Technical brief suggests ESD protection schemes for

RS-232 transceivers. Basic diode clamps, TranZorbs., and zener diodes are considered. Internal

latch-up is explained. Rate this linko ESD Protection for I/O Ports - This application note describes how ESD threatens electronic

systems, type of damage inflicted, how ESD is generated, test methods and waveforms used,

human body and machine models for testing, IEC compliance levels, contact and air discharge.Protection methods are described and Maxim.s approach to ESD protection is detailed.
http://www.epanorama.net/counter.php?url=http://www.interfaces.de/e-8wwww-14-apus-000.php3http://www.epanorama.net/rate.php?url=http://www.interfaces.de/e-8wwww-14-apus-000.php3http://www.epanorama.net/rate.php?url=http://www.interfaces.de/e-8wwww-14-apus-000.php3http://www.epanorama.net/counter.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/counter.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/rate.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/rate.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/counter.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/counter.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/rate.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/rate.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/counter.php?url=http://www.maxim-ic.com/appnotes.cfm/appnote_number/651http://www.epanorama.net/counter.php?url=http://www.maxim-ic.com/appnotes.cfm/appnote_number/651http://www.epanorama.net/counter.php?url=http://www.maxim-ic.com/appnotes.cfm/appnote_number/651http://www.epanorama.net/rate.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/counter.php?url=http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number=564http://www.epanorama.net/rate.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/counter.php?url=http://www.epl.co.uk/info16.htm#Dohttp://www.epanorama.net/rate.php?url=http://www.interfaces.de/e-8wwww-14-apus-000.php3http://www.epanorama.net/counter.php?url=http://www.interfaces.de/e-8wwww-14-apus-000.php3


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Guidelines are given for selecting ICs with high resistance to ESD. RS-232 I/O ports are

specifically considered. Rate this link

o Interfacing a Mac Serial Port to RS-232 Lab Equipment Rate this linko New IC Caps Two Decades of UART Development - Maxim has introduced a tiny universal

asynchronous receiver/transmitter (UART) MAX3100 Rate this linko RS232 Connections, and wiring up serial devices - includes wiring for loopback and

monitoring Rate this link

o RS-232C/422/485 line isolation solves more than fault problems Rate this linko RS-232 Diagnosis Tech Bulletin Rate this linko RS-232C/422/485 line isolation solves more than fault problems - galvanic, or electrical,

isolation is a well-established technique for noise reduction and works best in computer and

industrial systems in which noise can seriously affect data transfer, isolation removes ground-

loop currents and their associated noise voltages from data line Rate this linko RS-232: Connectors and Cabling - DB-25 pinput. DB-9 pinout, information on RJ-45 wiring

and RS-232 port setting in UNIX systems Rate this link

o RS232 Data Interface - a tutorial Rate this linko RS232 on RJ45 (RS-232D) - More properly EIA/TIA-561. Use when connecting to or from a

serial port with a 8 position Modular Jack (RJ45). Rate this link

oRS232 on DB9 (EIA/TIA 574)

Rate this link

o Serial Communications - information on Mac Serial Port, PC serial port, keyboard port andUSB Rate this link

o The RS-232 Standard - A tutorial with Signal Names and Definitions Rate this linko What Is RS-232 Communication? - introduction to RS-232 communications Rate this link

RS-422

RS-422 is a serial interface standard for communicating data at relatively high speed over twisted pair

wiring. It uses differential signaling. RS-422 communications can provide longer range and better noisimmunity than RS-232. RS-422 supports bus configurations where there is one transmitter and many

receivers. The bus cable shall be terminated at both ends with a suitable termination resistor thatmatches the impedance of the used medium (twisted pair wiring). The purpose of the termination is toprevent the reflection of data at the ends of the cable.120 ohm resistance is recommended for this

termination. It matches well the commonly used twisted pair communcation cables. The RS-422

standard defines only the electrical characteristicsof the bus system. The actual protocol and thecommunication speedvary between different application.In some basic applications asyncronous signal

similar to RS-232 are used.RS-422 is often used with one wire pair for transmitting data from one

transmitter to one or more receivers. RS-422 interface can be also used for bidirections communication

between two devices: one wire pair is for one transmitted data (one RS-422 interface) and other for

received data (other RS-422 interface).

o AN-214: Transmission Line Drivers and Receivers for EIA Standards RS-422 and RS-423 - pddocument Rate this link

o AN-759: Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in MultipointApplications - pdf document Rate this link

o AN-979: The Practical Limits of RS - pdf document Rate this linko Cable election for RS-422 and RS-485 systems - selecting the cable is not difficult, but case

hould be taken because intermittent problems caused by marginal cable can be very difficult to

troubleshoot Rate this link

o Quick reference for RS485, RS422, RS232 AND RS423 Rate this linko RS-232C/422/485 line isolation solves more than fault problems Rate this link
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9/22

o RS-422/485 Application Note - contains lots of information Rate this linko RS422 Connection (37pin D-Sub) - connector pinout Rate this link

RS-485

RS-485 is a serial interface standard for communicating data at relatively high speed over twisted pair

wiring. It uses differential signaling. RS-485 communications can provide longer range and better nois

immunity than RS-232. RS-485 is often used in configurations where there is one transmitter and manyreceivers. RS-485 has also a flixible multidrop capability, where there can be multiple transmitters on

the bus (only one transmitting at a time).

Because typical RS-485 system does not have separate transmit and receive lines, RS-485 requires

special flow control mechanism implemented in the protocol if this multidrop capability is used. Thereare also systems that use RS-485 interface through 4-wire interface, where one line is sent data from

master and other pair is multi-drop data line fro data from the slaves back to master.

The RS-485 standard defines only the electrical characteristics of the bus system. The actual protocol

and the communication speed(usually between 20 kbps and 12 Mbps) vary between different

application.

In some basic applications asyncronous signals similar to RS-232 are used (just RS-485 electrical

signal levels), or some slight modification of it. For example system like Profibus and DMX-512 usethis kind of method. In some multidrop network implementations the protocol carries 9-bit data words,

in which the ninth (parity) bit identifies each word as address or data. There are also RS-485 networks

that use data formats completely different from "normal serial communications".

RS-485 interface is used in very many control networking applications(including for example

Profibus). RS-485 is a bus type network. It can be used for bidirectional communication between manydevices connected to a bus consistong of one wire pair. EIA-485-A (commonly refered to as RS485)

which is a balanced system that uses differential signaling. The signal voltage is is between the 2 datalines (let's call them here A and B). The difference between the A and B wire voltages is what

isimportant: data high (digital 1) is if pin B is at a higher voltage than pin A, data low (digital 0) is ifpin A is at a higher voltage than pin B. Typicaly the pin A and B are at either +5 or 0 Volts, but the

RS485 limits are +12 and -7 Volts. In other words, the data is carried over a twisted pair. The

transmitting device has a RS485 driver (transmitter) connected to A & B wires, and transmits uses +5and 0 volt levels with respects to the transmitting devices ground. There must be a difference of at leas

200mV between A and B wires for the logic state to be reliably detected. In receiver side wires A & B

need to be within a few volts with respect tothe receivers 0V reference. The RS485 reciever pins willtolerate -7V to +12V relative to signal ground, but you must have the signal ground connected for

reference, or the voltages can easily float outside this range and blow up the RS485 chip (which

happens frequently in real life because people think that the common connection on pin 1 is just ascreen and don't connect it through).

Differential signals radiate less than single-ended signals doand work better in noisy environments. If

the two complementary signals of a differential pair are perfectly balanced, the separation between buswires determines the degree of field cancellation. If two complementary signals are not perfectly

balanced, then the degree of attainable field cancellation is limited to a minimum value determined by

the common-mode balance of the differential pair. Because the common-mode balance of most digitaldrivers is not particularly good, differential pairs often radiate far more power in the common mode

than in the differential mode. In receiving end the receivers are designed to cancel the common mode
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10/22

signal, so with a poor transmitter you might get lots of radiated EMI but signals are still get good

shielding agains external interference on entering the signals on the wire.

The RS-485 bus cable shall be terminated at both ends with a suitable termination resistor that matches

the impedance of the used medium (twisted pair wiring). The purpose of the termination is to preventthe reflection of data at the ends of the cable 120 ohm resistance is recommended for this termination.

matches well the commonly used twisted pair communcation cables (they have typically 100-140 ohm

impedance). With this termination resistor the RS-485 system works well with twisted pair cables that

has impedance of 100-120 ohms (like typical telephone pairs, CAT5 wiring, some shielded twisted paicables). If there is no active driver on the network the line can be forced into a known idle state with a

fail-safe circuit. This fail-safe circuit is complemented by a pulldown resistor and a pullup resistor. Thi

supplement forces the differential mode voltage (i.e. the voltage between the conductors) to a welldefined value when no station is transmitting (during the idle periods). Without this circuit it is likely

that the line will pick up noise and falsely trigger the receivers leading to problematic communications

Usually RS-485 systems are built so that there is the differential pair which carries the signal plus som

ground (usually a shield on cable, can be also mains ground). The ground is usually needed to keep the

common voltage within limits (signals must be between +12 and -7V in receiver) and connected toreceiver. Theoretically you could get the same effect by averaging the line voltage (2 resistorsand a

cap) and then subtracting 2.5V to re create a virtual 0V referencefor the recever chip which wouldallow the use of a simple 2 wire line, but I do not know of anyone doing this.)

Remeber that RS-485 is just electrical specification and can run a large variety of communication

protocols vat various speeds. In simplest applications you can usually just use standard serial protocolsthat are used on rs-232 ports andyou will do just fine. A RS-232 is more susceptible tonoise than RS-

485 and RS-485 can be just used to move the same protocol over greater distances than RS-232. In cas

you have many transmitters and receivers at the same bus, you might soemtimes need something else

than simple serial communications to manage this situation.

In many RS-485 communications you ca just get an RS-232 to RS-485 convertor and then write code

that opens the serial portsand does serial things like would using RS-232. When using an RS-232 toRS-422/485 converter it is important to remember that an RS-485 driver sometimes has to enter tri-

state or become a receiver. Normally the RTS signal from the RS-232 circuit is used to control the stat

of the converter. To work correctly the RTS signal from the RS-232 device must go high for theduration that data is being transmitted from it and go low to allow the converter to receive any message

back. If this signal is not available then it is necessary to use converters that can control the data

direction from received data alone. If you use a converter that does automatically the data direction

handling, don't drive it too fast. Those converters have some time they need to switch the direction(some this kind of converters take up to around 1/2 - 3/4 second to switch direction). Unless you

acknowledge this, you can run into problems. The switchign speeds vary from converter to converter,

but generally with almost any converter you get switching problems at anything over 9600. Some 232-

485 converters with automatic switching are VERY bad. A simple RS-232 to RS4xx Converter shouldnot do any switching. But, you have to user 4 wires (2 pairs) instead of 3 (RX, TX and GND) to allow

full-duplex communications. If someone expects switching, this behaviour has to be programmed in

some way; either by RTS and obying CTS or by timing-loops or in any other fashion. The intelligentexternal converter counts the number of bits after the start bit to find out when the (last) stop bit ended

and then disablethe transmitter, if no new start bit appear immediately after the laststop bit of the

previous character. The converter needs to know theline speed and character length (number of databits, parity bit andstop bits) in order to know when the character has been completed. Usually these are

set by a DIL switches. The quick and dirty (non conforming) alternative is to control the RTS(Transmi

enable) pin directly by the RS-232 TX data stream. The transmitter is activated only when the Space


11/22

("0") bit is to betransmitted. When the RS-232 Tx pin is in the Mark ("1") bit state,the RS-485

transmitter is disabled and it goes to tri-state. The pull-up/pull-down resistors will set the line to theMark state and the receiver(s) will sense the "1" bit. This trick does of course notconform to the RS-48

standard, but it works in many situations. You may have to do your own echo cancellation in software

if the RTS pindoes not disable the receiver. With windows (standard NT or W2K - no extensions or

other hacks needed), you can set the com port to activate RTS when something is being transmitted,and deactivate it when there is nothing being transmitted. I haven't measured the jitter in the RTS signa

compared to the end of transmission, but it seems to work in many applications. When designing such

systems, I make apoint of having the slave device wait a few milliseconds before replying, tocater forslight jitters, but it is not as much as 10 msecs. To make this work well you either need a special UART

in which the Tx shift register empty status bit directly controls the RTS signal, which then controls

thetransmit enable/Rx disable pin, use an external intelligent RS-232/485controller or use the tri-statetrick, which is not standard compliant. Using a proper RS-485 card with proper hardware handshake is

the simplest solution.

The official name for RS485 sepecification is standard ANSI/TIA/EIA-485-A-1998 ElectricalCharacteristics of Generators & Receivers for Use in Balanced Digital Multipoint Systems. EIA-485-A

is compatible with: ISO/IEC 8482:1993 Information Technology - Telecommunications and

information exchange between systems - Twisted pair multipoint interconnections.

The transmission line of choice for RS485 applications is a twisted pair. There are coaxial cables

(twinaxial) made for this purpose that contain straight pairs, but these are less flexible, more bulky, andmore costly than twisted pairs. Many cable manufacturers offer a broad range of 120W cables designed

for RS485 applications. Losses in a transmission line are a complex combination of DC conductor loss

AC losses (skin effect), leakage and AC losses in the dielectric. In good polyethylene cables such as thBelden 9841, the conductor losses and dielectric losses are of the same order of magnitude, leading to

relatively low over all loss (for example 0.2 dB / 100 feet at 100 kHz, 0.6 dB / 100 feet at 1 MHz and

around 2 dB / 100 feet at 10 MHz). When using low loss cables, speeds up to 100 kbit/s can use cable

distances up to 4 kilofeet, when speed increases the cable distance drops according it, being 200 feet at1 Mbit/s and around 40 feet at 2.5 Mbit/s. Those digures can be used as a guideline for choosing the

maximum line length for a given data rate. With lower quality PVC cables, the dielectric loss factor canbe 1000 times worse. PVC twisted pairs have terrible losses at high data rates (>100kBs), and greatlyreduce the maximum cable length. At low data rates however, they are acceptable and much more

economical. The twisted pair cables (CAT5, CAT5e, CAT6) designed for LAN applications and

structured cabling systems are suitable for use with RS-485 system. Those cables work have impedancof 100 ohms, and work well when the system terminated is matched to 100 ohms (usually work

acceptably with original 120 ohm termination).

The proper termination of the cable is very important. If the cable is not terminated with it?s

characteristic impedance, distorted waveforms will result. In severe cases, distorted (false) data and

nulls will occur. If the cable is loaded excessively (much lower than 120 ohm termination), the signal

initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travelsdown the cable and is reflected back out of phase because of the mistermination. When the reflected

signal returns to the driver, the amplitude will be lowered. The width of the pedestal is equal to twice

the electrical length of the cable (about 1.5ns/foot). If the cable is lightly loaded (much higher than 120ohm termination or no termination at all), , the signal reflects in phase and increases the amplitude at

the driver output. You can teast the quality of termination by feeding suitable square wave signal to the

cable from RS-485 driver and looking at the signal waveform on the line with an oscilloscope. An inpufrequency of 30kHz is adequate for tests out to 4000 feet of cable.


12/22

Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. Th

typical differential output voltage of the driver is 2V when the cable is terminated with two 120Wresistors, causing 33mA of DC current to flow in the cable when no data is being sent. In cases where

this loss cannot be tolerated one way to eliminate the unwanted current is by AC coupling the

termination resistors. The coupling capacitor must allow high-frequency energy to flow to the

termination, but block DC and low frequencies. The dividing line between high and low frequencydepends on the length of the cable. The coupling capacitor must pass frequencies above the point wher

the line represents an electrical one-tenth wavelength. The value of the coupling capacitor should

therefore be set at 16.3pF per foot of cable length for 120W cables. With the coupling capacitors inplace, power is consumed only on the signal edges, and not when the driver output is idling at a 1 or 0

state. A 100nF capacitor is adequate for lines up to 4000 feet in length. The power savings start to

decrease once the data rate goes up. Most real-life RS-485 systems are DC terminated.

Grounding is essential to reliable operation of any RS485 network. It is also the most overlooked and

least understood. The differential signal does not require a signal ground to communicate, but the

ground connection serves aa other important purpose. With distances of up to thousands of feetsignificant differences in the ground voltage level are possible. The RS-485 networks can mostly

supply a correct data thransmission with a voltage difference of -7 to +12 Volts. If the ground voltages

differ more than that, data could get lost and the ports could get damaged.

The proper way is to do the siognal grounding though the signal cable shield. The signal ground

connection in the cable ties the signal grounds of each of the nodes together to one single commonground. Using a shielded cable and connecting the shield to signal ground on both ends is the preferred

method. It gives normally the best ground reference and best EMC performance.

Usually the easiest way to ground your RS485 network is to simply use "Earth" ground as your return

path. Although easy this may not be the best method for grounding your application, because current

leaking from equipment, electro-static discharge (ESD), and lightning all drive current through thispath which results in high noise content. The reason for this increased noise level is due to the fact that

"Earth" ground presents a relatively high resistance. RS485 is designed to operate normally with a

ground potential difference of +/- 7 Volts. During normal operations this is typically not a problem,however during fault conditions or lightning strikes even within ? mile the ground potential difference

can reach hundreds and in some cases thousands of volts. This will most likely result in damage or

failure of one or more devices on the RS-485 network. If the differences in signal grounds exceeding,

optical isolation is the best choice.

Many RS485 products are protected against ESD transients (for example up to 2kV using the human

body model 100pF + 1.5kohm). However, some applications need more protection. Uually the bestprotection method is to connect a bidirectional TransZorb? from each line side pin to ground. A

TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities, fas

response time, and low series resistance.

o AN-409: Transceivers and Repeaters Meeting the EIA RS-485 Interface Standard - pdfdocument Rate this link

o AN-759: Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in MultipointApplications - pdf document Rate this link

o AN-979: The Practical Limits of RS - pdf document Rate this linko AN-1057: Ten ways to Bulletproof RS-485 Interfaces - pdf document Rate this linko Basics of the RS-485 standard Rate this link
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o Cable election for RS-422 and RS-485 systems - selecting the cable is not difficult, but casehould be taken because intermittent problems caused by marginal cable can be very difficult to

troubleshoot Rate this link

o Designing RS-485 Circuits - This is a technical article published originally in Circuit Cellar Inkmamagzine. Rate this link

o Explanation of Maxim RS-485 Features - Maxim makes what seems like a bewildering array ofRS-485 drivers and receivers. This application note explains the different features withinMaxim's product line and thus will help you in selecting the most appropriate part for your

needs. Below are explanations of the various features and other important things to keep inmind. Rate this link

o Microcontroller discerns addresses in RS-485 systems - One of the many benefits of using theRS-485 data-interface system, unlike the RS-232 system, is its ability to implement multidrop

networks. Such networks usually carry 9-bit data words, in which the ninth (parity) bit identifie

each word as address or data. When using small microcontrollers without a hardware UART,designers must decide whether to add an external hardware UART or to configure a UART in

software. Ths circuit uses a PIC16F84 microcontroller and MAX3100 UART to impelement

256 address RS-485 bus transceiver. Rate this link

o Pre-emphasis improves RS-485 communications - intersymbol interference places the mainlimitation on the maximum allowable distance for RS-485 communications, but pre-emphasis i

extremely effective in reducing this interference Rate this link

o Quick Reference for RS485, RS422, RS232 and RS423 Rate this linko RS-485 101 - This article examines some general characteristics of a network built with drivers

and receivers compliant with TIA/EIA-485-A and gives a good overview of the whole RS-485

system. Rate this link

o RS-232C/422/485 line isolation solves more than fault problems Rate this linko RS-232C/422/485 line isolation solves more than fault problems - galvanic, or electrical,

isolation is a well-established technique for noise reduction and works best in computer and

industrial systems in which noise can seriously affect data transfer, isolation removes ground-

loop currents and their associated noise voltages from data lines Rate this link

o RS-422/485 Application Note - contains lots of information Rate this linko RS-485 serial port - basic information Rate this linko What is RS485 ? - short description of RS485 interface details Rate this linko Ten ways to bulletproof RS-485 interfaces - dispite its widespread use, RS-485 is not as well

understood as it should be Rate this linko The Art and Science of RS-485 - This article descrubes how successfully implement an RS-485

network. Rate this link

o Yhteysk?yt?nt? RS-422 ja RS-485 - information in Finnish Rate this link

Other RS standards

o AN-216: Summary of Electrical Characteristics of Some Well Known Digital InterfaceStandards - mentions RS-232, RS-422, RS-423, RS-485, EIA/TIA-485, EIA/TIA-562,

EIA/TIA-694, EIA/TIA-612, EIA/TIA-644 (LVDS) and more, pdf document Rate this linko An overview of the LA-1 specification - The LA-1 specification is the look-aside interface to

the network-processing element (NPE). The LA-1 interface primarily targets (though not

exclusively) look-up- and memory-based coprocessors and emphasizes as much as possible theuse of existing technology. It is based on QDR and SigmaRAM technologies. The LA-1

interface operates at speeds of 133 to 200 MHz. Although modeled on an SRAM interface, the

LA-1 specification aims to accommodate other device types; it targets devices such as ternary

content-addressable memories (TCAMs), classifiers, and encryption coprocessors. The LA-1
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interface aims to support the transaction requirements for OC-48 through OC-192 line

rates. Rate this link

o EIA-449 Secondary Interface Rate this linko EIA-530 Interface Reference Rate this linko RS449 Interface Rate this linko RS-449 Interface - The EIA RS-449 standard specifies the functional and mechanical

characteristics of the interconnection between Data Terminal Equipment (DTE) and complying

to EIA electrical interface standards RS-422 and RS-423. Rate this link

o RS530 Interface Rate this link

V.35

The V.35 interface was originally specified by CCITT as an interface for 48kbps line transmissions. It

has been adopted for all line speeds above 20kbps, and seems to have acquired a life of its own. V.35 i

a mixture of balanced (like RS422) and common earth (like RS232) signal interfaces.

The control lines including DTR, DSR. DCD, RTS and CTS are single wire common earth interfaces,

functionally compatible with RS-232 level signals. The data and clock signals are balanced, RS-422-

like signals. The control signals in V.35 are common earth single wire interfaces because these signallevels are mostly constant or vary at low frequencies. The high-frequency data and clock signals are

carried by balanced lines. Thus single wires are used for the low frequencies for which they are

adequate, while balanced pairs are used for the high-frequency data and clock signals.

The V.35 interface is typically found on DTE and DCE equipment interfacing to high speed digital

carrier services. V.35 interface has been around for quite some time and was originally designed for a48K bps modem but it has been shown to cope speeds up to 2 Mbit/s and even faster. This interface is

most often used with equipment which connect data circuits to syncronous telecommunication network

and with some fixed line modems.

The V.35 plug is standard. It is a black plastic plug about 20mm by 70mm, often with gold-platedcontacts and built-in hold down and mating screws. The V.35 plug is an expensive special plug, makineverything to do with V.35 somewhat expensive. The V.35 plug costs roughly 30 times the price of a

DB25. V.35 plug is too large to fit on many add-in cards, such as those used by PCs, thus there is very

often a non standard cable used to connect a V.35 system, terminating in a DB25 at one end and a V.35

plug at the other.

Please note that V.35 no longer officially exists as a ITU standard. It has been replaced by V.10/V.11.You can still though see references to this interface.V.35 was specified to use a 37 pin connector (the

chunkiest in the world). Nowadays for high speed serial connections (that use the term V.35) most

manufacturers use a DB25 connector with EIA/TIA RS-530A standard pinout or a non-standard pinout

V.35 on DB25 (RS-530-A) pinout:

Pin

No.

Name Notes/Description

1 Shield -

2 Transmit Data (A) -

3 Received Data (A) -
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4 RTS (A) Request To Send

5 CTS (A) Clear To Send

6 DCE Ready Data Communications Equipment Ready (modem/CSU)

7 Signal Common -

8 DCD (A) Data Carrier Detect (a.k.a CD or RLSD)

9 Receiver Signal Element Timing (B) RX Clock

10 DCD (B) Data Carrier Detect (a.k.a CD or RLSD)

11 Ext Transmit Clock (B) -

12 Tramsmit Signal Element Timing (B) TX CLOCK

13 CTS (B) Clear to Send

14 Transmitted Data (B) Secondary Transmit Data

15 Transmit Signal element Timing (A) TX CLOCK

16 Received Data (B) RX Data

17 Receiver Signal Element Timing (A) RX CLOCK

18 Local Loopback -

19 RTS (B) Request to Send

20 DTE Ready Host end

21 Remote Loopback -

22 RI Ring Indicator

23 Signal Common -24 Ext TX Clock (A) -

25 Test Mode -

The A and B on the table refer to each signal pair used in balanced serial interfaces. Please note thatthis 530-A pinout specification, the earlier RS-530 (without the A suffix) standard is wired differently.

o RS-232, RS-422 and V.35 interfaces - short description of interfaces Rate this linko V.35 Interface - description of the interface Rate this linko V.35 Interface - connector picture and pinout Rate this linko V.35 on DB25 (RS-530-A) Rate this linko V.35 Technical Reference - V.35 has been around for quite some time. It was originally

designed for a 48 kbps synchronous modem - that's right, officially its top rated speed is 48

kbps. However, V.35 has been used for many years in applications running from 20 kbps up toand past 2 Mbps. In 1989, CCITT BLUE BOOK (ITU) recommended the interface become

obsolete and replaced it with the V.10/V.11 standard. However, V.35 still remains popular, and

has evolved to using the specifications from V.11 for the differential signals, while the control

signals remain unbalanced. The V.11/V.35 BLUE is fully interoperable with the old V.35 RED

interface, except V.35 RED may not handle the speed and distance of the newer spec. Rate

this link
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X.21

The X.21 interface was recommended by the CCITT in 1976. It is defined as a digital signallinginterface between customers (DTE) equipment and carrier's equipment (DCE). And thus primarally

used for telecom equipment. X.21 is a digital signaling interface recommended by ITU-T that includes

specifications for DTE/DCE physical interface elements, alignment of call control characters and errorchecking, elements of the call control phase for circuit switched services, data transfer at up to 2 Mbps

and test loops. 64 kbps is the most commonly used transfer rate. All signals are balanced. Electricallythe X.21 signals are the same as RS422 (X.27 / V.11). This interface normally uses 15-pin D-connector

as the interface. ISO 4903 standard defines this 15-pole DTE/DCE interface connector and contactnumber assignments.Recommendations X.24, X.26 and X.27 are applicable. X.26 support speeds up to

100 kbps. X.27 extends possible speeds up to 10 Mbit/s.

o The X.21 Interface a Tutorial Rate this linko X.21 - brief introduction and pinout Rate this linko X.21 Overview Rate this link

Current loop

20mA transmission has its origins in the telex interfaces used by the post office. The technology was

mainly applied to industrial terminal devices as well as computers in small and larger business systemsDevices of newer design no longer use the 20mA interface, generally favoring instead an RS485 bus

interface.Until the early 1960?s, military teleprinters used 60 ma current loops to communicate over

long distances. In 1962, the Model 33 teletype was introduced and 20 mA current loop interfacesbecame widely used. Throughout the 60?s, 70?s, and early 80?s, 20 mA current loop interfaces were

applied in many types of equipment. Current loop interfaces became popular at this time because they

offered the most cost effective approach to long distance, noise immune data transmission. The 20 mA

current loop is suitable for distances to 2000 feet (650 meters) at data rates up to 19.2k baud withcareful attention to interface design. It can be used at longer distances when data rates are as low as 300

baud.When the EIA 422 Standard (December 1978) and the EIA 485 Standard (April 1983) broughtforth the application of balanced differential digital data transmission, the popularity of 20 mA currentloop rapidly diminished.Current loops are still used widely in process automation. 20 mA are wirely

used for transmitting serial communication data to programmable process controlling devices. The

reason for their use there is that they are quite immune to noise and easy to opto-isolate. The 20mA or

current loop interface transmits the serial data by switching a 20mA current on and off in a loop insynch with the data bits. In the rest state or during transmission of "1" bits, a constant current of 20 mA

flows, whereas "0" bits are indicated by an interrupted current flow. Within each current loop only one

connected device is allowed to provide the required 20mA loop current. This device is referred to as

active, and the other as passive. Decoupling of the user signals from the current loop is generallyaccomplished using optocouplers. This ensures in most applications that there is galvanic isolation

between the connected devices.In digital communications 20 mA current loop is a "de-facto"standard.The transmiter will only source 20 mA and the receiver will only sink 20 mA. Current loops often useopto-couplers. Current is current which matter, the voltages don't matter. Unfortunately there is no

standard regulating the 20mA interface, so that the market is characterized by a number of differently

designed current loop interfaces. The connector form factor and configuration likewise differ amongmanufacturers, for example the signal names and the options for active and passive mode. The data

trasmitted though this kind of onterface is usually a standard RS-232 signal just converted to current

pulses: current on and off depending the logic state of RS-232 circuit. The standard use of current is

such that MARK means that 20 mA current is flowing and space tells that no current is flowing.CurrenLoop is a reliable low baud rate, 20kp/s or less. All devices one the bus or "in the current loop" are
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wired in a series circuit. Up to 10 devices can be wired in a typical current loop circuit. Current loop

interfaces are usually divided to two types: active and passive. Many devices have ports that can bewired to both modes. In "passive mode" the receiver just has one optoisolator input LED wired to the

line. Passive mode transmitter just either passes through the 20 mA current or does not pass any

current. Passive transmitters and receivers are usually opto-isolated. Active transmitter actively

transmit the 20 mA current from some power source or transmit no current depending on logic stateneeded to be transmitted. Active receiver is basically just a combination of passive receiver and 20 mA

constant current source.Current loops can be wired in such way that they have one active interface and

one or more passive interfaced wired to one loop. Current loops can be also built using just passiveinterfaces and an external 20 mA current source.

o Background Information: 20mA Interfaces Rate this linko Circuit translates TTY current loop to RS-232C - This circuit provides signal translation

between a passive current-loop (TTY) interface and a duplex RS-232C ports Rate this linko Current Loop Application Note - The purpose of this current loop application note is to

introduce the reader to the physical aspects of 20 mA current loop communications. Rate

this link

o Current Loop Application Note Rate this linko Current loop (TTY) - THis document describes 10 mA balanced current loop (?10 mA)

technology used by some short-haul modems. Rate this linko HCPL-4100 Optically Coupled 20 mA Current Loop Transmitter Technical Data - The HCPL-

4100 optocoupler is designed to operate as a transmitter in equipment using the 20 mA current

loop. 20 mA current loop systems conventionally signal a logic high state by transmitting 20mA of loop current (MARK), and signal a logic low state by allowing no more than a few

milliamperes of loop current (SPACE). Optical coupling of the signal from the logic input to th

20 mA current loop breaks ground loops and provides very high immunity to common mode

interferences. Rate this link

o Self-Powered Current Loop Converter Model 232CL3 - This circuit converts between RS-232and 20 mA current loop. Rate this link

o The Basics of Current Loop - This page introduces the basic 20 mA current loopinterfaces. Rate this link

Low Voltage Differential Signaling (LVDS)

Low Voltage Differential Signaling (LVDS) is an electrical interfacethat can quickly handle a largeamount of data over long cable lengths. LVDS is a Low Voltage Differential Signaling device which

extends the performance of the commonly used RS-422 differential data bus. RS-422 limits the

frequency to the 20 MHz range. However, LVDS clock support is over 65MHz (66 MHz NSC 65 MHzTI) and improves the signal transmission

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