National Instruments GPIB-PC User manual

© Copyright 1984, 1994 National Instruments Corporation.
All Rights Reserved.
GPIB-PC User Manual
for the
IBM Personal Computer and
Compatibles
April 1988 Edition
Part Number 320014-01

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Limited Warranty
The GPIB-PC 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.
The media on which you receive National Instruments software are
warranted not to fail to execute programming instructions, due to defects in
materials and workmanship, for a period of 90 days from date of shipment,
as evidenced by receipts or other documentation. National Instruments will,
at its option, repair or replace software media that do not execute
programming instructions if National Instruments receives notice of such
defects during the warranty period. National Instruments does not warrant
that the operation of the software shall be uninterrupted or error free.
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.
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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
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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.

©National Instruments Corp. v GPIB-PC User Manual
Preface
Introduction to the GPIB
The GPIB is a link, or bus, or interface system, through which
interconnected electronic devices communicate.
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 from 250 kilobytes to 1 megabyte per second, 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 specialized 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.
The GPIB-PC Family
The GPIB-PC family consists of GPIB interface hardware products,
software, documentation, and other items for several types of personal
computers.
What Your Package Should Contain
Unless you have a special application, your GPIB-PC package consists
of the following:
• A GPIB-PC interface board for your personal computer. Each
board has a model name such as GPIB-PCIIA. This manual uses
GPIB-PC to refer generally to all models of the GPIB-PC
interface board.

Preface
GPIB-PC User Manual vi ©National Instruments Corp.
•AGetting Started with your GPIB-PC pamphlet. The pamphlet
contains the directions with a minimum of explanations for
installing your hardware and software in your GPIB system.
• A GPIB-PC distribution diskette. The distribution diskette is part
of the GPIB-PC package. It contains the DOS handler, BASICA
and QuickBASIC language interfaces, and other programs.
•AGPIB-PC User Manual. The manual contains descriptions of
the GPIB-PC handler functions, BASICA, and QuickBASIC
language interfaces to the handler.
•AProgrammer Reference Guide for BASIC.
• A supplement to Section Two of the manual describing your
particular interface board and how to install it in your personal
computer.
For a language other than BASICA and QuickBASIC, you also need:
• An additional GPIB-PC distribution diskette containing the
software for that language.
• A supplement to Section Four describing the GPIB functions in
the syntax and semantics of that language.
• A Programmer Reference Guide for that language.
Who Are Our Users?
Most of our users have experience in technological fields and with
computers.
How to Get Started
If you already have experience with the GPIB, you may wish to turn
directly to the Getting Started with your GPIB-PC pamphlet that was
shipped with your hardware. It contains directions, with a minimum of
explanations, for installing your hardware and software in your GPIB
system.
If you are less experienced or want more information than the pamphlet
provides, read this GPIB-PC User Manual. It explains in detail all of the
information you will need for the proper operation of the GPIB-PC.

Preface
©National Instruments Corp. vii GPIB-PC User Manual
About the Manual
This manual is written specifically for a GPIB-PC which is to be
installed in an IBM Personal Computer or compatible PC which is
operating under PC-DOS or MS-DOS and programmed using BASICA
and QuickBASIC. With appropriate supplements to the manual, other
GPIB-PC interfaces can be installed in other computers, using other
programming languages.
Organization of the Manual
Section One - Operation of the GPIB describes the operation of the
GPIB.
Section Two - Installation and Configuration describes the installation of
the software and the configuration program IBCONF. A supplement
contains instructions for installing your particular board into your
computer.
Section Three - GPIB-PC Functions — Introduction introduces you to the
functions used by your GPIB-PC. The features are divided into groups
as a means of helping you understand the uses of the functions.
Section Four - GPIB-PC Functions — Overview introduces you to
programming information common to all languages.
Section Four A - Function Reference — Language Interface(s) pertains to
BASICA and QuickBASIC. The descriptions are listed alphabetically
for easy reference.
Section Five - IBIC introduces you to IBIC, the interactive control
program that allows you to control and communicate with the GPIB
through functions you enter at your keyboard. IBIC is designed to help
you learn how to use the GPIB-PC functions to program your devices.
Section Six - Applications Monitor introduces you to the applications
monitor, a resident program that is useful in debugging sequences of
GPIB calls from within your application.
Appendix A - Multiline Interface Messages is a listing of Multiline
Interface Command Messages.
Appendix B - Common Errors and Their Solutions singles out the most
common errors users have encountered and some probable solutions.

Preface
GPIB-PC User Manual viii ©National Instruments Corp.
Appendix C - Differences Between Software Revisions points out
differences between revisions of the GPIB-PC handler.
Appendix D - Using your Printer with the GPIB-PC gives some quick
steps to connect your GPIB-PC with your printer.
Appendix E - Application Notes is an application note about computer-
to-computer transfers.
Appendix F - Customer Communication contains forms you can use to
request help from National Instruments or to comment on our products
and manuals.
The Glossary contains an alphabetical list and description of terms used
in this manual, including abbreviations, acronyms, metric prefixes,
mnemonics, and symbols.
The Index contains an alphabetical list of key terms and topics in this
manual, including the page where you can find each one.
Now, with your personal computer, your GPIB-PC, your manuals and
supplements, and these instructions, you are ready to get started with
your GPIB. We hope your experience will be a rewarding one.
Customer Support
National Instruments wants to receive your comments on our products
and manuals. We are interested in the applications you develop with
our products, and we want to help if you have problems with them. For
information on how to contact us, refer to Appendix F, Customer
Communication, at the end of this manual.

©National Instruments Corp. ix GPIB-PC User Manual
Contents
Section One - Operation of the GPIB................................................1-1
Types of Messages......................................................................................1-1
Talkers, Listeners, and Controllers.....................................................1-1
The Controller-In-Charge and System Controller.........................1-2
GPIB Signals and Lines...........................................................................1-3
Data Lines.....................................................................................1-3
Handshake Lines........................................................................1-3
NRFD (not ready for data)..................................1-3
NDAC (not data accepted).................................1-4
DAV (data valid).....................................................1-4
Interface Management Lines...............................................1-4
ATN (attention)........................................................1-4
IFC (interface clear)..............................................1-4
REN (remote enable)............................................1-4
SRQ (service request)...........................................1-5
EOI (end or identify)..............................................1-5
Physical and Electrical Characteristics............................................1-5
Configuration Requirements...................................................................1-9
Related Documents....................................................................................1-9
Section Two - Installation and Configuration............................2-1
Installing the Hardware.............................................................................2-1
The GPIB-PC Software Package.........................................................2-1
Additional Programs and Files............................................2-2
Installing the Software..............................................................................2-3
Step 1 - Preparation..................................................................2-3
Booting from a Floppy Disk...............................2-3
Booting from a Hard Disk....................................2-3
Step 2 - Run IBSTART...........................................................2-4
Step 3 - Run IBCONF (optional).......................................2-5
Step 4 - Reboot...........................................................................2-5
Step 5 - Test Software Installation....................................2-5
More About IBCONF.................................................................................2-6
Characteristics of the Instruments.....................................2-7
Characteristics of each GPIB-PC......................................2-7
Default Configurations..............................................................................2-8
Primary Default Characteristics.........................................2-8

Contents
GPIB-PC User Manual x ©National Instruments Corp.
Running IBCONF........................................................................................2-9
Upper and Lower Levels of IBCONF............................2-10
Upper Level - Device Map
for Board GPIBx....................................................2-10
Device Map Concepts and Terms................2-11
Lower Level - Device/Board
Characteristics.......................................................2-11
Device and Board Characteristics..................................2-12
Primary GPIB Address.......................................2-12
Secondary GPIB Address..................................2-12
Timeout Settings...................................................2-12
EOS Byte..................................................................2-14
EOS Modes..............................................................2-14
Set EOI with last byte of Write.....................2-14
GPIB-PC Model....................................................2-14
Board is System Controller
(Boards Only).........................................................2-14
Local Lockout on all Devices
(Boards Only).........................................................2-15
Disable Auto Serial Polling
(Boards Only).........................................................2-15
High-Speed Timing
(Boards Only).........................................................2-15
Interrupt Jumper Setting
(Boards Only).........................................................2-15
Base I/O Address (Boards Only)...................2-15
DMA Channel (Boards Only).........................2-16
Internal Clock Frequency
(Boards Only).........................................................2-16
Exiting IBCONF......................................................................2-16
Using Your GPIB-PC.............................................................................. 2-18
Section Three - GPIB-PC
Functions — Introduction............................................................................3-1
Introduction to the GPIB-PC Functions............................................3-1
High-Level Functions..............................................................3-1
Low-Level Functions...............................................................3-1
Calling Syntax............................................................................3-1
Group I..............................................................................................................3-2
IBRD (bd,buf,cnt).....................................................................3-2
IBWRT (bd,buf,cnt).................................................................3-2
IBFIND (bdname,bd)...............................................................3-3

Contents
©National Instruments Corp. xi GPIB-PC User Manual
Group II.............................................................................................................3-3
IBRSP (bd,spr)...........................................................................3-3
IBCLR (bd)..................................................................................3-4
Clearing the Device Versus
Clearing the GPIB...................................................3-4
Clearing the Device.............................3-4
Clearing the GPIB.................................3-4
IBTRG (bd)..................................................................................3-4
IBLOC (bd)..................................................................................3-4
Placing a Device in Remote Mode................3-4
Placing a Device in Local Mode.....................3-5
Group III...........................................................................................................3-5
IBRDA (bd,buf,cnt) and.........................................................3-6
IBRDF (bd,buf,cnt) and.........................................................3-6
IBWAIT (bd,mask)...................................................................3-6
IBSTOP (bd)................................................................................3-6
IBTMO (bd,v).............................................................................3-7
IBONL (bd,v)..............................................................................3-7
IBPCT (bd)...................................................................................3-7
Group IV...........................................................................................................3-8
Purpose of Board Functions..................................................3-9
Multiboard Capability.............................................................3-9
IBFIND (bdname,bd)............................................................3-10
IBCMD (bd,buf,cnt) and.....................................................3-10
IBRD (bd,buf,cnt) and.........................................................3-10
IBWRT (bd,buf,cnt) and.....................................................3-12
IBSTOP (bd)............................................................................. 3-12
IBWAIT (bd,mask)................................................................3-12
IBTMO (bd,v)..........................................................................3-12
IBONL (bd,v)...........................................................................3-12
IBSIC (bd).................................................................................3-12
IBSRE (bd,v)............................................................................3-13
IBGTS (bd,v)............................................................................3-13
IBCAC (bd,v)...........................................................................3-13
IBRPP (bd,buf)........................................................................3-13
IBPPC (bd,v)............................................................................3-13
More About Device and Board Functions....................................3-14
Group V......................................................................................................... 3-15
IBRSV (bd,v)...........................................................................3-15
IBLOC (bd)...............................................................................3-15
IBPPC (bd,v)............................................................................3-16
IBIST (bd,v)..............................................................................3-16
IBWAIT (bd,mask)................................................................3-16

Contents
GPIB-PC User Manual xii ©National Instruments Corp.
Group VI........................................................................................................3-17
IBEOT (bd,v)............................................................................ 3-17
IBEOS (bd,v)............................................................................3-18
IBBNA (bd,"GPIBn")...........................................................3-18
IBDMA (bd,v)..........................................................................3-18
IBPAD (bd,v)...........................................................................3-18
IBSAD (bd,v)...........................................................................3-18
IBRSC (bd,v)...........................................................................3-18
IBTMO (bd,v)..........................................................................3-18
Section Four - GPIB-PC
Functions — Overview................................................................................. 4-1
General Programming Information...................................................4-1
Status Word.................................................................................................4-2
Error Codes.................................................................................................. 4-6
Count Variable.........................................................................................4-11
Read and Write Termination............................................................4-11
Device Function Calls.........................................................................4-12
Automatic Serial Polling....................................................................4-13
Section Four A - BASICA/QuickBASIC
GPIB-PC Function Calls......................................................................... 4A-1
BASICA Files.......................................................................................... 4A-2
QuickBASIC Files.................................................................................4A-2
Programming Preparations for BASICA.................... 4A-3
Programming Preparations for QuickBASIC...........4A-4
BASICA/QuickBASIC GPIB-PC I/O Functions......................4A-5
BASICA/QuickBASIC "ON SRQ" Capability..........................4A-6
New GPIB-PC Functions..................................................................4A-12
GPIB-PC Function Descriptions....................................................4A-15
IBBNA.........................................................................................................................4A-16
IBCAC.........................................................................................................................4A-17
IBCLR..........................................................................................................................4A-19
IBCMD........................................................................................................................4A-20
IBCMDA.....................................................................................................................4A-23
IBDMA........................................................................................................................4A-25
IBEOS..........................................................................................................................4A-26
IBEOT..........................................................................................................................4A-30
IBFIND........................................................................................................................4A-32
IBGTS..........................................................................................................................4A-34
IBIST............................................................................................................................4A-36
IBLOC..........................................................................................................................4A-38
IBONL..........................................................................................................................4A-40
IBPAD..........................................................................................................................4A-42

Contents
©National Instruments Corp. xiii GPIB-PC User Manual
IBPCT..........................................................................................................................4A-44
IBPPC..........................................................................................................................4A-45
IBRD.............................................................................................................................4A-47
IBRDA ........................................................................................................................4A-50
IBRDF..........................................................................................................................4A-54
IBRDI...........................................................................................................................4A-57
IBRDIA........................................................................................................................4A-60
IBRPP..........................................................................................................................4A-64
IBRSC..........................................................................................................................4A-66
IBRSP..........................................................................................................................4A-68
IBRSV..........................................................................................................................4A-70
IBSAD..........................................................................................................................4A-71
IBSIC............................................................................................................................4A-73
IBSRE..........................................................................................................................4A-74
IBSTOP.......................................................................................................................4A-76
IBTMO.........................................................................................................................4A-78
IBTRAP.......................................................................................................................4A-81
IBTRG..........................................................................................................................4A-83
IBWAIT.......................................................................................................................4A-84
IBWRT........................................................................................................................4A-87
IBWRTA.....................................................................................................................4A-90
IBWRTF.....................................................................................................................4A-93
IBWRTI.......................................................................................................................4A-95
IBWRTIA ..................................................................................................................4A-99
BASICA/QuickBASIC GPIB
Programming Examples...................................................................4A-103
BASICA Example Program - Device......................4A-105
BASICA Example Program - Board........................4A-108
QuickBASIC Example Program - Device.............4A-111
QuickBASIC Example Program - Board...............4A-114
Section Five - IBIC..............................................................................................5-1
Running IBIC.................................................................................................5-2
Using HELP..................................................................................5-3
Using IBFIND..............................................................................5-3
Using IBWRT..............................................................................5-4
Using IBRD..................................................................................5-4
How to Exit IBIC.......................................................................5-5
Important Programming Note...............................................5-5
Using SET.....................................................................................5-6
IBIC Functions and Syntax.....................................................................5-7
Other IBIC Functions and Syntax........................................................5-8
Status Word.................................................................................................5-10
Error Code.................................................................................................... 5-11

Contents
GPIB-PC User Manual xiv ©National Instruments Corp.
Byte Count...................................................................................................5-12
Auxiliary Functions.................................................................................5-12
SET (Select Device or Board).........................................5-13
HELP (Display Help Information)..................................5-13
! (Repeat Previous Function)...........................................5-14
- (Turn OFF Display)............................................................5-14
+ (Turn ON Display).............................................................5-15
n* (Repeat Function n Times).........................................5-16
$ (Execute Indirect File)....................................................5-17
PRINT (Display the ASCII String)................................5-18
E or Q (exit or quit)...............................................................5-18
IBIC Sample Programs.......................................................................... 5-19
Device Function Calls.........................................................5-19
Board Function Calls............................................................5-22
Section Six - Applications Monitor.......................................................6-1
Installing the Applications Monitor....................................................6-2
IBTRAP..........................................................................................6-2
Applications Monitor Options................................................................6-5
Main Commands..........................................................................................6-6
Session Summary Screen........................................................................6-7
Configuring the Trap Mask.....................................................................6-7
Configuring the Monitor Mode..............................................................6-7
Hiding and Showing the Monitor.......................................6-8
Appendix A - Multiline Interface Messages.................................A-1
Multiline Interface Messages................................................................A-2
Interface Message Reference List.......................................................A-4
Appendix B - Common Errors
andTheirSolutions.............................................................................................B-1
EDVR(0)..........................................................................................................B-1
ECIC(1)............................................................................................................B-1
ENOL(2)..........................................................................................................B-2
EADR(3)..........................................................................................................B-3
EARG(4)..........................................................................................................B-3
ESAC(5)..........................................................................................................B-4
EABO(6)..........................................................................................................B-4
ENEB(7)..........................................................................................................B-5
EOIP(10)..........................................................................................................B-5
ECAP(11)........................................................................................................B-5
EFSO(12)........................................................................................................B-5
EBUS(14)........................................................................................................B-6
ESTB(15)........................................................................................................B-6

Contents
©National Instruments Corp. xv GPIB-PC User Manual
ESRQ(16)........................................................................................................B-6
Other Error Conditions...............................................................................B-7
Appendix C - Differences Between
SoftwareRevisions...............................................................................................C-1
Revision B and Revision C....................................................................C-1
Interrupts........................................................................................C-1
Startup Program..........................................................................C-1
Configuration Program............................................................C-1
Interface Bus Interactive Control Program (IBIC)....C-1
New Functions............................................................................C-2
Modified Functions...................................................................C-2
Language Interfaces.................................................................C-2
General...........................................................................................C-2
Revision C and Revision D....................................................................C-2
Device Functions.......................................................................C-2
Non-Interrupt Mode...................................................................C-2
Asynchronous I/O.......................................................................C-3
DMA on the GPIB-PCIII........................................................C-3
Local Lockout.............................................................................C-3
SRQI Status Bit..........................................................................C-3
ATN and/or TIMO.....................................................................C-3
DCAS and DTAS Status Bits..............................................C-3
Printer Support............................................................................C-3
Appendix D - Using your Printer
with the GPIB-PC...............................................................................................D-1
Installation......................................................................................................D-1
Appendix E - Application Notes..............................................................E-1
Application Note 1 - Computer to Computer
Data Transfer.................................................................................................E-1
Step 1. Configure the Computers......................................E-1
Step 2. Establish Communication....................................E-1
Step 3. Transfer Data..............................................................E-2
Appendix F - Customer Communication.......................................F-1
Glossary.........................................................................................................................G-1
Index.................................................................................................................................I-1

©National Instruments Corp. xvii GPIB-PC User Manual
Illustrations
List of Figures
Figure 1.1 - GPIB Connector and the Signal Assignment.........................1-6
Figure 1.2 - Linear Configuration..........................................................................1-7
Figure 1.3 - Star Configuration...............................................................................1-8
Figure 3.1 - Multiboard GPIB System...............................................................3-11
Figure 6.1 - Applications Monitor Popup Screen...........................................6-1
List of Tables
Table 2.1 - Timeout Settings.................................................................................2-13
Table 2.2 - Functions that Alter Default Characteristics.........................2-17
Table 4.1 - Status Word Layout.............................................................................4-2
Table 4.2 - GPIB Error Codes..................................................................................4-6
Table 4A.1 - BASICA GPIB-PC Functions..................................................4A-7
Table 4A.2 - QuickBASIC GPIB-PC Calls...................................................4A-8
Table 4A.3 - QuickBASIC GPIB-PC Calls.................................................4A-10
Table 4A.4 - QuickBASIC Version 4.0 GPIB-PC Functions..............4A-14
Table 5.1 - Syntax of GPIB Functions in IBIC...............................................5-8
Table 5.2 - Status Word Layout...........................................................................5-11
Table 5.3 - Auxiliary Functions that IBIC Supports...................................5-12

©National Instruments Corp. 1-1 GPIB-PC User Manual
Section One - Operation of the GPIB
Communication between interconnected devices is achieved by passing
messages through the interface system.
Types of Messages
The GPIB carries two types of messages — 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 functions 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.
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
computer's CPU, but a better analogy is to the switching center of a city
telephone system.

Operation of the GPIB Section One
GPIB-PC User Manual 1-2 ©National Instruments Corp.
The switching center (Controller) monitors the communications network
(GPIB). When the 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 usually addresses a Talker and a Listener before the
Talker can send its message to the Listener. After the message is
transmitted, the Controller usually unaddresses 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 the GPIB-PC interface board and its software, your personal
computer plays all three roles:
• Controller - to manage the GPIB,
• Talker - to send data, and
• 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.
The GPIB-PC is usually the System Controller.

Section One Operation of the GPIB
©National Instruments Corp. 1-3 GPIB-PC User Manual
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:
• 8 data lines,
• 3 handshake lines, and
• 5 interface management lines.
Data Lines
The eight data lines, DIO1 through DIO8, carry both data and command
messages. All commands and most data use the 7-bit ASCII or ISO
code set, in which case the 8th bit, DIO8, is unused or used for parity.
Handshake Lines
Three lines asynchronously control the transfer of message bytes
between devices:
• NRFD,
• NDAC, and
• DAV.
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.

Operation of the GPIB Section One
GPIB-PC User Manual 1-4 ©National Instruments Corp.
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 tells when the signals on the data lines are stable (valid) and can
be accepted safely by devices. The Controller drives DAV lines when
sending commands and the Talker drives DAV lines when sending data
messages.
Interface Management Lines
Five lines are used to manage the flow of information across the
interface:
• ATN,
• IFC,
• REN,
• SRQ, and
• EOI.
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.