National Instruments Network Router GPIB 120A User Manual

GPIB-120A

User Manual

October 1994 Edition

Part Number 370893A-01

Limited Warranty

The GPIB-120A is warranted against defects in materials and workmanship for a period of two years from the date

of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or

replace equipment that proves to be defective during the warranty period. This warranty includes parts and labor.

A Return Material Authorization (RMA) number must be obtained from the factory and clearly marked on the

outside of the package before any equipment will be accepted for warranty work. National Instruments will pay the

shipping costs of returning to the owner parts which are covered by warranty.

National Instruments believes that the information in this manual is accurate. The document has been carefully

reviewed for technical accuracy. In the event that technical or typographical errors exist, National Instruments

reserves the right to make changes to subsequent editions of this document without prior notice to holders of this

edition. The reader should consult National Instruments if errors are suspected. In no event shall National

Instruments be liable for any damages arising out of or related to this document or the information contained in it.

EXCEPT AS SPECIFIED HEREIN, NATIONAL INSTRUMENTS MAKES NO WARRANTIES, EXPRESS OR IMPLIED,

AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR

PURPOSE. CUSTOMER'S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART

OF NATIONAL INSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER.

NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING FROM LOSS OF DATA, PROFITS,

USE OF PRODUCTS, OR INCIDENTAL OR CONSEQUENTIAL DAMAGES, EVEN IF ADVISED OF THE POSSIBILITY

THEREOF. This limitation of the liability of National Instruments will apply regardless of the form of action,

whether in contract or tort, including negligence. Any action against National Instruments must be brought within

one year after the cause of action accrues. National Instruments shall not be liable for any delay in performance due

to causes beyond its reasonable control. The warranty provided herein does not cover damages, defects,

malfunctions, or service failures caused by owner's failure to follow the National Instruments installation, operation,

or maintenance instructions; owner's modification of the product; owner's abuse, misuse, or negligent acts; and

power failure or surges, fire, flood, accident, actions of third parties, or other events outside reasonable control.

Under the copyright laws, this publication may not be reproduced or transmitted in any form, electronic or

mechanical, including photocopying, recording, storing in an information retrieval system, or translating, in whole

or in part, without the prior written consent of National Instruments Corporation.

Trademarks

Product and company names listed are trademarks or trade names of their respective companies.

WARNING REGARDING MEDICAL AND CLINICAL USE

OF NATIONAL INSTRUMENTS PRODUCTS

National Instruments products are not designed with components and testing intended to ensure a level of reliability

suitable for use in treatment and diagnosis of humans. Applications of National Instruments products involving

medical or clinical treatment can create a potential for accidental injury caused by product failure, or by errors on

the part of the user or application designer. Any use or application of National Instruments products for or involving

medical or clinical treatment must be performed by properly trained and qualified medical personnel, and all

traditional medical safeguards, equipment, and procedures that are appropriate in the particular situation to prevent

serious injury or death should always continue to be used when National Instruments products are being used.

National Instruments products are NOT intended to be a substitute for any form of established process, procedure, or

equipment used to monitor or safeguard human health and safety in medical or clinical treatment.

FCC/DOC Radio Frequency Interference Compliance

This equipment generates and uses radio frequency energy and, if not installed and used in strict accordance with the

instructions in this manual, may cause interference to radio and television reception. This equipment has been tested

and found to comply with the following two regulatory agencies:

Federal Communications Commission

This device complies with Part 15 of the Federal Communications Commission (FCC) Rules for a Class A digital

device. Operation is subject to the following two conditions:

1. This device may not cause harmful interference in commercial environments.

2. This device must accept any interference received, including interference that may cause undesired operation.

Canadian Department of Communications

This device complies with the limits for radio noise emissions from digital apparatus set out in the Radio

Interference Regulations of the Canadian Department of Communications (DOC).

Le présent appareil numérique n’émet pas de bruits radioélectriques dépassant les limites applicables aux appareils

numériques de classe A prescrites dans le règlement sur le brouillage radioélectrique édicté par le ministère des

communications du Canada.

Instructions to Users

These regulations are designed to provide reasonable protection against harmful interference from the equipment to

radio reception in commercial areas. Operation of this equipment in a residential area is likely to cause harmful

interference, in which case the user will be required to correct the interference at his own expense.

There is no guarantee that interference will not occur in a particular installation. However, the chances of

interference are much less if the equipment is installed and used according to this instruction manual.

If the equipment does cause interference to radio or television reception, which can be determined by turning the

equipment on and off, one or more of the following suggestions may reduce or eliminate the problem.

•

Operate the equipment and the receiver on different branches of your AC electrical system.

Move the equipment away from the receiver with which it is interfering.

Reorient or relocate the receiver’s antenna.

Be sure that the equipment is plugged into a grounded outlet and that the grounding has not been defeated with

a cheater plug.

Notice to user: Changes or modifications not expressly approved by National Instruments could void the user’s

authority to operate the equipment under the FCC Rules.

If necessary, consult National Instruments or an experienced radio/television technician for additional suggestions.

The following booklet prepared by the FCC may also be helpful: How to Identify and Resolve Radio-TV

Interference Problems. This booklet is available from the U.S. Government Printing Office, Washington, DC

20402, Stock Number 004-000-00345-4.

Danger

The GPIB-120A does not generate high voltages; however, you must use extreme caution

if your system or application can cause high voltages on the shield or logic ground of the

GPIB cable you attach to the electrically isolated connector (GPIB Port B) located on the

rear panel of the GPIB-120A. Many GPIB cables use a metal shell to enclose the GPIB

piggyback connectors. This metal shell is normally connected to the cable shield.

Therefore, if the shield is at a high potential with respect to the chassis ground of the

GPIB-120A, the high voltage is also present, in most cases, on the cable connector

housing.

The chassis, back panel, and front panel of the GPIB-120A are electrically connected to

the earth ground pin of the AC power connector on the back panel. If the AC wiring in

your building is installed correctly, you can safely operate the GPIB-120A. Because the

EMI shield and jack screws of rear panel connector ‘GPIB A’ are electrically shorted to

the rear panel, a GPIB cable attached to GPIB Port A has its shield and connector

housing connected to earth ground, as well, and no danger should exist associated with

GPIB Port A. There may be, however, a hazardous voltage between the cable connector

housings of the GPIB cable attached to GPIB Port A and the GPIB cable attached to

GPIB Port B.

Do not touch the GPIB-120A case, power cord, or any GPIB cable connected to the

GPIB-120A unless you are absolutely sure that there are no hazardous voltages present.

In addition, never open the GPIB-120A case unless you have removed the power cord

and all GPIB cables from the back panel.

The GPIB-120A is not recommended for use when ground potential differences in excess

of 30 V are present between the two rear panel GPIB connectors and associated cables,

unless sufficient precautions are taken to ensure human safety.

Contents

About This Manual ...............................................................................................................ix

Organization of This Manual ...........................................................................................ix

Conventions Used in This Manual...................................................................................ix

Customer Communication ...............................................................................................x

Chapter 1

Introduction............................................................................................................................1-1

Description of the GPIB-120A ........................................................................................1-1

What You Need to Get Started ........................................................................................1-3

Optional Equipment .........................................................................................................1-3

Unpacking Your GPIB-120A...........................................................................................1-3

Chapter 2

Connection...............................................................................................................................2-1

Isolation Considerations...................................................................................................2-1

Placing Your Instruments.................................................................................................2-2

Connecting the GPIB-120A .............................................................................................2-2

Connecting to Hewlett-Packard Controllers ........................................................2-2

Mounting the GPIB-120A................................................................................................2-2

Chapter 3

Theory of Operation ............................................................................................................3-1

Circuitry of the GPIB-120A.............................................................................................3-1

The GPIB-120A Front Panel............................................................................................3-2

Power On..............................................................................................................3-2

System Controller Detection................................................................................3-2

Active Controller Detection .................................................................................3-2

Source Handshake Detection ...............................................................................3-2

Data Direction Control.........................................................................................3-3

Parallel Poll Detection .........................................................................................3-3

Acceptable Identification Codes ......................................................................................3-4

Appendix A

Operation of the GPIB........................................................................................................A-1

History of the GPIB .........................................................................................................A-1

GPIB Operation................................................................................................................A-1

Types of Messages ...............................................................................................A-1

Talkers, Listeners, and Controllers ......................................................................A-2

The Controller-In-Charge and System Controller ...............................................A-2

GPIB Signals and Lines .......................................................................................A-3

Data Lines ................................................................................................A-3

Handshake Lines ......................................................................................A-3

NRFD (Not Ready For Data) .......................................................A-3

vii

GPIB-120A User Manual

Contents

NDAC (Not Data Accepted) ........................................................A-3

DAV (Data Valid)........................................................................A-3

Interface Management Lines....................................................................A-4

ATN (Attention)...........................................................................A-4

IFC (Interface Clear)....................................................................A-4

REN (Remote Enable) .................................................................A-4

SRQ (Service Request) ................................................................A-4

EOI (End Or Identify) ..................................................................A-4

Physical and Electrical Characteristics ................................................................A-5

Configuration Restrictions: The Role of Expanders and Extenders ...................A-8

Appendix B

Specifications..........................................................................................................................B-1

Appendix C

Multiline Interface Messages ............................................................................................C-1

Appendix D

Customer Communication.................................................................................................D-1

Figures

Figure 1-1.

Figure 1-2.

Typical GPIB-120A Extension System (Physical Configuration) .....................1-1

Typical GPIB-120A Extension System (Logical Configuration).......................1-2

Figure 3-1.

GPIB-120A Block Diagram ...............................................................................3-1

Figure A-1.

Figure A-2.

Figure A-3.

GPIB Connector and the Signal Assignment......................................................A-5

Linear Configuration of the GPIB Devices ........................................................A-6

Star Configuration of the GPIB Devices ............................................................A-7

Tables

Table 3-1.

IEEE 488 Capability Identification Codes .........................................................3-4

System Configuration .........................................................................................B-1

Performance Characteristics ...............................................................................B-1

Electrical Characteristics ....................................................................................B-2

Environmental Characteristics............................................................................B-2

Physical Characteristics ......................................................................................B-2

GPIB-120A User Manual

viii

About This Manual

This manual describes how to use the National Instruments GPIB-120A bus expander/isolator.

Organization of This Manual

This manual is organized as follows:

•

Chapter 1, Introduction, contains a description of the GPIB-120A, lists what you need to get

started and optional equipment you can order, and explains how to unpack the GPIB-120A.

•

Chapter 2, Connection, contains information for connecting your GPIB-120A.

Chapter 3, Theory of Operation, describes the operational theory of the GPIB-120A.

Appendix A, Operation of the GPIB, contains a brief history of the GPIB and describes the

operation of the GPIB.

•

Appendix B, Specifications, lists the specifications of the GPIB-120A.

Appendix C, Multiline Interface Messages, contains an interface message reference list,

which describes the mnemonics and messages that correspond to the interface functions.

•

Appendix D, Customer Communication, contains forms you can use to request help from

National Instruments or to comment on our products or manuals.

The Glossary contains an alphabetical list and a description of terms used in this manual,

including abbreviations, acronyms, metric prefixes, mnemonics, and symbols.

Conventions Used in This Manual

The following conventions are used in this manual.

italic

Italic text denotes emphasis, a cross reference, or an introduction to a key

concept.

bold italic

Bold italic text denotes a note, caution, warning, or danger statement.

Abbreviations, acronyms, metric prefixes, mnemonics, symbols, and terms are listed in the

Glossary.

GPIB-120A User Manual

About This Manual

Related Documentation

The following document contains information that you may find helpful as you read this manual.

•

ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable

Instrumentation

Customer Communication

National Instruments wants to receive your comments on our products and manuals. We are

very interested in the applications you develop with our products, and we want to help if you

have problems with them. To make it easy for you to contact us, this manual contains comment

and configuration forms for to you complete. These forms are in Appendix D, Customer

Communication, at the end of this manual.

GPIB-120A User Manual

Chapter 1

Introduction

This chapter contains a description of the GPIB-120A, lists what you need to get started and

optional equipment you can order, and explains how to unpack the GPIB-120A.

Description of the GPIB-120A

The GPIB-120A is a high-speed bus expander/isolator with the following features:

•

It is transparent to user software.

It electrically isolates two GPIB systems.

It expands the GPIB to interface up to 28 devices.

It extends the GPIB by effectively doubling the 20 m cable limit.

It has optional rack-mount hardware.

The high-speed GPIB-120A bus expander connects two GPIB (IEEE 488) bus systems in a

functionally transparent manner.

The two bus systems are electrically isolated from each other. Isolating an instrument or group

of instruments from an IEEE 488 bus Controller can eliminate ground loop noise and induced

common-mode noise, which may cause measurement problems in both analog and digital

systems. The two isolated bus systems are physically separate, as shown in Figure 1-1; however,

the devices logically appear to be located on the same bus, as shown in Figure 1-2.

GPIB #1

GPIB-120A

GPIB #2

Computer

(Controller,

Talker, and Listener)

Printer

(Listener)

Multimeter

(Talker and

Listener)

Signal Generator

(Listener)

Unit Under Test

Figure 1-1. Typical GPIB-120A Extension System (Physical Configuration)

1-1

GPIB-120A User Manual

Introduction

Chapter 1

GPIB

Computer

(Controller,

Talker, and Listener)

Printer

(Listener)

Multimeter

(Talker and

Listener)

Signal Generator

(Listener)

Unit Under Test

Figure 1-2. Typical GPIB-120A Extension System (Logical Configuration)

With the GPIB-120A, it is possible to overcome the following two configuration restrictions

imposed by the ANSI/IEEE Standard 488.1-1987:

•

An electrical loading limit of 15 devices per contiguous bus.

A cable length limit of 20 m total per contiguous bus or 2 m times the number of devices on

the bus, whichever is smaller.

With each GPIB-120A, you can add up to 14 additional devices to the bus. The GPIB-120A

appears as a device load on each side of the expansion; therefore, one GPIB-120A increases the

maximum load limit from 15 devices to 28 devices. The cable length limit for the system is also

increased an additional 4 m to 20 m, depending on the number of devices on that side of the

expansion.

All signal expansion is bidirectional, meaning that Controllers, Talkers, and Listeners can be on

either side of the expander. The GPIB-120A front panel light-emitting diodes (LEDs) indicate

the location of the System Controller, Active Controller, and Source Handshaker, with respect to

the two sides of the expansion.

Because the GPIB-120A is a functionally transparent expander, the same GPIB communications

and control programs that work with an unexpanded system can work unmodified with an

expanded system.

GPIB-120A User Manual

1-2

Chapter 1

Introduction

What You Need to Get Started

One of the following GPIB-120A Bus Expanders/Isolators:

GPIB-120A (100 to 120 VAC)

GPIB-120A (220 to 240 VAC)

2.2 m, 125 VAC Power Cord (When connected to a power source, this cord connects the

equipment chassis to power ground.)

Optional Equipment

You can contact National Instruments to order any of the following optional equipment.

•

Rack-Mount Kit

Single Rack-Mount Kit

Dual Rack-Mount Kit

Shielded GPIB cables*

Type X1 single-shielded GPIB cables (1 m, 2 m, or 4 m)

Type X2 double-shielded GPIB cables (1 m, 2 m, or 4 m)

* To meet FCC emission limits for this Class A device, you must use a shielded (Type X1 or

X2) GPIB cable. Operating this equipment with a non-shielded cable may cause interference

to radio and television reception in commercial areas.

Unpacking Your GPIB-120A

Follow these steps when unpacking your GPIB-120A.

1. Verify that the package you received contained everything you need to get started, as listed

earlier in this chapter.

2. Inspect the shipping container and contents for damage. If the container is damaged and the

damage appears to have been caused in shipment, file a claim with the carrier. If the

equipment is damaged, do not attempt to operate it. Contact National Instruments for

instructions. Retain the shipping material for possible inspection by carrier or reshipment of

the equipment.

3. Verify that the voltage you will be using is the same as that marked on the rear panel of the

GPIB-120A.

Note: The GPIB-120A is shipped from the factory set at a certain operating voltage, either

100 to 120 VAC or 220 to 240 VAC. Operating the unit at the wrong voltage can

damage the unit. If the GPIB-120A is set at a voltage other than the one you will be

using, contact National Instruments for further instructions.

1-3

GPIB-120A User Manual

Chapter 2

Connection

This chapter contains information for connecting your GPIB-120A.

Danger: The GPIB-120A does not generate high voltages; however, you must use extreme

caution if your system or application can cause high voltages on the shield or logic

ground of the GPIB cable you attach to the electrically isolated connector (GPIB

Port B) located on the rear panel of the GPIB-120A. Many GPIB cables use a

metal shell to enclose the GPIB piggyback connectors. This metal shell is normally

connected to the cable shield. Therefore, if the shield is at a high potential with

respect to the chassis ground of the GPIB-120A, the high voltage is also present, in

most cases, on the cable connector housing.

The chassis, back panel, and front panel of the GPIB-120A are electrically

connected to the earth ground pin of the AC power connector on the back panel. If

the AC wiring in your building is installed correctly, you can safely operate the

GPIB-120A. Because the EMI shield and jack screws of rear panel connector

‘GPIB A’ are electrically shorted to the rear panel, a GPIB cable attached to GPIB

Port A has its shield and connector housing connected to earth ground, as well,

and no danger should exist associated with GPIB Port A. There may be, however,

a hazardous voltage potential between the cable connector housings of the GPIB

cable attached to GPIB Port A and the GPIB cable attached to GPIB Port B.

Never touch the GPIB-120A case, power cord, or any GPIB cable connected to the

GPIB-120A unless you are absolutely sure that there are no hazardous voltages

present. In addition, never open the GPIB-120A case unless you have removed the

power cord and all GPIB cables from the back panel.

The GPIB-120A is not recommended for use when ground potential differences in

excess of 30 V are present between the two rear panel GPIB connectors and

associated cables unless sufficient precautions are taken to ensure human safety.

Isolation Considerations

Noise and ground loop problems are common when performing small signal analysis. You can

solve such problems by isolating a group of sensitive GPIB instruments from other noisier

instruments.

Your GPIB-120A has two GPIB connectors that are electrically isolated from each other: Bus A

and Bus B. Bus B is also isolated from the chassis and back panel of the GPIB-120A.

2-1

GPIB-120A User Manual

Connection

Chapter 2

Placing Your Instruments

Identify the instruments or devices in your system that are sensitive to ground loop noise. In the

next section, Connecting the GPIB-120A, you will connect these instruments or devices to Bus B

of your GPIB-120A. Then, you will connect the other instruments or devices to Bus A. Usually

you connect your computer or other device acting as the GPIB System Controller to Bus A.

Connecting the GPIB-120A

To connect the GPIB-120A, follow these instructions:

1. Make sure that the power switch on the back panel is in the off position.

2. Plug the utility power cord into an acceptable electrical outlet (100 to 120 VAC or 220 to

240 VAC, depending upon which model of the GPIB-120A you have). Plug the other end

of the power cord into the back panel of the GPIB.

3. Link your GPIB instrument(s), board(s), and other device(s) to the GPIB-120A with

appropriate cables (type X1 or X2). Use Bus A for devices that do not need to be electrically

isolated. Use Bus B for devices that need to be electrically isolated from devices on Bus A.

4. Move the power switch to the on position.

Connecting to Hewlett-Packard Controllers

To achieve very high data transfer rates and long cable spans between devices, many HP

Controllers and computers, such as the 64000 series, use a preload technique on the unit

designated Master Controller. When preloaded, the GPIB lines of the Master Controller are

terminated to represent six device loads. HP has two types of preloading: Class A, in which all

16 GPIB lines are loaded, and Class B, in which all lines, except NRFD and NDAC lines, are

loaded.

Preloading increases ringing on signal transitions and can cause improper operation of the

GPIB-120A. If this happens, set all signals on the Master Controller to normal (1 unit) load.

This is done using a back panel switch on your HP Controller. In addition, you must also strictly

conform to the cabling rule of no more than two meters per device.

Mounting the GPIB-120A

The GPIB-120A is designed for table-top or rack-mount operation. Single- and dual-unit

rack-mount kits are available from National Instruments. Refer to the Optional Equipment

section of Chapter 1, Introduction, for ordering information on the rack-mount kits.

GPIB-120A User Manual

2-2

Chapter 3

Theory of Operation

This chapter describes the operational theory of the GPIB-120A. This chapter assumes that you

have a basic knowledge of the GPIB. If you are a first-time user or you would like to review the

basics, refer to Appendix A, Operation of the GPIB, for a history and the basic operation of the

GPIB.

Circuitry of the GPIB-120A

The GPIB-120A consists of two sides: Bus A and Bus B. The circuitry for both sides is

logically identical and the two sides are electrically isolated from each other. The only

difference between the two sides is that logic ground on Bus A is connected to the chassis

ground while the logic ground for Bus B is not. Thus, Bus B is the isolated side of the

expansion. To reduce measurement problems caused by noise and ground loops, the

measurement instruments must be located on the isolated Bus B and all other devices must be

located on Bus A. A block diagram for the GPIB-120A is shown in Figure 3-1.

Controller

Detection

Controller

Detection

Source

Handshake

Detection

Source

Handshake

Detection

Data

Direction

Control

Data

Direction

Control

Parallel

Poll

Detection

Parallel

Poll

Detection

Side A

Side B

Figure 3-1. GPIB-120A Block Diagram

3-1

GPIB-120A User Manual

Theory of Operation

Chapter 3

The circuitry on each side of the expansion monitors local GPIB states and converts the signals

monitored into X signals (for transmit) which drive the R signals (for receive) on the opposite

side of the expansion. The X signals are optically isolated from the R signals. Each GPIB signal

is sensed or driven depending on the System Controller, Active Controller, and Source

Handshake states of each side of the expansion.

The GPIB-120A Front Panel

The GPIB-120A front panel has seven light-emitting diodes (LEDs). The POWER LED on the

left-hand side of the front panel is lit whenever you power on the GPIB-120A.

For each bus, an LED indicates the status of the System Controller, Active Controller, or Source

Handshake state.

Power On

When you power on the GPIB-120A, all circuitry is cleared to an initialized state. The

GPIB-120A expansion system is fully operational when you power on the GPIB-120A and your

instruments are connected. Where there is GPIB activity, it is recommended that you keep at

least two-thirds of the devices on both buses powered on.

System Controller Detection

After you power on, Bus A and Bus B System Controller states are false.

If a GPIB device on Bus A asserts IFC or REN, the Bus A System Controller state becomes true

and the Bus B System Controller state becomes false.

If a GPIB device on Bus B asserts IFC or REN, the Bus B System Controller state becomes true

and the Bus A System Controller state becomes false.

Active Controller Detection

After you power on, Bus A and Bus B Active Controller states are false.

If a GPIB device on Bus A asserts ATN, the Bus A Active Controller state becomes true and the

Bus B Active Controller state becomes false.

If a GPIB device on Bus B asserts ATN, the Bus B Active Controller state becomes true and the

Bus A Active Controller state becomes false.

Source Handshake Detection

A device is considered a source handshaker if it is an active Controller sourcing command bytes

or if it is a Talker sourcing data bytes.

After you power on, Bus A and Bus B Source Handshake states are false.

GPIB-120A User Manual

3-2

Chapter 3

Theory of Operation

If a GPIB device on Bus A asserts DAV, the Bus A Source Handshake state becomes true and

the Bus B Source Handshake state becomes false.

If a GPIB device on Bus B asserts DAV, the Bus B Source Handshake state becomes true and

the Bus A Source Handshake state becomes false.

Bus A and Bus B Source Handshake states also become false when a parallel poll begins or

when the ATN signal changes states.

Data Direction Control

Bus B sends the data lines to Bus A if the Bus B Source Handshake state is true or if a Controller

on Bus A is conducting a parallel poll.

Bus A sends the data lines to Bus B if the Bus A Source Handshake state is true or if a Controller

on Bus B is conducting a parallel poll.

Parallel Poll Detection

Controllers can conduct parallel polls on Bus A or Bus B with no modification of user software.

Devices on both Bus A and Bus B can respond to parallel polls.

If a Controller on Bus A conducts a parallel poll, the parallel poll detection circuitry on side B

conducts a parallel poll on Bus B. The result of the parallel poll is driven on the data lines of

Bus A.

If a Controller on Bus B conducts a parallel poll, the parallel poll detection circuitry on side A

conducts a parallel poll on Bus A. The result of the parallel poll is driven on the data lines of

Bus B.

3-3

GPIB-120A User Manual

Theory of Operation

Chapter 3

Acceptable Identification Codes

Bus A and Bus B of the GPIB-120A are each capable of appearing as a GPIB device having the

GPIB capabilities listed in Table 3-1. For a complete description of each code, consult the

ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable

Instrumentation.

Table 3-1. IEEE 488 Capability Identification Codes

Capability Code

SH1

Description

Source Handshake

AH1

Acceptor Handshake

Talker, Extended Talker

Listener, Extended Listener

Service Request

Remote/Local

T5, TE5

L3, LE3

SR1

RL1

PP1, PP2

DC1

Parallel Poll

Device Clear

DT1

Device Trigger

C1, C2, C3, C4, C5

E1, E2

Controller

Three-state bus drivers with automatic switch

to open collector during parallel poll

GPIB-120A User Manual

3-4

Appendix A

Operation of the GPIB

This appendix contains a brief history of the GPIB and describes the operation of the GPIB.

History of the GPIB

The original GPIB was designed by Hewlett-Packard (where it is called the HP-IB) to connect

and control programmable instruments manufactured by Hewlett-Packard. Because of its high

data transfer rates of up to 1 Mbytes/s, the GPIB quickly gained popularity in other applications

such as intercomputer communication and peripheral control. It was later accepted as the

industry standard IEEE 488. The versatility of the system prompted the name General Purpose

Interface Bus.

National Instruments expanded the use of the GPIB among users of computers manufactured

by companies other than Hewlett-Packard. National Instruments specializes both in high-

performance, high-speed hardware interfaces and in comprehensive, full-function software that

helps users bridge the gap between their knowledge of instruments and computer peripherals and

of the GPIB itself.

GPIB Operation

The GPIB is a link, or interface system, through which interconnected electronic devices

communicate. Communication among interconnected GPIB devices is achieved by passing

messages through the interface system.

Types of Messages

The GPIB carries device-dependent messages and interface messages.

•

Device-dependent messages, often called data or data messages, contain device-specific

information such as programming instructions, measurement results, machine status, and data

files.

Interface messages manage the bus itself. They are usually called commands or command

messages. Interface messages perform such tasks as initializing the bus, addressing and

unaddressing devices, and setting device modes for remote or local programming.

The term command as used here should not be confused with some device instructions which can

also be called commands. Such device-specific instructions are actually data messages.

A-1

GPIB-120A User Manual

Operation of the GPIB

Appendix A

Talkers, Listeners, and Controller

There are three types of GPIB communicators: Talkers, Listeners, and Controllers. A Talker

sends data messages to one or more Listeners. The Controller manages the flow of information

on the GPIB by sending commands to all devices.

Devices can be Listeners, Talkers, and/or Controllers. A digital voltmeter, for example, is a

Talker and may be a Listener as well.

The GPIB is a bus like an ordinary computer bus, except that the computer has its circuit cards

interconnected via a backplane bus, whereas the GPIB has standalone devices interconnected via

a cable bus.

The role of the GPIB Controller can also be compared to the role of the CPU of a computer, but

a better analogy is to the switching center of a city telephone system.

The switching center (Controller) monitors the communications network (GPIB). When the

center (Controller) notices that a party (device) wants to make a call (send a data message), it

connects the caller (Talker) to the receiver (Listener).

The Controller addresses a Talker and a Listener before the Talker can send its message to the

Listener. After the message is transmitted, the Controller may unaddress both devices.

Some bus configurations do not require a Controller. For example, one device may always be a

Talker (called a Talk-only device) and there may be one or more Listen-only devices.

A Controller is necessary when the active or addressed Talker or Listener must be changed. The

Controller function is usually handled by a computer.

With a GPIB interface board and its software, your personal computer plays all three roles.

•

Controller – to manage the GPIB

Talker – to send data

Listener – to receive data

The Controller-In-Charge and System Controller

Although there can be multiple Controllers on the GPIB, only one Controller at a time is active

or Controller-In-Charge (CIC). Active control can be passed from the current CIC to an idle

Controller. Only one device on the bus, the System Controller, can make itself the CIC. A GPIB

interface board in a computer is usually the System Controller.

GPIB-120A User Manual

A-2

Appendix A

Operation of the GPIB

GPIB Signals and Lines

The interface system consists of 16 signal lines and 8 ground return or shield drain lines.

The 16 signal lines are divided into the following three groups.

•

Eight data lines

Three handshake lines

Five interface management lines

Data Lines

The eight data lines, DI01 through DI08, carry both data and command messages. All

commands and most data use the 7-bit ASCII or ISO code set, in which case the eighth bit, DI08,

is unused or used for parity.

Handshake Lines

Three lines asynchronously control the transfer of message bytes among devices. The process is

called a three-wire interlocked handshake, and it guarantees that message bytes on the data lines

are sent and received without transmission error.

NRFD (Not Ready For Data)

NRFD indicates when a device is ready or not ready to receive a message byte. The line is

driven by all devices when receiving commands and by Listeners when receiving data messages.

NDAC (Not Data Accepted)

NDAC indicates when a device has or has not accepted a message byte. The line is driven by all

devices when receiving commands and by Listeners when receiving data messages.

DAV (Data Valid)

DAV indicates when the signals on the data lines are stable (valid) and can be accepted safely by

devices. The Controller drives DAV when sending commands and the Talker drives it when

sending data messages.

A-3

GPIB-120A User Manual

Operation of the GPIB

Appendix A

Interface Management Lines

Five lines are used to manage the flow of information across the interface.

ATN (Attention)

The Controller drives ATN true when it uses the data lines to send commands and false when it

allows a Talker to send data messages.

IFC (Interface Clear)

The System Controller drives the IFC line to initialize the bus and become CIC.

REN (Remote Enable)

The System Controller drives the REN line, which is used to place devices in remote or local

program mode.

SRQ (Service Request)

Any device can drive the SRQ line to asynchronously request service from the Controller.

EOI (End Or Identify)

The EOI line has two purposes: the Talker uses the EOI line to mark the end of a message

string, and the Controller uses the EOI line to tell devices to identify their response in a parallel

poll.

GPIB-120A User Manual

A-4

Appendix A

Operation of the GPIB

Physical and Electrical Characteristics

Devices are usually connected with a cable assembly consisting of a shielded 24 conductor cable

with both a plug and receptacle connector at each end. With this design, devices can be linked in

either a linear configuration (shown in Figure A-2) or a star configuration (shown in

Figure A-3), or a combination of the two.

The standard connector is the Amphenol or Cinch Series 57 Microribbon or Amp Champ type.

An adapter cable using a non-standard cable and/or connector is used for special interconnecting

applications.

The GPIB uses negative logic with standard TTL logic levels. When DAV is true, for example,

it is a TTL low level ( ≤ 0.8 V), and when DAV is false, it is a TTL high level ( ≥ 2.0 V).

DIO1

DIO2

DIO3

DIO4

EOI

DAV

NRFD

NDAC

IFC

SRQ

ATN

DIO5

DIO6

DIO7

DIO8

REN

GND (TW PAIR W/DAV)

GND (TW PAIR W/NRFD)

GND (TW PAIR W/NDAC)

GND (TW PAIR W/IFC)

GND (TW PAIR W/SRQ)

10 22

11 23

12 24

GND (TW PAIR W/ATN)

SIGNAL GROUND

SHIELD

Figure A-1. GPIB Connector and the Signal Assignment

A-5

GPIB-120A User Manual

Operation of the GPIB

Appendix A

Figure A-2. Linear Configuration of the GPIB Devices

GPIB-120A User Manual

A-6

Appendix A

Operation of the GPIB

Figure A-3. Star Configuration of the GPIB Devices

A-7

GPIB-120A User Manual

Operation of the GPIB

Appendix A

Configuration Restrictions: The Role of Expanders and Extenders

To achieve the high data transfer rate for which the GPIB is designed, the physical distance

between devices and the number of devices on the bus are limited.

The following restrictions are typical:

•

A maximum separation of 4 m between any two devices and an average separation of 2 m

over the entire bus.

•

A maximum total cable length of 20 m.

No more than 15 devices connected to each bus, with at least two-thirds of the devices

powered on.

It is usually possible to connect a cluster of lab instruments without exceeding these restrictions,

but many applications require longer cable spans or additional loading. From the time the GPIB

was invented, the need has existed for bus extenders and expanders (repeaters).

Extenders connect two separate buses via a transmission medium and the distance between the

buses can be quite long. Expanders generally buffer one bus from an adjacent bus, allowing a

doubling of the total cable and loading limits.

National Instruments offers several products that accept longer cable spans. These extenders can

functionally connect GPIB systems separated by distances from 20 m to over 2 km. For more

information about which extender may be right for your application, contact National

Instruments.

The GPIB-120A is a high-speed bus expander which allows up to 14 additional devices to be

connected to the bus and 20 m of cable to be added to the system. In addition, the two connected

GPIBs are optically isolated to prevent ground loop and noise problems. Only one GPIB-120A

is required per expansion.