National Instruments GPIB-100A User manual

© Copyright 1985, 1991 National Instruments Corporation.
All Rights Reserved.
GPIB-100A
User Manual
March 1990 Edition
Part Number 320063-01

National Instruments Corporation
6504 Bridge Point Parkway
Austin, TX 78730-5039
[phone]
(800) IEEE-488 (toll-free U.S. and Canada)
Technical support fax: [phone]

Limited Warranty
The GPIB-100A 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 Instrument
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 book may not be copied, photocopied, reproduced, or translated, in whole or in
part, without the prior written consent of National Instruments Corporation.
Trademarks
Product names listed are trademarks of their respective manufacturers. Company names listed are trademarks
or trade names of their respective companies.

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 (1) the limits for a Class A
computing device, in accordance with the specifications in Subpart J of Part 15 of U.S. Federal
Communications Commission (FCC) Rules, and (2) the limits for radio noise emissions from
digital apparatus set out in the Radio Interference Regulations of the Canadian Department of
Communication (DOC). These regulations are designed to provide reasonable protection against
interference from the equipment to radio and television reception in commercial areas.
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 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.
• Relocate the equipment with respect to the receiver.
• Reorient 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.
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.

© National Instruments Corporation v GPIB-100A User Manual
Preface
Organization of the Manual
This manual is divided into the following chapters:
Chapter 1, Description of the GPIB-100A, contains a brief description of the GPIB-100A Bus
Extender and how it is used. This section also lists all components and accessories. In addition, it
provides system configuration, performance, operating, electrical, environmental, and physical
specifications for the GPIB-100A.
Chapter 2, Installation, contains instructions for configuring and connecting the GPIB-100A into
your system at your operating voltage.
Chapter 3, Configuration and Operation, describes how to configure and operate a GPIB-100A
system.
Chapter 4, Theory of Operation, contains descriptions of how the GPIB-100A circuitry operates.
Appendix A, Operation of the GPIB, describes GPIB terminology and protocol for users
unfamiliar with the GPIB.
Appendix B, Schematic Diagram, contains a detailed schematic diagram of the GPIB-100A.
Appendix C, GPIB-100A Parts Locator Diagram, contains the parts locator diagram for the
GPIB-100A.
Appendix D, Cable Assembly Wire List, contains the listing of wire connections for the
GPIB-100A transmission cable.
Appendix E, Multiline Interface Messages, contains an ASCII chart and a list of the corresponding
GPIB messages.
Appendix F, Mnemonics Key, contains a mnemonics key that defines the mnemonics used
throughout the manual.
Related Document
The following document is a reference that covers in greater detail specific topics introduced in this
manual:
•ANSI/IEEE Standard 488-1978, IEEE Standard Digital Interface for Programmable
Instrumentation.

Preface
GPIB-100A User Manual vi © National Instruments Corporation
Abbreviations Used in the Manual
The following abbreviations are used in the text of this manual.
C centigrade
F Fahrenheit
Hz hertz
in. inch
kbytes thousand bytes
m meter
mA milliamperes
Mbytes million bytes
mm millimeter
µsec microsecond
nsec nanosecond
sec second
V volts
VAC Volts Alternating Current
W watt

© National Instruments Corporation vii GPIB-100A User Manual
Contents
Chapter 1
Description of the GPIB-100A.......................................................................................1-1
Introduction......................................................................................................................1-1
GPIB-100A Specifications.............................................................................................. 1-3
Chapter 2
Installation.............................................................................................................................2-1
Inspection.........................................................................................................................2-1
Power Connection............................................................................................................ 2-1
Grounding Configuration.................................................................................................2-2
Disassembly........................................................................................................2-2
Mounting.........................................................................................................................2-2
Connecting to Hewlett-Packard Controllers.....................................................................2-2
Chapter 3
Configuration and Operation..........................................................................................3-1
Operating Modes.............................................................................................................3-1
Talker/Listener/Controller (TLC) Mode............................................................... 3-1
Talker/Listener (TL) Mode................................................................................... 3-2
Setting the Operating Mode................................................................................. 3-2
Parallel Poll Response (PPR) Modes............................................................................... 3-2
Buffered PPR Mode (Approach 1)......................................................................3-3
Unbuffered PPR Mode (Approach 2)..................................................................3-3
Mixed Mode Operation....................................................................................................3-4
Operating the GPIB-100A System.................................................................................. 3-4
Chapter 4
Theory of Operation........................................................................................................... 4-1
Diagrams.........................................................................................................................4-1
Power-On........................................................................................................................4-1
System Controller Detection............................................................................................ 4-2
Active Controller Detection.............................................................................................. 4-2
Source Handshake Detection........................................................................................... 4-2
Parallel Polling.....................................................................................................4-3
Data Direction Control......................................................................................... 4-4
EOI..................................................................................................................................4-4

Contents
GPIB-100A User Manual viii © National Instruments Corporation
Appendix A
Operation of the GPIB....................................................................................................... A-1
History of the GPIB.........................................................................................................A-1
Types of Messages..........................................................................................................A-1
Talkers, Listeners, and Controllers...................................................................................A-1
System Controller and Active Controller.........................................................................A-2
GPIB Signals................................................................................................................... A-2
Data Lines........................................................................................................................A-3
Handshake Lines.............................................................................................................. A-3
NRFD (not ready for data)................................................................................... A-3
NDAC (not data accepted)................................................................................... A-4
DAV (data valid)................................................................................................. A-4
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............................................................................................... A-7
Appendix B
Schematic Diagram............................................................................................................B-1
Appendix C
GPIB-100A Parts Locator Diagram.............................................................................C-1
Appendix D
Cable Assembly Wire List...............................................................................................D-1
Appendix E
Multiline Interface Messages.........................................................................................E-1
Appendix F
Mnemonics Key....................................................................................................................F-1

Contents
© National Instruments Corporation ix GPIB-100A User Manual
Figures
Figure 1-1. The Model GPIB-100A Bus Extender..................................................................1-1
Figure 1-2. Typical GPIB-100A Extension System (Physical Configuration)........................1-2
Figure 1-3. Typical GPIB-100A Extension System (Logical Configuration).......................... 1-2
Figure 2-1. Voltage Selection....................................................................................................2-1
Figure 3-1. Switch Settings for Operating Mode......................................................................3-2
Figure 3-2. Switch Settings for Parallel Poll Response Mode...................................................3-4
Figure 4-1. GPIB-100A Block Diagram...................................................................................4-1
Figure A-1. GPIB Cable Connector..........................................................................................A-3
Figure A-2. Linear Configuration of the GPIB Devices............................................................A-5
Figure A-3. Star Configuration of GPIB Devices.....................................................................A-6
Figure C-1. GPIB-100A Parts Locator Diagram...................................................................... C-1
Tables
Table 1-1. System Configuration Characteristics......................................................................1-3
Table 1-2. Performance Characteristics.....................................................................................1-4
Table 1-3. Operating Characteristics.........................................................................................1-4
Table 1-4. Electrical Characteristics..........................................................................................1-5
Table 1-5. Environmental Characteristics.................................................................................1-5
Table 1-6. Physical Characteristics............................................................................................1-6
Table 1-7. Components and Accessories..................................................................................1-6
Table D-1. Cable Assembly Wire List......................................................................................D-1

© National Instruments Corporation 1-1 GPIB-100A User Manual
Chapter 1
Description of the GPIB-100A
Introduction
The high-speed GPIB-100A Bus Extender (Figure 1-1) is used in pairs with a special parallel data
transmission cable to connect two separate GPIB or IEEE-488 bus systems in a functionally
transparent manner.
Figure 1-1. The Model GPIB-100A Bus Extender
While the two bus systems are physically separate, as shown in Figure 1-2, devices logically
appear to be located on the same bus as shown in Figure 1-3. Thus, with the GPIB-100A it is
possible to overcome two configuration restrictions imposed by ANSI/IEEE Standard 488-l978,
namely:
• Cable length limit of 20 m total per contiguous bus or 2 m times the number of devices on the
bus, whichever is smaller.
• Electrical loading limit of 15 devices per contiguous bus.
Each GPIB-100A system extends the distance limit by 300 m and the loading limit to 30 devices
including the extenders, without sacrificing speed or performance. These point-to-point extender
systems can be connected in series for longer distances or in star patterns for additional loading.
At short distances, the data transfer rate over the extension can exceed 250 kbytes/sec, degrading
with distance only by the propagation delay along the cable. Furthermore, regardless of the
distance, there is no speed degradation at all for transfers between devices on the same side of the

Description of the GPIB-100A Chapter 1
GPIB-100A User Manual 1-2 © National Instruments Corporation
extension. And because the GPIB-100A is a functionally transparent extender, the same GPIB
communications and control programs that work with an unextended system will work
unmodified with an extended system. There is one minor exception to this transparency in
conducting parallel polls, as explained in Chapter 3 in the paragraph Parallel Poll Response (PPR)
Modes.
GPIB #2
GPIB #1 GPIB-100A
Computer
(System Controller,
Talker, and Listener)
Printer
(Listener) Multimeter
(Talker and Listener) Signal Generator (Listener)
Unit Under Test
GPIB-100A
RS-232 Compatible
Transmission
Lines
Figure 1-2. Typical GPIB-100A Extension System (Physical Configuration)
Computer
(System Controller,
Talker, and Listener)
Printer
(Listener) Multimeter (Talker and Listener) Signal Generator (Listener)
Unit Under Test
GPIB
Figure 1-3. Typical GPIB-100A Extension System (Logical Configuration)

Chapter 1 Description of the GPIB-100A
© National Instruments Corporation 1-3 GPIB-100A User Manual
GPIB-100A Specifications
The following tables show the system configuration; the performance, operating, electrical,
environmental, and physical characteristics of the GPIB-100A, as well as providing a list of
available GPIB-100A components and accessories.
Table 1-1. System Configuration Characteristics
Characteristic Specification
distance per extension up to 300 m
loading per extension up to 14 additional devices
multiple extensions permitted in any combination of star or linear pattern
GPIB driver output no restrictions (automatic conversion to 2 µsec
circuit and T1 timing T1 delay on remote side is built in)
of source device
Note: T1 is the data settling time (DIO valid to DAV) and varies according to the type of
drivers and the system configuration used.

Description of the GPIB-100A Chapter 1
GPIB-100A User Manual 1-4 © National Instruments Corporation
Table 1-2. Performance Characteristics
Characteristic Specification
speed 250 to 135 kbytes/sec (approximately 4 µsec per byte
degraded at 10.5 nsec per meter of distance) no
limitation to device speeds when there are no
listeners on remote side of extension
functionality transparent GPIB operation except for pulsed parallel
polls
interlocked maintained across the extension
IEEE-488 handshake (message-grams not used)
IEEE-488 capability SH1 complete Source Handshake
identification AH1 complete Acceptor Handshake
codes T5,TE5 complete Talker
L3,LE3 complete Listener
SR1 complete Service Request
RL1 complete Remote Local
PP1,2 complete Parallel Poll
DC1 complete Device Clear
DT1 complete Device Trigger
C1-5 complete Controller
E1 open collector GPIB drivers
Table 1-3. Operating Characteristics
Characteristic Specification
architecture asynchronous (no clock) parallel design
point-to-point (not multi-drop) transmission
operating modes Talker/Listener/Controller or
Talker/Listener (Talk Only)
Parallel Poll Response Buffered Parallel Poll Response or
modes Unbuffered Parallel Poll Response

Chapter 1 Description of the GPIB-100A
© National Instruments Corporation 1-5 GPIB-100A User Manual
Table 1-4. Electrical Characteristics
Characteristic Specification
GPIB interface circuit duplex transceivers with open
collector drivers (MC3441A)
transmission interface RS-422 drivers and receivers
circuit (MC3487 and AM26LS32) connected with patented
data transmission cable for minimum skewing
(<= 3%) between any two pairs
power supply 50 to 60 Hz
selectable (fuse) 110 V, 160 mA (250 mA, 250 V, Slow Blow)
220 V, 80 mA (200 mA, 250 V, Slow Blow)
GPIB interface load one standard load, AC and DC
power 18 W typical
Table 1-5. Environmental Characteristics
Characteristic Specification
operating temperature 0 to 55 C
humidity 5 to 95% non-condensing conditions
FCC Class A verified
110V Version UL Listed
220V Version UL Listed and also classified by Underwriters
Laboratories Inc. in accordance with International
Electrotechnical Commission publication 950

Description of the GPIB-100A Chapter 1
GPIB-100A User Manual 1-6 © National Instruments Corporation
Table 1-6. Physical Characteristics
Characteristic Specification
case style CS2
size 3.5 x 8.5 x 13 in. (89 x 216 x 330 mm)
case material UL94V-0 flame retardant polystyrene
Dow 60875 F or equivalent
rack mounting single or dual kits available
GPIB cable Hewlett Packard 10833 style or equivalent
Transmission cable Dynatronics D-200-24 cable with
AMP Amplimite connectors AMP HDP-20 50 pin
connector with RFI/EMI shield
Table 1-7. Components and Accessories
Item Part Number
Model GPIB-100A Bus Extender (110V) 776107-01
(two required per extension)
Model GPIB-100A Bus Extender (220V) 776107-31
(two required per extension)
Type T2 Transmission Cable 178056-xxx
(xxx = length in meters)
Type X2 GPIB Cable
1 meter 763061-01
2 meters 763061-02
4 meters 763061-03
Single Rack-Mount Kit 180304-01
Dual Rack-Mount Kit 180304-02
Note: All part numbers in this table are National Instruments part numbers.

© National Instruments Corporation 2-1 GPIB-100A User Manual
Chapter 2
Installation
Inspection
Inspect the shipping container and contents for evidence of physical damage or stress. If damage
is discovered and appears to have been caused in shipment, file a claim with the carrier. If the
equipment is damaged, do not attempt to operate it before contacting National Instruments for
instructions. Retain the shipping material for possible inspection by carrier or reshipment of the
equipment.
Power Connection
The GPIB-100A Bus Extender is shipped from the factory set at a certain operating voltage, either
110 VAC or 220 VAC. Verify that the voltage you are using is the same as that selected on the
rear panel of the GPIB-100A. Operating at a voltage other than the one selected may damage the
unit. If the GPIB-100A is set at a voltage other than the one you are using, follow the steps below
to change the operating voltage.
1. Remove the power cord from the unit.
2. Pull out the fuse holder and replace the fuse with one that has the type and rating specified in
Table 1-4 for your operating voltage.
3. Using a small flat-head screwdriver, rotate the voltage selector to point to your operating
voltage.
Figure 2-1. Voltage Selection

Installation Chapter 2
GPIB-100A User Manual 2-2 © National Instruments Corporation
Grounding Configuration
A U.S. standard three-wire power cable is provided with the GPIB-100A. When connected to a
power source, this cable connects the equipment chassis to the power ground.
The GPIB-100A is shipped from the factory with chassis and power grounds connected to the
logic ground of the digital circuitry and the shields of the interfacing cables. If it is necessary to
isolate these grounds to prevent current loops between units, disassemble the unit according to the
following instructions and remove jumper W1 located on the circuit card assembly near the back
panel.
Disassembly
The case consists of two identical sections. Before disassembling, remove power from the unit.
Then remove the two screws on each side of the case and lift the top section. When reassembling,
it may be necessary to adjust the two trim panels on the case side for proper fit in their grooves.
Mounting
The GPIB-100A enclosure is designed for table top operation or for rack mounting. Single and
dual unit rack mounting kits are available from National Instruments for field installation.
Connecting to Hewlett-Packard Controllers
To achieve very high data transfer rates and long cable spans between devices, many Hewlett-
Packard (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 Not Ready For Data (NRFD) and
Not Data Accepted (NDAC) are loaded.
Preloading increases ringing on signal transitions and may cause improper operation of the GPIB-
100As. If this happens, all signals on the Master Controller should be set to normal (1 unit) load.
This is done by means of a back panel switch when working from the exterior. In addition, the
cabling rule of no more than 2m/device must be strictly enforced.

© National Instruments Corporation 3-1 GPIB-100A User Manual
Chapter 3
Configuration and Operation
Users who are unfamiliar with the GPIB should first read Appendix A, Operation of the GPIB, to
become familiar with GPIB terminology and protocol.
In the following discussions, the terms local and remote refer to certain states of the two GPIB-
100A Bus Extenders in the system. When one extender is in a local state, meaning that the state in
question originated on the local state's side, the other extender is in the corresponding remote state.
The three states in question are the System Controller, Active Controller, and Source Handshake
states.
Operating Modes
The GPIB-100A has two operating modes: Talker/Listener/Controller mode and Talker/Listener
mode. Both units in the extension system must be set to the same mode.
Talker/Listener/Controller (TLC) Mode
The GPIB-100A is set at the factory to the more common TLC operating mode. The TLC mode
requires a System Controller on one side of the extension. There may be any number of Talkers,
Listeners, and other Controllers in the system.
In the TLC mode, the two GPIB-100As expect to see in order: first the Interface Clear (IFC)
signal from the System Controller; second the Attention (ATN) signal from the Active Controller;
and third the Data Valid (DAV) signal from the Active Controller or Talker. A brief description of
this mode is in the following paragraph.
Both units power up in a quiescent condition with no local or remote state active. They remain that
way until one unit detects an IFC pulse from the System Controller which is on the same
contiguous bus. That unit enters the Local System Controller (LSC) state and causes the other unit
to enter the Remote System Controller (RSC) state. The IFC and Remote Enable (REN) signals
are switched to flow from the local to the remote unit. Next, one unit detects the ATN signal from
the Active Controller, enters the Local Active Controller (LAC) state, and places the other unit in
the Remote Active Controller (RAC) state. The ATN signal is switched to flow from local to
remote side and the Service Request (SRQ) is switched to flow in the opposite direction. Finally,
one unit detects the DAV from the Source Handshake function of the Talker or Active Controller.
That unit enters the Local Source (LS) state and places the other unit in the Remote Source (RS)
state. The DAV and Data (DIO) signals are switched to flow from local to remote side, and the
Not Ready for Data (NRFD) and Not Data Accepted (NDAC) signals are switched to flow from
remote to local side.
As the source side for these three key signals–IFC, ATN, and DAV–change, the local/remote
states of each extender and the directions of the other GPIB signals change accordingly. Chapter 4,
Theory of Operation, contains a more thorough discussion of this.

Configuration and Operation Chapter 3
GPIB-100A User Manual 3-2 © National Instruments Corporation
Talker/Listener (TL) Mode
There is no Controller and only one Talker in the TL mode of operation, sometimes called talk only
mode. Usually, there is just one Listener as well. In the TL mode, the System and Active
Controller states remain inactive and the IFC, REN, ATN, and SRQ signals are unused. The
directions of the other signals are set the first time the Talker asserts DAV.
Setting the Operating Mode
Both GPIB-100As in the extension system must be set to the same operating mode. Use switch
S1, position 1, on the back panel of each GPIB-100A to set the operating mode. Set the switches
as shown in Figure 3-1.
T/L/C
A. Talker/Listener/Controller Mode B. Talker/Listener Mode
O
NO
N
T/L
123 123
represents the side of the switch you press down
Figure 3-1. Switch Settings for Operating Mode
Parallel Poll Response (PPR) Modes
According to ANSI/IEEE Standard 488-l978, devices must respond to a parallel poll within 200
nsec after the Identify (IDY) message (Attention (ATN) and End Or Identify (EOI)) is asserted by
the Active Controller, which then waits until 2 µsec or more to read the Parallel Poll Response
(PPR). It is not possible for a remote device on an extended system to respond to this quickly
because of cable propagation delay. GPIB extender manufacturers have approached this in three
ways:
Approach 1: Respond to IDY within 200 nsec with the results of the previous poll of the
remote bus.
Approach 2: Ignore the 200 nsec rule and assume the Controller will wait sufficiently long to
capture the response.
Approach 3: Do not support parallel polling at all.
The GPIB-100A uses either Approach 1 or 2, selected at switch S1, position 3. Set this switch as
shown in Figure 3-2.

Chapter 3 Configuration and Operation
© National Instruments Corporation 3-3 GPIB-100A User Manual
Buffered PPR Mode (Approach 1)
Most Controllers pulse the IDY signal for a period of time exceeding 2 µsec and expect a response
within that time. When used with this type of Controller, the GPIB-100A should be left in the
Buffered PPR mode as set at the factory.
In this mode, the local GPIB-100A extender responds to IDY by outputting the contents of the
PPR data register. At the same time, a parallel poll message is sent to the remote bus and the poll
response is returned to the local unit in due course. When the local IDY signal is unasserted, the
register is loaded with the new remote response. Consequently the register contains the response
of the previous poll. To obtain the response of both local and remote buses, the control program
executes two parallel polls back-to-back and uses the second response.
The software driver library of most Controllers contains an easy-to-use parallel poll function. If,
for example, the function is called PPOLL and the control program is written in BASIC, the
sequence to conduct a poll in Buffered PPR mode might be like this:
CALL PPOLL(PPR)
CALL PPOLL(PPR)
IF PPR > 0 GOTO NNN
If two GPIB extender systems are connected in series, three polls are necessary to get responses
from the local, middle, and far buses.
Unbuffered PPR Mode (Approach 2)
Many Hewlett-Packard GPIB Controllers remain in a parallel poll state with IDY asserted
whenever they are not performing another function. A change in the response causes an interrupt
of the control program. In other Controllers, the IDY signal is toggled on and off and the duration
of the signal can be varied to accommodate delayed responses over extenders. When used with
these types of Controllers, the GPIB-100A should be set to Unbuffered PPR mode. This means
that the IDY message is sent to the remote bus and the response is returned as fast as propagation
delays allow. The Controller must allow time to receive the response.