National Instruments GPIB-140B User manual

USER MANUAL
GPIB-140B
Fiber Optic GPIB Extender
This document provides installation, configuration, and reference information for the
GPIB-140B fiber optic GPIB extender.
Contents
Basic Information......................................................................................................................2
Unpacking......................................................................................................................... 3
Hardware Symbol Definitions.......................................................................................... 3
Hardware Overview.................................................................................................................. 4
GPIB-140B Description....................................................................................................4
GPIB-140B Dimensions................................................................................................... 5
Grounding the GPIB-140B .............................................................................................. 6
LED Indicators..................................................................................................................8
DIP Switches.....................................................................................................................8
Hardware Connections..............................................................................................................9
Connecting the Cables...................................................................................................... 9
Connecting the External Power Supply.......................................................................... 10
Verifying the Connection................................................................................................ 10
Hardware Configuration..........................................................................................................11
Data Transfer Modes....................................................................................................... 11
HS488 Mode................................................................................................................... 12
Parallel Poll Response Modes.........................................................................................13
Theory of Operation................................................................................................................15
GPIB Basics.................................................................................................................... 16
Introduction to HS488.....................................................................................................21
Multiline Interface Messages.......................................................................................... 25
Product Certifications and Declarations................................................................................. 29
NI Services..............................................................................................................................29

Basic Information
This section provides general information about the GPIB-140B.
Conventions
GPIB-140B Refers to a GPIB extender that extends the GPIB to a maximum
distance of 1 km.
GPIB extender Refers to the bus extender.
IEEE 488 and IEEE
488.2
Refers to the ANSI/IEEE Standard 488.1-1987 and the ANSI/
IEEE Standard 488.2-1992, respectively, which define the GPIB.
Kit Contents
• A GPIB-140B bus extender
• 12 V DC power supply
This power adapter can be used with an input AC voltage between 100 V AC and
240 V AC. Verify that the voltage you will be using is in the input range of this power
adapter.
Note If using a different adapter than what ships with the GPIB-140B, ensure
that the adapter provides 9 V DC to 15 V DC and has appropriate safety
certification marks for country of use.
Optional Equipment
The following table lists some cables available for the GPIB-140B. For a complete list of
GPIB accessories and ordering information, refer to the pricing section of the Fiber-Optic
Cable and GPIB Cable product pages at ni.com.
Note The GPIB-140B is designed for use with multi-mode fiber-optic cable. Do
not use with single-mode cable.
Table 1. GPIB-140B Optional Cables
Cable/Accessory Part Number
GPIB T7 fiber-optic cable - extends up to 1 km
(10 m to 1000 m lengths)
182805-010/020/030/050/100/200/500/01K
Type X2 double-shielded cable with shielded
plug/receptacles
0.5 m: 763061-005 3 m: 763061-003
1 m: 763061-01 4 m: 763061-04
2 m: 763061-02
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Note To meet FCC emission limits for this device, use only with shielded cables
and accessories. If you operate this equipment with a non-shielded cable, it may
interfere with radio and television reception.
Related Documentation
The following documents contain information that you may find helpful as you read this
manual:
• GPIB 140B Specifications
• GPIB-140B Safety, Environmental, and Regulatory Information
• 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 Commands
Unpacking
The GPIB-140B ships in an antistatic package to prevent electrostatic discharge (ESD). ESD
can damage several components on the device.
To avoid ESD damage in handling the device, take the following precautions:
• Ground yourself with a grounding strap or by touching a grounded object.
• Touch the antistatic package to a metal part of your computer chassis before removing the
device from the package.
Remove the device from the package and inspect it for loose components or any other signs of
damage. Notify NI if the device appears damaged in any way. Do not install a damaged
device.
Store the device in the antistatic package when the device is not in use.
Hardware Symbol Definitions
The following symbols are marked on the GPIB-140B.
Caution Take precautions to avoid injury. Refer to the GPIB-140B Safety,
Environmental, and Regulatory Information for safety guidelines.
At the end of the product life cycle, all NI products must be disposed of
according to local laws and regulations. For more information about how to
recycle NI products in your region, visit ni.com/environment/weee.
540
e
NI 符合中国电子信息产品中限制使用某些有害物质指令(RoHS)。关于 NI
中国 RoHS 合规性信息，请登录 ni.com/environment/rohs_china。
(For information about China RoHS compliance, go to ni.com/
environment/rohs_china.)
GPIB-140B User Manual | © National Instruments Corporation | 3

Hardware Overview
This section describes the GPIB-140B fiber optic GPIB extender.
GPIB-140B Description
Note The GPIB-140B bus extender can communicate with either a GPIB-140B or
a GPIB-140A bus extender. The GPIB-140B cannot communicate with a GPIB-140,
a GPIB-140/2 or a GPIB-140A/2 bus extender since it uses a different protocol to
communicate across the fiber-optic cable.
The GPIB-140B is a high-speed bus extender that you can use in pairs with multi-mode fiber-
optic cable to connect two separate GPIB systems in a functionally transparent manner.
The two bus systems are physically separate, as shown in the following figure.
Figure 1. Typical Extension System (Physical Configuration)
Printer
(Listener)
GPIB Cable
Computer
(System Controller,
Talker, and Listener)
GPIB Cable
Multi-Mode
Fiber-Optic Cable
Signal Generator
(Listener) Unit Under Test
Multimeter
(Talker and Listener)
GPIB-140BGPIB-140B
GPIB Cable
The devices logically appear to be located on the same bus, as shown in the following figure.
Figure 2. Typical Extension System (Logical Configuration)
Computer
(System Controller,
Talker, and Listener)
Printer
(Listener)
GPIB
Multimeter
(Talker and Listener)
Signal Generator
(Listener)
Unit Under Test
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The bus extender complies with the specifications of the ANSI/IEEE Standard 488.1-1987 and
the ANSI/IEEE Standard 488.2-1992, including the Find Listeners protocol. With the GPIB
extenders, you can overcome the following two configuration restrictions imposed by IEEE
488:
• A cable length limit of 20 m total per contiguous bus or 2 m per each device on the bus,
whichever is smaller.
• An electrical loading limit of 15 devices per contiguous bus.
Each GPIB-140B system extends the GPIB to a maximum distance of 1 km, and extends the
loading limit to 28 devices (including the GPIB extenders), without sacrificing speed or
performance. You can connect these point-to-point extension systems in series for longer
distances or in star patterns for additional loading.
Using the HS488 protocol, the maximum data transfer rate over the extension is greater than
2.8 MBytes/s. The GPIB extenders use a buffered transfer technique with a serial extension
bus, which maximizes performance and minimizes the cabling cost. Furthermore, the extender
does not affect the transfer rate between devices on the same side of the extension. The GPIB
extender can also check for errors to make sure that the data transmitted successfully over the
fiber-optic link.
Because the GPIB-140B is a functionally transparent extender, the GPIB communications and
control programs that work with an unextended system also work with an extended system.
However, the Parallel Poll Response Modes section describes one exception to this
transparency in conducting parallel polls.
GPIB-140B Dimensions
The following dimensional drawings apply to the GPIB-140B. To find detailed dimensional
drawings and 3D models visit ni.com/dimensions and search for the device number.
GPIB-140B User Manual | © National Instruments Corporation | 5

Figure 3. GPIB-140B Top and Bottom Dimensions
93.67 mm
(3.688 in.)
133.88 mm (5.271 in.)
50.00 mm
(1.969 in.)
Figure 4. GPIB-140B Side Dimensions
POWER
OFF ON
LINK STATUS PARALLEL POLL IMMEDIATE
GPIB-140B
FIBER OPTIC GPIB EXTENDER
HS488 ENABLED
BUFFERED TRANSFER
ON
OFF
28.80 mm
(1.134 in.)
RECEIVE TRANSMIT
INPUT - - - +
9 to 15 V - - - 3 W MAX
Grounding the GPIB-140B
You must connect the GPIB-140B grounding terminal to the grounding electrode system of the
facility.
Note For more information about ground connections, visit ni.com/r/emcground.
6| ni.com | GPIB-140B User Manual

What to Use
• Standard ring lug
• Wire, 1.3 mm2 (16 AWG) or larger
• Screwdriver, Phillips #2
What to Do
Complete the following steps to ground the GPIB-140B.
Figure 5. Installing the Ground Wire
POWER
OFF ON
LINK STATUS PARALLEL POLL IMMEDIATE
GPIB-140B
FIBER OPTIC GPIB EXTENDER
HS488 ENABLED
BUFFERED TRANSFER
ON
OFF
1. Attach the ring lug to the wire.
2. Remove the grounding screw from the grounding terminal on the side panel of the
product.
3. Fasten the ring lug to the grounding terminal.
4. Tighten the grounding screw to 1.3 N · m (11.5 in-lb) of torque.
5. Attach the other end of the wire to the chassis safety ground using a method that is
appropriate for your application.
GPIB-140B User Manual | © National Instruments Corporation | 7

LED Indicators
Table 2. LED State/Device Status
LED Color Behavior Description
POWER
Green Solid GPIB-140B is powered on.
Red Solid GPIB-140B is powered on, but the input supply voltage
is either out of the operating range or the overcurrent
protection is active.
— Off The supply voltage is connected in reverse polarity
(when the device is powered on), or the device is
powered off.
LINK
Green Solid Both GPIB extenders are powered on and the fiber-optic
transmission cable is properly connected between them.
The GPIB-140B bus extenders are ready to use.
— Off The fiber-optic cable is defective or disconnected, or the
remote GPIB-140B is turned off.
STATUS
Green Flashing, 10 Hz Activity is present on the GPIB bus.
Red Solid The fiber-optic cable is either defective or disconnected,
or the GPIB-140B is turned off.
Red Flashing, 10 Hz The GPIB-140B is receiving corrupted data, and starts
re-transmission.
The red LED turns off after 50 ms and flashes green
when the extender receives retransmitted data bytes
without error.
— Off There is no activity on the GPIB bus.
DIP Switches
The 3-bit DIP switch sets the operation mode of the GPIB extender. The default switch setting
is for unbuffered transfer mode, latched parallel poll response (PPR), and HS488 disabled
mode, as shown here.
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Figure 6. Default DIP Switch Setting
OFF
PARALLEL POLL IMMEDIATE
HS488 ENABLED
BUFFERED TRANSFER
Verify that the DIP switches on your GPIB extender are in these default positions. If you need
to change these settings, refer to Hardware Configuration on page 11 for instructions about
how to set the operation mode for your application.
Hardware Connections
This section explains how to connect the GPIB-140B to the fiber-optic cables and power
supply, and how to run a self test to verify operation.
Connecting the Cables
Note The GPIB-140B is designed for use with multi-mode fiber-optic cable. Do
not use with single-mode cable.
Complete the following steps to connect a multi-mode fiber-optic cable to both GPIB
extenders.
1. Make sure that each GPIB-140B extender is powered off.
2. Connect the two connectors on each end of the fiber-optic cable to your GPIB extenders,
as follows:
a. As shown in the following figure, align the connector marked T (transmit) with the
connector labeled TRANSMIT on the side of the GPIB extender. Align the
connector marked R (receive) with the connector labeled RECEIVE on the side of
the GPIB extender.
Figure 7. Connecting the Fiber-Optic Cable to Both GPIB Extenders
GPIB-140BGPIB-140B
Multi-Mode
Fiber-Optic Cable
Transmit
Transmit
Receive
Receive
b. Remove the caps on the connectors.
GPIB-140B User Manual | © National Instruments Corporation | 9

c. Align the notch on each cable connector to the slot of the fiber-optic connector on
the GPIB extender.
d. Firmly push in the cable connector and rotate the sleeve clockwise until it locks on
to the side notch of the fiber-optic connector on the GPIB extender.
3. Connect the end of the extender with the GPIB connector to your GPIB system. Make
sure to follow all IEEE 488 cabling restrictions. For typical restrictions, refer to
Configuration Requirements on page 20.
Connecting the External Power Supply
Complete the following steps to connect the external power supply.
1. Plug the utility power cord of your 12 V DC power supply into a 100 V AC to 240 V AC
electrical outlet.
2. Plug the other end of the power cord into the power supply.
3. Connect the 12 V DC output of the power supply into the DC power connector on the
GPIB-140B by rotating the sleeve by hand until it is firmly screwed in place.
4. Power on the GPIB-140B extender; the POWER LED lights green. If the LED does not
light green make sure that the supply voltage is in the acceptable range.
The LINK LED lights green when multiple GPIB extenders are properly connected and turned
on.
Note Refer to LED Indicators on page 8 for detailed information about the
behavior of each LED.
Verifying the Connection
Each GPIB extender has a self test that determines whether the GPIB extender receivers,
transmitters, and packet transmission and reception circuitry are working properly.
Complete the following steps to run the self-test.
1. Power off the GPIB extender.
2. Disconnect the fiber-optic cable from the GPIB extender.
3. Power on the GPIB extender.
The POWER LED lights green, indicating that power is supplied to the extender. The
LINK LED remains off, and the STATUS LED lights red.
4. Connect the connector marked T (transmit) on one end of the fiber-optic cable to the
connector marked TRANSMIT on the side of the GPIB extender.
5. Connect the connector marked R (receive) on the opposite end of the fiber-optic cable to
the connector marked RECEIVE on the side of the GPIB extender.
10 | ni.com | GPIB-140B User Manual

Figure 8. GPIB Extender Self-Test Configuration
Transmit GPIB-140B
Multi-Mode
Fiber-Optic Cable
Receive
R
R
T
T
The LINK LED lights green, indicating that a properly working cable is connected. The
STATUS LED remains off during the self-test.
If the LINK LED is off and the STATUS LED is either solid or flashing red, there may be an
issue with the fiber-optic transmission cable. Complete the following steps to troubleshoot:
1. Verify that the fiber-optic cable is properly connected to the GPIB extender as described
in steps 4 and 5 above. If the problem persists, continue to the next step.
2. Repeat steps 4 and 5 using the unconnected ends of the fiber-optic cable. If switching the
fiber-optic cable connectors solves the problem, you need to replace your fiber-optic
cable. Refer to the Fiber-Optic Cable product page at ni.com for cable information.
If switching the fiber-optic cable connectors does not solve the problem, continue to the
next step.
3. Repeat steps 4 and 5 using a different fiber-optic cable. If the problem persists, you might
need to replace your GPIB extender. For more information, contact NI for support.
Note Refer to LED Indicators on page 8 for detailed information about the
behavior of each LED.
Hardware Configuration
This section describes how to configure the operation modes supported by the GPIB-140B.
Data Transfer Modes
The GPIB extender has two data transfer modes—unbuffered mode and buffered mode. The
data transfer mode determines how data is transmitted across the extension.
Selecting a Data Transfer Mode
Refer to the following descriptions when selecting a data transfer 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 extension is transparent in unbuffered mode.
GPIB-140B User Manual | © National Instruments Corporation | 11

Buffered Mode
In buffered mode, the GPIB extenders use FIFO (first-in-first-out) buffers to buffer data
between the remote and local units. 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 extension is addressed to talk, another device on the remote side
is addressed to listen. When the Talker sources data bytes, the GPIB extenders accept the data
bytes and store them in a FIFO buffer. At the same time, the GPIB extenders read data from
the FIFO buffer and source 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.
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
The two GPIB extenders in your extension system must use the same data transfer mode.
To use buffered mode, set the BUFFERED TRANSFER DIP switch to the ON position, as
shown in the following figure. To use unbuffered mode, set this switch to the OFF position.
Figure 9. DIP Switch Setting for Buffered Mode
OFF
PARALLEL POLL IMMEDIATE
HS488 ENABLED
BUFFERED TRANSFER
= Not used to set data transfer mode
HS488 Mode
The GPIB extender can handle data transfers using the HS488 protocol. HS488 transfers data
between two or more devices using a noninterlocked handshaking protocol. You can use
HS488 to transfer data at rates higher than rates possible using the IEEE 488 protocol. For
more information about HS488, refer to Introduction to HS488 on page 21.
Selecting an HS488 Mode
Refer to the following descriptions when selecting the HS488 mode.
HS488 Disabled
If you disable HS488, the GPIB extender sources and accepts data using a three-wire
handshaking protocol, even if both the Talker and Listener can transfer data using the HS488
protocol.
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HS488 Enabled
After the Talker indicates that it wants to issue HS488 transfers, HS488 is enabled and the
GPIB extender accepts data using the HS488 protocol. Also, when talking, the GPIB extender
always tries to use the HS488 mode. In HS488 mode, FIFO buffers buffer data during HS488
transfers, even if the data transfer mode is set to unbuffered. When you use the HS488
protocol with the GPIB extender, you should set the GPIB cable length to 5 m for both the
local and the remote system. To do so, use your IEEE 488.2 software configuration utility.
Setting the HS488 Mode
The two GPIB extenders in your extension system do not need to use the same HS488 mode,
however, the system uses the maximum data transfer rate when both sides in your extension
system use HS488.
To enable HS488, set the HS488 ENABLED DIP switch to the ON position, as shown in the
following figure. To disable HS488, set this switch to the OFF position.
Figure 10. DIP Switch Setting for Enabled HS488
OFF
= Not used to set HS488 mode
PARALLEL POLL IMMEDIATE
HS488 ENABLED
BUFFERED TRANSFER
Parallel Poll Response Modes
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
many cases, a remote device on an extended system cannot respond to parallel polls this
quickly because of cable propagation delays. To solve this problem, use one of the following
two solutions in your application:
• If possible, specify in your application that the CIC must allow enough time to receive the
response. For more information, refer to Immediate PPR Mode on page 14.
If you are using NI-488.2 software, you can use the NI-488.2 Configuration utility to set
the amount of time that the CIC waits.
• Execute two consecutive parallel polls and use the second response. For more
information, refer to Latched PPR Mode on page 14.
PPR Mode Considerations
When selecting a PPR mode, consider the type of Controller present in your GPIB system and
the length of cable between the GPIB-140B extenders. However, if your application does not
use parallel polls, you do not need to select a PPR mode.
Some Hewlett Packard GPIB Controllers remain in a parallel poll state with IDY asserted if
they are not performing another function. A change in the response interrupts the application.
GPIB-140B User Manual | © National Instruments Corporation | 13

In some Controllers, the IDY signal is toggled on and off, and you can change the duration of
the signal to accommodate delayed responses over extenders. If you are using these types of
Controllers, you should set the GPIB extender to immediate PPR mode.
Most other Controllers pulse the IDY signal for approximately 2 µs and expect a response
within that time. If you are using this type of Controller and if the cable between the extenders
is longer than 60 m, you should set the GPIB extender to latched PPR mode. For shorter cable
distances, use immediate PPR mode.
The two GPIB extenders in your extension system do not need to use the same PPR mode.
Select the PPR mode of the local GPIB extender based on the Controllers on the local GPIB
system. Likewise, select the PPR mode of the remote GPIB extender based on the Controllers
on the remote GPIB system. If no Controllers are physically connected to one of the GPIB
extenders, the PPR mode of that GPIB extender has no effect on your system.
Selecting a PPR Mode
Refer to the following descriptions when selecting the PPR mode.
Immediate PPR Mode
In immediate PPR mode, the GPIB extenders do not use the internal PPR data register. When a
Controller on the local system asserts IDY, the local extender sends the IDY message to the
remote bus and the response is returned as fast as propagation delays permit. Your application
must allow enough time to receive the response.
Latched PPR Mode
In latched PPR mode, the GPIB extenders use an internal PPR data register. When a Controller
on the local system asserts IDY, the local extender sends the contents of the PPR data register
to the local data lines. At the same time, a parallel poll message is sent to the remote bus.
When the local system unasserts IDY, the PPR from the remote system is loaded into the
internal PPR data register. Consequently, the register always contains the response of the
previous complete poll. To obtain the response of both local and remote systems, your
application should execute two consecutive parallel polls and use the second response.
The software driver library of most Controllers contains an easy-to-use parallel poll function.
For example, if the function is called ibrpp and your application is written in BASIC, the
sequence to execute a poll in latched PPR mode might be similar to the following sequence:
CALL ibrpp (brd0%, ppr%)
CALL ibrpp (brd0%, ppr%)
IF ppr > 0 GOTO 300
Setting the PPR Mode
To enable immediate PPR mode, set the PARALLEL POLL IMMEDIATE DIP switch to
the ON position, as shown in the following figure. To enable latched PPR mode, set this
switch to the OFF position.
14 | ni.com | GPIB-140B User Manual

Figure 11. DIP Switch Setting for Immediate PPR Mode
OFF
= Not used to set Parallel Poll Response (PPR) mode
PARALLEL POLL IMMEDIATE
HS488 ENABLED
BUFFERED TRANSFER
Theory of Operation
This section describes how the GPIB extender circuitry operates.
This section assumes that you are familiar with GPIB. If you are a first-time user or if you
would like to review the basics about GPIB, refer to GPIB Basics on page 16.
The following figure shows the five layers of a GPIB extender. To form a complete link, you
can connect each layer to the corresponding layer of another extender at the remote side.
Figure 12. GPIB Extender Block Diagram
Message Interpreter
Layer
Message Interpreter
Layer
Packet Translation
Layer
Packet Translation
Layer
Link Management
Layer
Link Management
Layer
Parallel-to-Serial
Conversion Layer Physical Layer
GPIB BUS #2
GPIB BUS #1
GPIB EXTENDER
GPIB EXTENDER
Transmission
Medium
Physical Layer
Parallel-to-Serial
Conversion Layer
GPIB-140B User Manual | © National Instruments Corporation | 15

Message Interpreter Layer
The Message Interpreter Layer handles the handshake between the GPIB extender and other
devices on the GPIB. At the same time, the layer monitors the activities that occur on the
GPIB, translates them into equivalent local and remote GPIB messages, and sends these
messages to the Packet Translation Layer.
Packet Translation Layer
The Packet Translation Layer converts the messages that it receives to packets and sends them
to the Link Management Layer. It can also receive packets from the Link Management Layer
and convert them back to local or remote GPIB messages.
Link Management Layer
The Link Management Layer receives packets from the Packet Translation Layer. It sends the
packets to the Parallel-to-Serial Conversion Layer and it stores them in a local buffer. If a
transmission error occurs, the Link Management Layer can re-send the packets from this local
buffer. The Link Management Layer also receives packets from the Parallel-to-Serial
Conversion Layer and checks the packets for transmission errors. If the Link Management
Layer does not detect an error, it sends the packets to the Packet Translation Layer. However,
if it detects a transmission error, then it re-transmits the packets.
Parallel-to-Serial Conversion Layer
The Parallel-to-Serial Conversion Layer accepts packets from the Link Management Layer,
converts them into serial data, and sends the data to the Physical Layer. It also extracts serial
bits from the Physical Layer, reconstructs them back into packets, and sends them to the Link
Management Layer.
Physical Layer
The Physical Layer transmits and receives serial data over the fiber-optic link.
GPIB Basics
This section 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.
16 | ni.com | GPIB-140B User Manual

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 are capable of playing more than one role. A digital voltmeter, for example, can
be a Talker and a Listener. If your system has a NI GPIB interface and software installed, it
can function as a Talker, Listener, and Controller.
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
GPIB-140B User Manual | © National Instruments Corporation | 17

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 and receive message bytes on the data lines without transmission error. The following
table summarizes the GPIB handshake lines.
Table 3. 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.
Interface Management Lines
Five hardware lines manage the flow of information across the bus. The following table
summarizes the GPIB interface management lines.
Table 4. GPIB Interface Management Lines
Line Description
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.
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Table 4. GPIB Interface Management Lines (Continued)
Line Description
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.
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 the following figure.
Figure 13. GPIB Connector and Signal Assignments
13
14
15
16
17
18
19
20
21
22
23
24
1
2
3
4
5
6
7
8
9
10
11
12
DIO5
DIO6
DIO7
DIO8
REN
GND (TW PAIR W/DAV)
GND (TW PAIR W/NRFD)
GND (TW PAIR W/NDAC)
GND (TW PAIR W/IFC)
GND (TW PAIR W/SRQ)
GND (TW PAIR W/ATN)
SIGNAL GROUND
DIO1
DIO2
DIO3
DIO4
EOI
DAV
NRFD
NDAC
IFC
SRQ
ATN
SHIELD
With this design, you can link devices in a linear configuration, a star configuration, or a
combination of the two configurations. The following figure shows both linear and star
configurations.
GPIB-140B User Manual | © National Instruments Corporation | 19

Figure 14. Linear and Star System Configuration
Device A
Device B
Device C
Device DDevice A
Device CDevice B
a. Linear Configuration b. Star Configuration
The standard connector is the Amphenol or Cinch Series 57 Microribbon or Amp Champ type.
For special interconnection applications, you use an adapter cable using a non-standard cable
and/or connector.
The GPIB uses negative logic with standard TTL (transistor-transistor logic) level. For
example, when DAV is true, it is a TTL low level (≤ 0.8 V), and when DAV is false, it is a
TTL high level (≥ 2.0 V).
Configuration Requirements
To achieve the high data transfer rate that the GPIB was designed for, you must limit the
number of devices on the bus and the physical distance between devices. The following
restrictions are typical:
• A maximum separation of 4 m between any two devices and an average separation of 2 m
over the entire bus.
• A maximum total cable length of 20 m.
• A maximum of 15 devices connected to each bus, with at least two-thirds powered on.
For high-speed operation, the following restrictions apply:
• All devices in the system must be powered on.
• Cable lengths must be as short as possible with up to a maximum of 15 m of cable for
each system.
• There must be at least one equivalent device load per meter of cable.
If you want to exceed these limitations, you can use a bus expander to increase the number of
device loads. You can order bus expanders from NI.
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