National Instruments GPIB-120A User manual

© Copyright 1991, 1994 National Instruments Corporation.
All Rights Reserved.
GPIB-120A
User Manual
October 1994 Edition
Part Number 370893A-01

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[phone]
Technical support fax: [phone]
[phone]
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Switzerland 056/20 51 51, Taiwan 02 377 1200, U.K. 0635 523545

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.
Copyright
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.
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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.

© National Instruments Corporation vii GPIB-120A User Manual
Contents
About This Manual...............................................................................................................ix
Organization of This Manual...........................................................................................ix
Conventions Used in This Manual...................................................................................ix
Related Documentation....................................................................................................x
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

Contents
GPIB-120A User Manual viii © National Instruments Corporation
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
Related Document............................................................................................................A-8
Appendix B
Specifications..........................................................................................................................B-1
Appendix C
Multiline Interface Messages............................................................................................C-1
Appendix D
Customer Communication.................................................................................................D-1
Figures
Figure 1-1. Typical GPIB-120A Extension System (Physical Configuration) .....................1-1
Figure 1-2. Typical GPIB-120A Extension System (Logical Configuration).......................1-2
Figure 3-1. GPIB-120A Block Diagram ...............................................................................3-1
Figure A-1. GPIB Connector and the Signal Assignment......................................................A-5
Figure A-2. Linear Configuration of the GPIB Devices ........................................................A-6
Figure A-3. Star Configuration of the GPIB Devices............................................................A-7
Tables
Table 3-1. IEEE 488 Capability Identification Codes .........................................................3-4
Table B-1. System Configuration.........................................................................................B-1
Table B-2. Performance Characteristics...............................................................................B-1
Table B-3. Electrical Characteristics....................................................................................B-2
Table B-4. Environmental Characteristics............................................................................B-2
Table B-5. Physical Characteristics......................................................................................B-2

© National Instruments Corporation ix GPIB-120A User Manual
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.

About This Manual
GPIB-120A User Manual x © National Instruments Corporation
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.

© National Instruments Corporation 1-1 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.
Printer
(Listener) Multimeter (Talker and
Listener)
Signal Generator
(Listener)
Computer
(Controller,
Talker, and Listener)
Unit Under Test
GPIB-120AGPIB #1 GPIB #2
Figure 1-1. Typical GPIB-120A Extension System (Physical Configuration)

Introduction Chapter 1
GPIB-120A User Manual 1-2 © National Instruments Corporation
Printer
(Listener) Multimeter (Talker and
Listener)
Signal Generator
(Listener)
Computer
(Controller,
Talker, and Listener)
Unit Under Test
GPIB
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.

Chapter 1 Introduction
© National Instruments Corporation 1-3 GPIB-120A User Manual
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.

© National Instruments Corporation 2-1 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.

Connection Chapter 2
GPIB-120A User Manual 2-2 © National Instruments Corporation
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.

© National Instruments Corporation 3-1 GPIB-120A User Manual
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.
GPIB
Transceivers
Optoisolators
Controller
Detection
GPIB A
GPIB B
GPIB
Transceivers
Controller
Detection
Side BSide A
Source
Handshake
Detection
Source
Handshake
Detection
Data
Direction
Control
Data
Direction
Control
Parallel
Poll
Detection
Parallel
Poll
Detection
Figure 3-1. GPIB-120A Block Diagram

Theory of Operation Chapter 3
GPIB-120A User Manual 3-2 © National Instruments Corporation
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.

Chapter 3 Theory of Operation
© National Instruments Corporation 3-3 GPIB-120A User Manual
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.

Theory of Operation Chapter 3
GPIB-120A User Manual 3-4 © National Instruments Corporation
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 Description
SH1 Source Handshake
AH1 Acceptor Handshake
T5, TE5 Talker, Extended Talker
L3, LE3 Listener, Extended Listener
SR1 Service Request
RL1 Remote/Local
PP1, PP2 Parallel Poll
DC1 Device Clear
DT1 Device Trigger
C1, C2, C3, C4, C5 Controller
E1, E2 Three-state bus drivers with automatic switch
to open collector during parallel poll

© National Instruments Corporation A-1 GPIB-120A User Manual
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.

Operation of the GPIB Appendix A
GPIB-120A User Manual A-2 © National Instruments Corporation
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.