Serial Communication Standards

8/2/2019 Serial Communication Standards

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Common Implementations of Interfaces

Parallel port (8 bits per shot)

Serial (RS-232, RS-485)

usually asynchronousGPIB (IEEE-488) parallel

General Purpose Interface (or Instrument) Bus

Originally HPIB; Hewlett PackardDAQ card (data acquisition)

Like national instruments A/D, D/A, digital I/O

CAMACComputer Automated Measurement And Control

VME bus / VXI bus

Modern CAMAC-like bus

Exchanging Data

Parallel: Fast and expensive

devices A, B simple, but cabling harder

strobe alerts to data valid state

Serial: Slow and cheap

but devices A and must convert between serial/parallel


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Serial Communication Standards

All long-haul communications and most computer networks use serial connections,

because the cost of cable and synchronization difficulties make parallel connections

impractical. The most significant advantage is simpler wiring. Also, serial cables can be

longer than parallel cables, because there is much less interaction (crosstalk) among the

conductors in the cable. In this chapter, we will confine our consideration

of serial communications to those connecting LANs to WANs.

The figure is a simple representation of a serial communication. Data is encapsulated by

the communications protocol used by the sending router. The encapsulated frame is sent on

a physical medium to the WAN. There are various ways to traverse the WAN, but thereceiving router uses the same communications protocol to de-encapsulate the frame when

it arrives.

There are many different serial communication standards, each one using a different

signaling method. There are three key serial communication standards affecting LAN-to-

WAN connections:

RS-232 - Most serial ports on personal computers conform to the RS-232C or

newer RS-422 and RS-423 standards. Both 9-pin and 25-pin connectors are used.

A serial port is a general-purpose interface that can be used for almost any type of

device, including modems, mice, and printers. Many network devices use RJ-45

connectors that also conform to the RS-232 standard. The figure shows an example

of an RS-232 connector.
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V.35 - Typically used for modem-to-multiplexer communication, this ITU standard

for high-speed, synchronous data exchange combines the bandwidth of several

telephone circuits. In the U.S., V.35 is the interface standard used by

most routers and DSUs that connect to T1 carriers. V.35 cables are high-

speed serial assemblies designed to support higher data rates and connectivity

between DTEs and DCEs over digital lines. There is more on DTEs and DCEs later

in this section.

HSSI - A High-Speed Serial Interface (HSSI) supports transmission rates up to 52

Mb/s. Engineers use HSSI to connect routers on LANs with WANs over high-speed

lines such as T3 lines. Engineers also use HSSI to provide high-speed connectivity

between LANs, using Token Ring or Ethernet. HSSI is a DTE/DCE interfacedeveloped by Cisco Systems and T3plus Networking to address the need for high-

speed communication over WAN links.

As well as using different signaling methods, each of these standards uses different types of

cables and connectors. Each standard plays a different role in a LAN-to-WAN topology.

While this course does not examine the details of V.35 and HSSI pinning schemes, a quick

look at a 9-pin RS-232 connector used to connect a PC to a modem helps illustrate the

concept. A later topic looks at V.35 and HSSI cables.

Pin 1 - Data Carrier Detect (DCD) indicates that the carrier for the transmit data is

ON.

Pin 2 - The receive pin (RXD) carries data from the serial device to the computer.

Pin 3 - The transmit pin (TxD) carries data from the computer to theserial device.

Pin 4 - Data Terminal Ready (DTR) indicates to the modem that the computer is

ready to transmit.

Pin 5 - Ground

Pin 6 - Data Set Ready (DSR) is similar to DTR. It indicates that the Dataset is ON.


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Pin 7 - The RTS pin requests clearance to send data to a modem

Pin 8 - The serial device uses the Clear to Send (CTS) pin to acknowledge the RTS

signal of the computer. In most situations, RTS and CTS are constantly ON throughout

the communication session.

Pin 9 - An auto answer modem uses the Ring Indicator (RI) to signal receipt of atelephone ring signal.

The DCD and RI pins are only available in connections to a modem. These two lines

are used rarely because most modems transmit status information to a PC when a

carrier signal is detected (when a connection is made to another modem) or when the

modem receives a ring signal from the telephone line.

IEEE-488 INTERFACE BUS (HP-IB/GP-IB)

In the early 1970's, Hewlett-Packard came out with a standard bus (HP-IB) to help support

their own laboratory measurement equipment product lines, which later was adopted by theIEEE in 1975. This is known as the IEEE Std. 488-1975. The IEEE-488 Interface Bus (HP-

IB) or general purpose interface bus (GP-IB) was developed to provide a means for variousinstruments and devices to communicate with each other under the direction of one or moremaster controllers. The HP-IB was originally intended to support a wide range of

instruments and devices, from the very fast to the very slow.

Description:

The HP-IB specification permits up to 15 devices to be connected together in any givensetup, including the controller if it is part of the system. A device may be capable of any

other three types of functions: controller, listener, or talker. A device on the bus may have

only one of the three functions active at a given time. A controller directs which deviceswill be talkers and listeners. The bus will allow multiple controllers, but only one may be

active at a given time. Each device on the bus should have a unique address in the range of

0-30. The maximum length of the bus network is limited to 20 meters total transmissionpath length. It is recommended that the bus be loaded with at least one instrument or device

every 2 meter length of cable (4 meters is maximum). The use of GP-IB extenders may be

used to exceed the maximum permitted length of 20 meters.

Electrical Interface:

The GP-IB is a bus to which many similar modules can be directly connected, as is shown

in Figure 1. A total of 16 wires are shown in the figure - eight data lines and eight control

lines. The bus cables actually have 24 wires, providing eight additional for shielding andgrounds.


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Serial Communication Standards