National Instruments GPIB-120B User manual

GPIB
GPIB-120B User Manual
GPIB-120B User Manual
February 2019
371851C-01

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• Reorient the antenna of the receiver (the device suffering interference).
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© National Instruments |vii
Contents
About This Manual
Related Documentation .................................................................................................... ix
Chapter 1
Introduction
Description of the GPIB-120B ......................................................................................... 1-1
What You Need to Get Started ......................................................................................... 1-3
Optional Equipment.................................................................................................. 1-3
Unpacking Your GPIB-120B ................................................................................... 1-4
Chapter 2
Hardware Overview
GPIB-120B LEDs............................................................................................................. 2-1
Power On (PWR)...................................................................................................... 2-1
System Controller Detection (SC)............................................................................ 2-1
Active Controller Detection (AC) ............................................................................ 2-1
Source Handshake Detection (SH) ........................................................................... 2-2
Data Transfer Modes ........................................................................................................ 2-2
Selecting a Data Transfer Mode ............................................................................... 2-2
Unbuffered Mode ............................................................................................. 2-2
Buffered Mode.................................................................................................. 2-2
Setting the Data Transfer Mode................................................................................ 2-3
Data Direction Control ..................................................................................................... 2-3
Parallel Poll Detection ...................................................................................................... 2-3
Parallel Poll Operation...................................................................................................... 2-4
Chapter 3
Configuring and Using Your Hardware
Connecting the GPIB-120B.............................................................................................. 3-1
Appendix A
GPIB Basics
Appendix B
Multiline Interface Messages
Appendix C
Specifications

Contents
viii |ni.com
Appendix D
NI Services
Index

© National Instruments |ix
About This Manual
This manual describes how to install, configure, and operate the National Instruments
GPIB-120B.
Related Documentation
The following documents contain information that you may find helpful as you read this manual:
• ANSI/IEEE Standard 488.1-1987, IEEE Standard Digital Interface for Programmable
Instrumentation
• ANSI/IEEE Standard 488.2-1992, IEEE Standard Codes, Formats, Protocols, and
Common Command

© National Instruments |1-1
1
Introduction
This chapter contains a description of the GPIB-120B, lists what you need to get started and
optional equipment you can order, and explains how to unpack the GPIB-120B.
Description of the GPIB-120B
The GPIB-120B is a high-speed bus isolator/expander with the following features:
• It is transparent to user software.
• It electrically isolates two GPIB systems from each other and from the power supply.
• 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 or DIN rail mount hardware accessories.
The high-speed GPIB-120B bus isolator/expander connects two GPIB (IEEE 488) bus systems
in a functionally transparent manner.
The two bus systems are electrically isolated from each other. The two bus systems are also
isolated from the power supply. Isolation is maintained up to 2500 VDC (withstand 5 s).
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 on the same bus, as shown in
Figure 1-2.
Figure 1-1. Typical GPIB-120B Expansion System (Physical Configuration)
Computer
(Controller, Talker,
and Listener)
Printer
(Listener)
GPIB-120BGPIB GPIB
Multimeter
(Talker and Listener)
Signal Generator
(Listener)
Unit Under Test

1-2 |ni.com
Chapter 1 Introduction
Figure 1-2. Typical GPIB-120B Expansion System (Logical Configuration)
With the GPIB-120B, you can 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 multiplied by the number of
devices on the bus, whichever is smaller.
With each GPIB-120B, you can add up to 14 additional devices to the bus. The GPIB-120B
appears as a device load on each side of the expansion; therefore, one GPIB-120B 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-120B 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-120B 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.
Computer
(Controller, Talker,
and Listener)
Printer
(Listener)
GPIB
Multimeter
(Talker and Listener)
Signal Generator
(Listener)
Unit Under Test

© National Instruments |1-3
GPIB-120B User Manual
What You Need to Get Started
*GPIB-120B
*10–18 VDC 9 W power supply
*Standard GPIB cables to connect both sides of the GPIB-120B to buses on either side
Optional Equipment
Contact National Instruments to order any of the following optional equipment.
• Rack-mount kit (part number 194906-01, shown in Figure 1-3)
Figure 1-3. Rack Mount Kit

1-4 |ni.com
Chapter 1 Introduction
• DIN rail mount kit (part number 779689-01, shown in Figure 1-4)
Figure 1-4. DIN Rail Mount Kit
• Shielded GPIB cables1
– Type X2 double-shielded GPIB cables (1 m, 2 m, or 4 m)
Unpacking Your GPIB-120B
Follow these steps when unpacking your GPIB-120B.
1. Verify that the package you received contains the following:
• GPIB-120B Isolator/Expander
• 12 VDC power supply
• Power cord appropriate for your location
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 in the input range of your power adapter. The
GPIB-120B ships with a power adapter capable of working with an input AC voltage
between 100 V and 240 V. This adapter provides 12 VDC to the GPIB-120B. This adapter
can be replaced as long as the replacement provides the GPIB-120B with a DC voltage
between 10 VDC and 18 VDC and has appropriate safety certification marks for country of
use. Refer to Appendix C, Specifications, for more information.
1 To meet FCC emission limits for this Class A device, you must use a shielded GPIB cable. Operating this
equipment with a nonshielded cable may cause interference to radio and television reception in
commercial areas.

© National Instruments |2-1
2
Hardware Overview
This chapter describes your GPIB isolator/expander.
GPIB-120B LEDs
The GPIB-120B has seven light-emitting diodes (LEDs). The POWER LED on the left side of
the isolator/expander is lit whenever you power on the GPIB-120B.
For each bus, an LED indicates the status of the System Controller, Active Controller, or Source
Handshake state, as shown in Figure 2-1. LEDs associated with Bus A are green, while those
associated with Bus B are amber.
Figure 2-1. Front View GPIB-120B
Power On (PWR)
When you power on the GPIB-120B, all circuitry is cleared to an initialized state. The
isolation/expansion system is fully operational when you power on the GPIB-120B and your
instruments are connected. Where there is GPIB activity, keep at least two-thirds of the devices
on both buses powered on.
System Controller Detection (SC)
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 (AC)
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.
I0
GPIB-120B
Isolator / Expander
PWR SC AC SH SC AC SH

2-2 |ni.com
Chapter 2 Hardware Overview
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 (SH)
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.
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.
Data Transfer Modes
The GPIB-120B isolator/expander has two data transfer modes—unbuffered mode and buffered
mode. The data transfer mode determines how data is transmitted across the expansion. The
switch on the back of the GPIB-120B sets the GPIB isolator/expander operation mode. The
default switch setting is for unbuffered transfer mode.
Selecting a Data Transfer Mode
To select a data transfer mode, refer to the following descriptions of each mode.
Unbuffered Mode
In unbuffered mode, each data byte is transmitted using the GPIB double-interlocked
handshaking protocol. For long data streams, transfers are slower than transfers using buffered
mode. However, the GPIB isolator/expander is transparent in unbuffered mode.
Buffered Mode
In buffered mode, the GPIB isolator/expander uses FIFO (first-in-first-out) buffers to buffer data
between the remote and local sides of the isolation barrier. For long data streams, the data
throughput is much higher than with unbuffered mode.
However, a few applications may not operate properly in buffered mode. For example, a GPIB
device on the local side of the isolator/expander is addressed to talk, another device on the
remote side is addressed to listen. When the Talker sources data bytes, the GPIB
isolator/expander accepts the data bytes and stores them in a FIFO buffer. At the same time, the
GPIB isolator/expander reads data from the FIFO buffer and sources data bytes to the Listener.
If the FIFO buffer contains data, the number of bytes sourced by the Talker differs from the
number of bytes accepted by the Listener. Therefore, there could be situations in which the talker
will assume the listener has accepted data which the listener has not yet received because it is

© National Instruments |2-3
GPIB-120B User Manual
still in the FIFO buffer. If this situation is unacceptable for your application, you must use
unbuffered mode, in which the 3-wire interlocked behavior of GPIB is maintained.
Buffered mode applies only to data transfers. GPIB command bytes are not stored in the FIFO
buffers; they are transmitted using the GPIB double-interlocked handshaking protocol.
Setting the Data Transfer Mode
To use buffered mode, set the switch to the ON position, as shown in Figure 2-2. To use
unbuffered mode, set the switch to the OFF position.
Figure 2-2. Switch Setting for Buffered Mode
Note The switch to select buffered or unbuffered mode is recessed to avoid
unintentional toggling during operation of the GPIB-120B. Flipping the switch may
require a flathead screwdriver or similar tool.
Verify that the switch on your GPIB isolator/expander is in the desired position before powering
on the unit.
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, and 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 parallel poll result 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 parallel poll result is driven on the data lines of Bus B.
Refer to the Parallel Poll Operation section in this chapter for important information about
conducting parallel polls with the GPIB-120B.
ON OFF
BUFFERED

2-4 |ni.com
Chapter 2 Hardware Overview
Parallel Poll Operation
According to IEEE 488, devices must respond to a parallel poll within 200 ns after the
Controller-In-Charge (CIC) asserts the Identify (IDY) message—Attention (ATN) and End or
Identify (EOI). The CIC waits at least 2 µs before reading the Parallel Poll Response (PPR). In
some cases, a remote device on an expanded system cannot respond to parallel polls this quickly
because of propagation delays across the expander and the longer cables.
When the GPIB-120B notices that a GPIB controller is conducting a parallel poll, it sends a
message to the secondary side to initiate a parallel poll. The parallel poll on the secondary side
is of the same duration as the poll on the primary side, but delayed by the time it takes to get the
message to the secondary side. In the GPIB-120B this time is approximately 400 ns. When the
poll on the primary side finishes, a message is sent to finish it on the secondary side, where the
poll finishes about 400 ns later. Therefore, if the secondary side of the GPIB-120B waited until
the end of its parallel poll to send the result of the poll to the primary side, it would be after the
primary side poll has ended. Thus the controller would miss the responses of the devices on the
secondary side.
To solve this problem, the secondary side of the GPIB-120B samples the state of the bus every
600 ns during parallel polls and sends that data back to the primary side. Therefore, taking the
start of the poll on the primary side as time 0, the state of the secondary bus is sent to the primary
side at times 600 ns, 1200 ns, 1800 ns and so on, and again when the poll actually ends.
For slow devices or topologies in which a device on the secondary bus responds to the parallel
poll after the last data packet was sent to the primary side, the controller would miss the response
from this device. If you encounter this situation, you must configure your controller to conduct
parallel polls longer than 2 µs.

© National Instruments |3-1
3
Configuring and Using
Your Hardware
This chapter describes how to configure and use your GPIB-120B.
Connecting the GPIB-120B
To connect the GPIB-120B, follow the steps below.
1. Make sure that the power switch on the isolator/expander is in the off (0) position.
2. Plug the utility power cord of your 12 VDC power supply into an acceptable electrical
outlet (100-240 VAC). Plug the other end of the power cord into the power supply.
Connect the 12 VDC output of the power supply into the GPIB-120B rear panel.
3. Link your GPIB instrument(s), board(s), and other device(s) to the GPIB-120B with
appropriate cables.
4. Verify that the switch is in the data transfer mode required for your application. Refer to the
Setting the Data Transfer Mode section in Chapter 2, Hardware Overview.
5. Move the power switch to the on (1) position.

© National Instruments |A-1
A
GPIB Basics
This appendix describes the basic concepts of GPIB, including its physical and electrical
characteristics, and configuration requirements.
The ANSI/IEEE Standard 488.1-1987, also known as General Purpose Interface Bus (GPIB),
describes a standard interface for communication between instruments and controllers from
various vendors. It contains information about electrical, mechanical, and functional
specifications. GPIB is a digital, 8-bit parallel communications interface with data transfer rates
of 1 Mbyte/s and higher, using a three-wire handshake. The bus supports one System Controller,
usually a computer, and up to 14 additional instruments. The ANSI/IEEE Standard 488.2-1992
extends IEEE 488.1 by defining a bus communication protocol, a common set of data codes and
formats, and a generic set of common device commands.
Types of Messages
Interconnected GPIB devices communicate by passing messages through the interface system,
including device-dependent messages and interface messages.
• Device-dependent messages, also called data or data messages, contain device-specific
information, such as programming instructions, measurement results, machine status, and
data files.
• Interface messages, also called commands or command messages, manage the bus itself.
Interface messages initialize the bus, address and unaddress devices, and set device modes
for remote or local programming.
The term command as used here does not refer to device instructions, which are also called
commands. Those device-specific instructions are data messages.
Talkers, Listeners, and Controllers
GPIB devices can be Talkers, Listeners, or Controllers. A Talker sends out data messages.
Listeners receive data messages. The Controller, usually a computer, manages the flow of
information on the bus. It defines the communication links and sends GPIB commands to
devices.
Some devices can play more than one role. A digital voltmeter, for example, can be a Talker and
a Listener. If your system has a National Instruments GPIB interface and software installed, it
can function as a Talker, Listener, and Controller.

A-2 |ni.com
Appendix A GPIB Basics
The GPIB is like a typical computer bus, except that the typical computer has circuit cards
interconnected via a backplane bus, whereas the GPIB has standalone devices interconnected via
a cable bus.
The role of the GPIB Controller is similar 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 the 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
Controller-In-Charge and System Controller
You can have multiple Controllers on the GPIB, but only one Controller at a time can be the
active Controller, or Controller-In-Charge (CIC). The CIC can be either active or inactive
(standby). Control can pass from the current CIC to an idle Controller, but only the System
Controller, usually a GPIB interface, can make itself the CIC.
GPIB Signals and Lines
Devices on the bus communicate by sending messages. Signals and lines transfer these messages
across the GPIB interface, which consists of 16 signal lines and 8 ground return (shield drain)
lines. The 16 signal lines are discussed in the following sections.
Data Lines
Eight data lines, DIO1 through DIO8, carry both data and command messages.
Handshake Lines
Three hardware handshake lines asynchronously control the transfer of message bytes between
devices. This process is a three-wire interlocked handshake, and it guarantees that devices send

© National Instruments |A-3
GPIB-120B User Manual
and receive message bytes on the data lines without transmission error. Table A-1 summarizes
the GPIB handshake lines.
Interface Management Lines
Five hardware lines manage the flow of information across the bus. Table A-2 summarizes the
GPIB interface management lines.
Physical and Electrical Characteristics
Devices are usually connected with a cable assembly consisting of a shielded 24-conductor cable
with both a plug and receptacle connector at each end, as shown in Figure A-1. With this design,
you can link devices in a linear configuration, a star configuration, or a combination of the
two configurations. Figure A-2 shows the linear and star configurations.
Table A-1. GPIB Handshake Lines
Line Description
NRFD (not ready for data) Listening device is ready/not ready to receive a message byte.
Also used by the Talker to signal high-speed GPIB transfers.
NDAC (not data accepted) Listening device has/has not accepted a message byte.
DAV (data valid) Talking device indicates signals on data lines are stable (valid)
data.
Table A-2. GPIB Interface Management Lines
Table Head Table Head
ATN (attention) Controller drives ATN true when it sends commands and false
when it sends data messages.
IFC (interface clear) System Controller drives the IFC line to initialize the bus and
make itself CIC.
REN (remote enable) System Controller drives the REN line to place devices in
remote or local program mode.
SRQ (service request) Any device can drive the SRQ line to asynchronously request
service from the Controller.
EOI (end or identify) Talker uses the EOI line to mark the end of a data message.
Controller uses the EOI line when it conducts a parallel poll.