the smart approach to instrumentation ™

Measurement Computing
10 Commerce Way
Norton, MA 02766(508) 946-5100
Fax: (508) 946-9500
info@mccdaq.com

www.mccdaq.com

Extender 488 User’s Manual
p/n Extender488-901

1996-1997 by Measurement Computing Corp.
Part No. Extender488-901
Printed in the United States of America

Warranty Information

Contact Measurement Computing by phone, fax, or e-mail in regard to warranty-related issues:
Phone: (508) 946-5100, fax: (508) 946-9500, e-mail: info@mccdaq.com

Limitation of Liability

Measurement Computing cannot be held liable for any damages resulting from the use or misuse of this product.

Copyright, Trademark, and Licensing Notice

All Measurement Computing documentation, software, and hardware are copyright with all rights reserved. No part ofthis product may be copied, reproduced or transmitted by any mechanical, photographic, electronic, or other method
without Measurement Computing’s prior written consent. IOtech product names are trademarked; other product names, asapplicable, are trademarks of their respective holders. All supplied IOtech software (including miscellaneous support
files, drivers, and sample programs) may only be used on one installation. You may make archival backup copies.

CE Notice

Many Measurement Computing products carry the CE marker indicating they comply with the safety and emissions
standards of the European Community. When applicable these products have a Declaration of Conformity stating which
specifications and operating conditions apply. You can view the Declarations of Conformity at
www.mccdaq.com/legal.aspx (CE Information page).

Warnings, Cautions, Notes, and Tips

Refer all service to qualified personnel. This caution symbol warns of possible personal injury or equipment damage
under noted conditions. Follow all safety standards of professional practice and the recommendations in this manual.
Using this equipment in ways other than described in this manual can present serious safety hazards or cause equipment
damage.
This warning symbol is used in this manual or on the equipment to warn of possible injury or death from electricalshock under noted conditions.

This ESD caution symbol urges proper handling of equipment or components sensitive to damage from electrostatic
discharge. Proper handling guidelines include the use of grounded anti-static mats and wrist straps, ESD-protective bags
and cartons, and related procedures.

This symbol indicates the message is important, but is not of a Warning or Caution category. These notes can be of great
benefit to the user, and should be read.
In this manual, the book symbol always precedes the words “Reference Note.” This type of note identifies the locationof additional information that may prove helpful. References may be made to other chapters or other documentation.
Tips provide advice that may save time during a procedure, or help to clarify an issue. Tips may include additionalreference.

Specifications and Calibration
Specifications are subject to change without notice. Significant changes will be addressed in an addendum or revision to
the manual. As applicable, the hardware is calibrated to published specifications. Periodic hardware calibration is notcovered under the warranty and must be performed by qualified personnel as specified in this manual. Improper
calibration procedures may void the warranty.

Table of Contents
Section 1
1.1
1.2
1.3
1.4
1.5

INTRODUCTION
Description
Available Accessories
Extender488 Specifications
Extender488/F Specifications
Abbreviations

Page
1.1
1.2
1.3
1.4
1.5

Section 2 GETTING STARTED
2.1
Inspection
2.2
Extender488 Configuration
2.2.1
Extender488 Serial Baud Rate Selection
2.2.2
Extender488 Serial Data Format Selection
2.2.3
Extender488 Serial Port Cabling
2.3
Extender488/F Configuration
2.3.1
Extender488 Fiber Optic Connections
2.4
Operation
2.5
Parallel Poll Response Times

Page
2.1
2.2
2.2
2.3
2.4
2.5
2.5
2.7
2.9

Section 3 IEEE 488 Primer
3.1
History
3.2
General Structure
3.3
Send It To My Address
3.4
Bus Management Lines
3.4.1
Attention (ATN)
3.4.2
Interface Clear (IFC)
3.4.3
Remote Enable (REN)
3.4.4
End Or Identify (EOI)
3.4.5
Service Request (SRQ)
3.5
Handshake Lines
3.5.1
Data Valid (DAV)
3.5.2
Not Ready For Data (NRFD)
3.5.3
Not Data Accepted (NDAC)
3.6
Data Lines
3.7
Multiline Commands
3.7.1
Go To Local (GTL)
3.7.2
Listen Address Group (LAG)

Page
3.1
3.1
3.4
3.4
3.4
3.5
3.5
3.5
3.5
3.6
3.6
3.6
3.6
3.7
3.7
3.7
3.8

Table of Contents
Section 3
3.7.3
3.7.4
3.7.5
3.7.6
3.7.7
3.7.8
3.7.9
3.7.103.7.11
3.7.12
3.7.133.7.143.7.15
3.8
3.8.1
3.8.2

IEEE 488 Primer
Unlisten (UNL)
Talk Address Group (TAG)
Untalk (UNT)
Local Lockout (LLO)
Device Clear (DCL)
Selected Device Clear (SDC)
Serial Poll Disable (SPD)Serial Poll Enable (SPE)Group Execute Trigger (GET)
Take Control (TCT)
Secondary Command Group (SCG)
Parallel Poll Configure (PPC)
Parallel Poll Unconfigure (PPU)
More On Service Requests
Serial Poll
Parallel Poll

Page
3.8
3.8
3.8
3.8
3.8
3.9
3.9
3.9
3.9
3.9
3.9
3.10
3.10
3.10
3.11
3.11

Section 4
4.1
4.2
4.3
4.4
4.4

SERVICE INFORMATION
Factory Service
Theory of Operation
Extender488 Component Layout
Extender488/F Fiber Optic I/O Layout
Replaceable Parts List

Page
4.1
4.1
4.2
4.3
4.4

Appendix A

IEEE Command and Address Messages

Section 1

Introduction

INTRODUCTION
1.1 DESCRIPTION
The Extender488s enable IEEE 488 devices to be co ntrolled up to 4,000 feet from the h ost
computer. This overcomes the limit imp osed by t he IEEE 488 specification of 6 feet between any two
devices, to a m aximum of 60 feet total chained length. I n addition, the Extender488s allows up to 30
devices to be on the bus, versus the 15 device limit imposed by the IEEE standard.
Two extenders are required to perform bus extension. The bus connecting the host computer, or
controller, is the "local" bus. T he bus to which control is extended is the "remote" bus. One of the
Extender488s is required on each bus to accomplish IEEE bus extension.
Communication between extenders is accomplished with serial data transmission, using RS-422 data
drivers and receivers with the Extender488. This differential serial data format provides high noise
immunity and long-distance capability using low cost twisted pair wire.
The Extender488/F communicates serially over duplex fiber optic cables. Fiber optic transmission
provides high noise immunity, low RFI emissions and long-distance capability.
Operation of the Extender488s is completely transparent to the system. The controller can accessboth local and remote devices in exactly the same manner, with the exception of Parallel Poll.

1-1

Section 1

Introduction

1.2 AVAILABLE ACCESSORIESThe following accessories are available from Measurement Computing for use with the Extender488s.
CA-7-1
CA-7-2CA-7-3CA-7-2
CA-18-X

1.5 foot IEEE 488 cable
6 foot IEEE 488 cable
6 foot IEEE 488 cable; shielded
6 foot IEEE 488 cable; reverse entry
Shielded serial cable with mating
connectors for the Extender488;
specify X in feet from 100 to 4000
100µm/140µm duplex fiber optic cablewith mating connectors for the
Extender488/F;
specify X in feet from 100 to 4000
Right angle IEEE 488 adapter
IEEE 488 connector extender
IEEE 488 multi-tap bus strip;
4 connectors in parallelIEEE 488 panel mount feed-thru connector
Additional Manual
Rack mount kit; one Extender488 package
Rack mount kit; two Extender488 packages
External power supply for Extender488
105-125v; 50/60 Hz
External power supply for Extender488
210-250v; 50 Hz

CA-24-X

CN-20
CN-21
CN-22
CN-23
114-0920
Rack488-1
Rack488-2
TR-2
TR-2E

Refer to the Measurement Computing catalog for those accessories not listed here.

1-2

Section 1

Introduction

1.3 Extender488 SPECIFICATIONSIEEE 488-1978
Implementation:
Connector:
Serial Interface
EIA RS-422A:
Connector:
Character Set:
Baud Rate:
Data Format:
Max. Cable Length:
GeneralData Rates:

Indicators:
Power:
Dimensions:
Weight:Environment:
Controls:

Proprietary transparent control and data transfers.
Standard IEEE 488 connector with metric studs.
Balanced voltage on TxD and RxD.
9-pin Sub-D female.
Proprietary asynchronous bit serial.
Selectable 300, 600, 1200, 2400, 4800, 9600, 19,200 and
115,200.
Selectable 7 or 8 data bits; 1 stop bit; odd parity.
1000 meters.
Specifications apply to data transfers after
addressing has been established.
Baud Rate
Data Rate (bytes per second, avg)
300
13
600
26
1200
53
2400
105
4800
204
9600
385
19.2K
691
115.2K
3400
LEDs for IEEE Talk, Listen, SRQ, Error and Power.
105-125V or 210-250V; 50-60 Hz, 10 VA Max.
188mm deep x 140mm wide x 68mm high (7.39" x 5.5" x1.35 kg. (3.1 lbs).
0¡ - 50¡C; 0 to 70% R.H. to 35¡C.
Linearly derate 3% R.H./¡C from 35¡ to 50¡C.
Power Switch (external),
Serial parameter switches (internal).
Specifications subject to change without notice.

1-3

2.68").

Section 1

Introduction

1.4 Extender488/F SPECIFICATIONSIEEE 488-1978
Implementation:
Connector:

Proprietary transparent control and data transfers.
Standard IEEE 488 connector with metric studs.

Fiber Optic Interface
Wavelength:
Connectors:
Required Cable:

820 nanometers typical.
SMA-905 series female.
Duplex 100µm, 140µm clad fiber optic.
200MHz-km bandwidth; 6 db/km max. attenuation.
1000 meters.

Max. Cable Length:
GeneralData Rates:
Indicators:
Power:
Dimensions:

3400 bytes per second typical.
LEDs for IEEE Talk, Listen, SRQ, Error and Power.
105-125V or 210-250V; 50-60 Hz, 10 VA Max.
188mm deep x 140mm wide x 68mm high(7.39" x 5.5" x 2.68").
1.55 kg. (3.6 lbs).
0¡ - 50¡C; 0 to 70% R.H. to 35¡C.
Linearly derate 3% R.H./¡C from 35¡ to 50¡C.
Power Switch (external).

Weight:Environment:
Controls:

Specifications subject to change without notice.

1-4Section 1

Introduction

1.5 ABBREVIATIONS
The following IEEE 488 abbreviations are used throughout this manual.
addr n
ATN
CA
CO
CR
data
DCL
GET
GTL
LA
LAG
LF
LLO
MLA
MTA
PE
PPC
PPU
SC
SDC
SPD
SPE
SRQ
TA
TAD
TCT
term
UNL
UNT
*

IEEE bus address "n"Attention line
Controller Active
Controller
Carriage Return
Data String
Device Clear
Group Execute Trigger
Go To Local
Listener Active
Listen Address GroupLine Feed
Local Lock Out
My Listen Address
My Talk Address
Peripheral
Parallel Poll Configure
Parallel Poll Unconfigure
System Controller
Selected Device Clear
Serial Poll Disable
Serial Poll Enable
Service Request
Talker Active
Talker Address
Take Control
Terminator
Unlisten
UntalkUnasserted

1-5Section 2

Getting Started

GETTING STARTED
2.1 INSPECTION
The Extender488s are carefully inspected, both mechanically and electrically, prior to shipment.
When you receive the inter face, carefully unpack all items from the s hipping carton and check for any
obvious signs of physical damage w hich may have occurred during shipment. Immediately report any
damage found to the shipping agent. Remember to retain all shipping materials in the event that shipment
back to the factory becomes necessary.
Every Extender488 is shipped with the following....
 Extender488

IEEE bus extender interface
Power Supply; TR-4 115V
TR-4E 220V
Instruction Manual TR-2 or TR-2E
 114-0920

Note: accessories ordered may be packaged and shipped separately

WARNINGThe Extender488 requires a regulated external power s upply with a maximum
output of 5.25 volts! Using voltages above this level will result in damage to theinterface. Use only power supplies provided by Measurement Computing.

2.1

Section 2

Getting Started

2.2 Extender488 CONFIGURATION
The following describes the configuration necessary for the RS-422 Extender488s. If you are using
the fiber optic extenders, refer to Section 2.3.
Two Extender488s are req uired to accomplish bus extension, with one c onnected directly to the
LOCAL IEEE bus, and the other connected to the REMOTE IEEE bus.
Both local and remote extenders are identical. At power up, each extender monitors the Attention(ATN) and Remote Enable (REN) lines. The Error LED on each extender will blink indicating that ATN or
REN has not yet been detected and that communication with the other extender has not yet been
established. The first extender to detect either of t hese lines asserted declares itself the LOCAL extenderand commands the other extender to declare itself the REMOTE extender. The Error LEDs will now stop
blinking. From this moment on, the LOCAL extender will monitor all local bus activity and transmit thisactivity to the REMOTE extender. All activity detected by the REMOTE extender is also communicatedback to the LOCAL extender.

2.2.1 Extender488 Serial Baud Rate Selection
The Extender488 is shipped with the serial baud rate set to 115.2K baud. For mostapplications, this is the most efficient operating rate. However, if the extenders are to be located at
extreme distances from each other, baud rates other than 115.2K can be selected. Selectable bau drates include 115.2K, 19.2K, 9600, 4800, 2400, 1200, 600, 300. Both extenders must be set to thesame baud rate for proper operation.
To modify the baud rate, follow this simple procedure: Disconnect the power supply fromthe AC line and from the interface. Disconnect any IEEE or serial cables prior to disassembly.

2.2

Section 2

Getting Started
WARNING
Never open the Extender488 case while it is connected to th e AC line.
Failure to obs erve this warning may result in equipment failure, personal
injury or death.

Remove the four screws located in each corner of the rear panel. Hold the case firmly andpull the rear panel outward, noting the slot location of the main circuit board. Modify thoseparameters which are appropriate for your installation and reassemble the unit. Slide the main
circuit board into the previously noted slot and finish reassembly by tightening the four screws into
the rear panel.

Extender488 Baud Rate Selection
0
1

1 2 3 4

OPEN

SwitchSide
View

Default = 115200

DOT

S102
1
0
1
0
1
0
1
0
1

2
0
0
1
1
0
0
1
1

Baud Rate
3
0
0
0
0
1
1
1
1

300
600
1200
2400
4800
9600
19200
1152002.2.2 Extender488 Serial Data Format Selection
The Extender488 is configurable for two data formats. The first, factory default, format is 8
data bits, 1 stop bit with odd parity. The number of data bits transmitted can be set to 7 by setting
S102 switch 4 to the closed position. Even though the seven bit format is chosen, binary data canstill be transferred over the Extender488's. There will, h owever, be a slight degradation in datatransfer speeds with this format.

2.3

Section 2

Getting Started
Extender488 Data Format Selection0

1 2 3 41

1 2 3 4

OPEN

OPEN

8 data Bits

7 data Bits

Switch
Side
View

DOT

Note: Factory default is 8 data bits.

2.2.3 Extender488 Serial Port Cabling
The Extender488 is shipped from the factory for RS-422 transmission medium, whichprescribes differential receivers for high noise immunity. T o insure signal integrity, only use
shielded cable for the serial port connections. If you are not using a cable provided by the factory,
below is the appropriate connections for proper operation.
Local Extender
SignalPin #Receive Data Receive Data +Ground (shield)
Transmit Data Transmit Data +Remote ExtenderPin #Signal

1
2
5
6
7

6
7
5
1
2

2.4

Transmit Data Transmit Data +Ground (shield)
Receive Data Receive Data +Section 2

Getting Started

2.3 Extender488/F CONFIGURATION
The following describes the configuration necessary for the fiber optic Extender488/Fs. If you areusing the RS-422 extenders, refer to Section 2.2.
Two Extender488/Fs are required to accomplish bus extension, with one connected directly to the
LOCAL IEEE bus, and the other connected to the REMOTE IEEE bus.
Both local and remote extenders are identical. At power up, each extender monitors the Attention(ATN) and Remote Enable (REN) lines. The Error LED on each extender will blink indicating that ATN or
REN has not yet been detected and that communication with the other extender has not yet been
established. The first extender to detect either of t hese lines asserted declares itself the LOCAL extenderand commands the other extender to declare itself the REMOTE extender. The Error LEDs will now stop
blinking. From this moment on, the LOCAL extender will monitor all local bus activity and transmit thisactivity to the REMOTE extender. All activity detected by the REMOTE extender is also communicatedback to the LOCAL extender.

WARNING
There are no user serviceable parts or us er selectable parameters inside theExtender488/F case. Refer all servicing to qualified repair personnel. R efer to
Section 4 for additional information.

2.3.1 Extender488/F Fiber Optic Connections
The Extender488/F requires duplex fiber optic cabling. Most cable manufacturers mark thetwo connectors at each end of t he duplex cable with a " T" and an " R" to distinguish which toconnect to the transmitter (T) and which to c onnect to the receiver (R). Connect the fiber optic
cables as shown below. The Transmitter of #1 is connected to the receiver of #2 and visa verse.

2.5Section 2

Getting Started

R
Extender488/F#1
TT
Extender488/F#2
R

There are several recommended precautions you should take to ensure years of trouble free
operation.
1.

Treat the fiber optic transmitter, receiver and cabling ends as you would treat an expensive
camera lens. Good system performance requires clean port optics and cable ferrules to avoid
obstructing the optical path.

2.

Clean compressed air is o ften all that's needed to remove dirt particles from the optic
transmitters and receivers. Sta ndard lens cleaning tissues or cotton swabs soaked withmethanol or Freon also work well for cleaning cable ferrules. Take care to avoid splashingmethanol or Freon on case parts.

3.

When not in us e, replace Extender488/F optical transmitter and receiver protection caps.
Also ensure cable ferrule protection caps are also replaced.

4.

When pulling optic cable through conduits of walls, NEVER u se the connectors as a lin k to
the pulling lead. Make sure that the cable is loose in the conduit after installation. If thecable is pulled tight on a corner, transmission loss may be increased significantly.

5.

When seating connectors in the transmitter and receiver, some rotation of the connector may
be necessary to achieve satisfactory transmission.

2.6

Section 2

Getting Started

2.4 OPERATION
After configuring the Extender488s attaching the serial port or fiber optic cabling, plug the power
supply connector into the rear jack on the interface.
CAUTION
Never install the power s upply into the interface while it is connected to AC line
power. Failure to observe this caution may result in damage to the Extender488.
Only a supply with regulated +5 volt output should be used.
WARNING
The power supply provided with the interface is int ended for INDOOR USE
ONLY. Failure to observe this warning could res ult in equi pment failure,personal injury or death.
After installing the power s upply connector into the interface, plug the power supply into the ACline power. Place the rear panel power switch in the ON [1] position. All front panel indicators should
light for a pproximately one second while the Extender488 performs an i nternal ROM and R AM self
check. At the end of the power-on sequence, the ERROR LED should continue to blink, indicating that
communications with another extender has not been established. Now repeat the above procedure with thesecond extender, and observe the same results. If any of the following LED conditions exist after poweron, a failure has occurred.
SYMPTOM
All lights remain on
All lights blink continually
No LEDs blink

2.7

FAILUREROM test has failed
RAM test has failed
Power supply has failed

Section 2

Getting Started

If any of the symptoms shown occur, cycle the power switch on the Extender488 to be sure of the
problem. If the problem is unresolved, refer to the Service Information section of this manual.
WARNINGThe Extender488 makes its earth ground connection through the I EEEinterface cable. Each extender must be connected to at least one IEEE device whichis earth ground referred. Failure to do s o may allow the Extender488 to float to a
voltage away from ground. This co uld result in da mage to the interface, personal
injury or death.
If proper operation is obtained, turn-off the Extender488 power switch and connect the other IEEE
devices to each of the Extender488's IEEE ports. Connect the local extender to the remote extender via theserial or fi ber optic cable. Apply power to all devices in the system. If the Extender488 ERROR LED
continues to blink after the power-on sequence, this indicates one of two possible conditions: either thesystem controller has not yet asserted the Attention line, or communication with the mating extender has
not yet been established. B e sure both extenders are powered-on, the cabling has been installed, and thecontroller has asserted Attention before assuming there is a problem.
Once all IEEE devices have been connected and powered on, the Extender488 will allo w thesystem controller to co mmand up to 13 IEEE devices on its local b us (in ad dition to itself and t he local
extender), and up to 14 IEEE devices on the remote bus (in addition to the remote extender). The extenders
have no address of their own, and therefore will operate completely transparent to the system (with the
exception of parallel poll). Be careful not to have two IEEE devices with the same address connected toeither remote or local buses. Failure to do so will result in the bus 'locking up' when one of the devices isaccessed.

2.8

Section 2

Getting Started

2.5 PARALLEL POLL RESPONSE TIMESThe Parallel Poll function is the only extended bus transaction which is not completely transparent
to your system. This is because the s peed at w hich data is transferred serially is slower than the IEEE
specification for Parallel Poll response. Consequently, it is n ecessary for the controller to perform two
Parallel Polls.
When the local extender detects the parallel poll, it outp uts the mos t recent Parallel Poll
information to the controller on the local bus. This data should be discarded by the controller because it is
not current. After receiving a Parallel Poll, the local exten der commands the remote extender to perform aParallel Poll, and returns the remote Parallel Poll data byte to the local extender. The bus controller should
now perform another Parallel Poll, and use this data as th e valid Parallel Poll byte. T he following table
describes the minimum times the controller should wait b etween consecutive Parallel Polls in order toassure valid data on the second poll.
Extender488 Parallel Poll Response Times
Baud Rate

Delay (typ)1152001 msec19200

3 msec9600

8 msec4800

13 msec

2400

24 msec

1200

48 msec

600

92 msec

300

200 msec

The minimum time the controller should wait with the Extender488/F between consecutive
Parallel Polls is 1 millisecond.

2.9

Section 3

IEEE 488 Primer

IEEE 488 Primer
3.1 HISTORYThe IEEE 488 bus is an instrumentation communication bus adopted by the Institute of Electricaland Electronic Engineers in 1975 and revised in 1978. The Extender488 and the Extender488/F conformto this most recent revision designated IEEE 488-1978.
Prior to the adoption of t his standard, most instrumentation manufacturers offered their own
versions of computer interfaces. This placed the burden of system hardware design on the end user. If his
application required the products of several different manufacturers, then he might need to design several
different hardware and software interfaces. The popularity of the IEEE 488 interface (sometimes called
the General Purpose Interface Bus or GPIB) is due to the t otal specification of the electrical andmechanical interface as well as the data tra nsfer and control protocols. The use of the IEEE 488 standard
has moved the responsibility of the user from design of the interface to design of the high level softwarethat is specific to the measurement application.
3.2 GENERAL STRUCTURE
The main purpose of the GPIB is to transfer information between two or more devices. A device
can either be an instrument or a computer. B efore any i nformation transfer can take place, it is firstnecessary to specify which will d o the talking (send data) and which devices will be allowed to listen(receive data). The decision of who will talk and who will listen usually falls on the System Controller
which is, at power on, the Active Controller.
The System Controller is similar to a committee chairman. On a well r un committee, only one
person may speak at a time and the chairman is responsible for recognizing members and allowing them tohave their say. On the bus, the device which is recognized to speak is the Active Talker. There can only
be one Talker at a tim e if the information transferred is to be clearly understood by all. The act of "giving
the floor" t o that de vice is called Addressing to Talk. If the committee chairman ca n not atten d themeeting, or if other matters require his attention, he can appoint an acting chairman to take control of theproceedings. For the GPIB, this device becomes the Active Controller.
At a committee meeting, everyone present usually listens. This is not the case with the GPIB. The
Active Controller selects which devices will listen and commands all other devices to ignore what is beingtransmitted. A device is instructed to listen by being Addressed to Listen. This device is then referred toas an Active Listener. Devices which are to ignore the data message are instructed to Unlisten.
The reason some devices are instructed to Unlisten is quite simple. Suppose a college instructor ispresenting the day's lesson. Each s tudent is told to rais e their hand if the instructor has exceeded their

3-1Section 3

IEEE 488 Primer

ability to keep up while taking notes. I f a hand is r aised, the instructor stops his discussion to allow theslower students the time to catch u p. I n this way, the instructor is certain that each and every student
receives all the informatio n he is trying to present. Since ther e are a l ot of students in the classroom, thisexchange of information can be very slow. In fact, the rate of information transfer is no faster than the rate
at which the slowest note-taker can keep up. The instructor, though, may have a message for one particular
student. The i nstructor tells the rest of the class to ignore this message (Unlisten) and tells it to that one
student at a rate which he can understand. Thi s information transfer can then happen much quicker,
because it need not wait for the slowest student.
The GPIB transfers information in a sim ilar way.
handshaking. More on this later.

This method of data transfer is called

For data transfer on the IEEE 488, the Active Controller mustÉ
a)
b)
c)
d)

Unlisten all devices to protect against eavesdroppers.
Designate who will talk by addressing a device to talk.
Designate all the devices who are to listen by addressing thosedevices to listen.
Indicate to all devices that the data transfer can take place.

3-2

Section 3

IEEE 488 Primer

To Other Devices

Device 1
System Controller
Able to Talk,
Listen, and Control
Data Bus
Device 2
DMM
Able to Talk
and Listen

Data Byte
Transfer
Control

Device 3
Printer
Only Able to Listen

GeneralInterfaceManagement

Device 4
Frequency Counter
Only Able to Talk}

DIO1-8

DAV
NRFD
NDAC

IEEE 488 Bus Structure

IFC
ATN
SRQ
REN
EOI

Figure 3.1

3-3Section 3

IEEE 488 Primer

3.3 SEND IT TO MY ADDRESSIn the previous discussion, the terms Addressed to Talk and Addressed to Listen were u sed.
These terms require some clarification.
The IEEE 488 standard permits up to 15 devices to be configured within one system. Each of thesedevices must have a unique address to avoid confusion. In a similar fashion, every building in town has aunique address to prevent one home from receiving another home's mail. E xactly how eac h device's
address is set is specific to the product's manufacturer. Some are set by DIP switches in hardware, othersby software. Consult the manufacturer's instructions to determine how to set the address.
Addresses are sent with universal (multiline) commands from the Active Controller. Thes ecommands include My Listen Address (MLA), My Talk Address (MTA), Talk Address Group (TAG),
and Listen Address Group (LAG).
3.4 BUS MANAGEMENT LINES
Five hardware lines on the GPIB are us ed for bus management. Signals on these lines are often
referred to as uniline (single line) commands. The signals are active low, i.e . a low v oltage represents alogic "1" (asserted), and a high voltage represents a logic "0" (unasserted).
3.4.1 Attention (ATN)
ATN is one of the most important lines for bus management. If Attention is asserted, then theinformation contained on the data lines is to be interpreted as a m ultiline command. If it is n ot, thenthat information is to be interpreted as data for the Active Listeners. The Active Controller is the
only bus device that has control of this line.

3-4Section 3

IEEE 488 Primer

3.4.2 Interface Clear (IFC)The IFC line is used only by the System Controller. It is used to place all bus devices in a
known state. Although device configurations vary, the IFC command usually places the devices in
the Talk and Listen Idle states (neither Active Talker nor Active Listener).
3.4.3 Remote Enable (REN)
When the System Controller sends the REN command, bus devices will res pond to remote
operation. Generally, the REN command should be issued before any bus programming is attempted.
Only the System Controller has control of the Remote Enable line.
3.4.4 End or Identify (EOI)The EOI line is used to s ignal the last byte of a multibyte data transfer. The device that issending the data asserts EOI during the transfer of the last data byte. The EOI signal is not always
necessary as the end of the data may be indicated by some special character such as carriage return.
The Active Controller also uses EOI to perform a Parallel Poll by simultaneously assertingEOI and ATN.
3.4.5 Service Request (SRQ)
When a device des ires the immediate attention of the Active Controller it asserts SRQ. It isthen the Controller's responsibility to determine which device requested service. This isaccomplished with a Serial Poll or a Parallel Poll.

3-5

Section 3

IEEE 488 Primer

3.5 HANDSHAKE LINESThe GPIB uses three handshake lines in an "I'm ready - Here's the data - I've got it" sequence. This
handshake protocol assures reliable data transfer, at the rate determined by the slowest Listener. One line iscontrolled by the Talker, while the other two are shared by all Active Listeners. The handshake lines, likethe other IEEE 488 lines, are active low.
3.5.1 Data Valid (DAV)The DAV line is controlled by the Talker. The Talker verifies that NDAC is asserted (activelow) which indicates that all Lis teners have accepted the previous data byte tra nsferred. The Talker
then outputs data on the bus and waits until NRFD is unasserted (high) which i ndicates that all
Addressed Listeners are ready to accept the information. When NRFD and NDAC are in the properstate, the Talker asserts DAV ( active low) to indicate that the data on the bus is valid.
3.5.2 Not Ready for Data (NRFD)
This line is u sed by the Listeners to inform the Talker when they a re ready to acce pt new
data. The Talker must wait for each Listener to unassert this line (high) which they will do at theirown rate when they are ready for more data. This assures that all devices that are to accept the
information are ready to receive it.
3.5.3 Not Data Accepted (NDAC)
The NDAC line is also controlled by the Listeners. This line indicates to the Talker that eachdevice addressed to lis ten has accepted the information. Each device releases NDAC (high) at its
own rate, but the NDAC will not go high until the slowest Listener has accepted the data byte.

3-6Section 3

IEEE 488 Primer

1st Data Byte

2nd Data Byte

DIO1-8(composite)
DAV

Valid

Not
Valid

SourceNRFD
Acceptor
NDAC
Acceptor

All
Ready
None
Accept

None
Ready
All
Accept

Valid

All
Ready

Not
Valid

None
Ready

None
AcceptAll
AcceptIEEE Bus Handshaking
3.6 DATA LINESThe GPIB provides eight data lines for a bit parallel/byte serial data transfer. These eight data lines
use the convention of DIO1 through DIO8 instead of the binary designation of D0 to D7. The data lines arebidirectional and are active low.
3.7 MULTILINE COMMANDS
Multiline (bus) commands are sent by the Active Controller over the data bus with ATN asserted.
These commands include addressing commands for talk, listen, Untalk and Unlisten.
3.7.1 Go To Local (GTL)
This command allows the selected devices to be manually controlled. ($01)

3-7Section 3

IEEE 488 Primer

3.7.2 Listen Address Group (LAG)
There are 31 (0 to 30) listen addresses associated with this group. The 3 most significant bitsof the data bus are set to 001 while the 5 least significant bits are the address of the device being told
to listen.
3.7.3 Unlisten (UNL)
This command tells all bus devices to Unlisten. The same as Unaddressed to Listen. ($3F)
3.7.4 Talk Address Group (TAG)
There are 31 (0 to 30) talk addresses associated with this group. The 3 most significant bits ofthe data bus are set to 010 while the 5 least significant bits are the address of the device being told totalk.
3.7.5 Untalk (UNT)This command tells bus devices to Untalk. The same as Unaddressed to Talk. ($5F)
3.7.6 Local Lockout (LLO)
Issuing the LLO command prevents manual control of the instrument's functions. ($11)
3.7.7 Device Clear (DCL)
This command causes all bus devices to be initialized to a pre-defined or power up state. ($14)

3-8Section 3

IEEE 488 Primer

3.7.8 Selected Device Clear (SDC)
This causes a single device to be initialized to a pre-defined or power up state. ($04)
3.7.9 Serial Poll Disable (SPD)The SPD command disables all devices from sending their Serial Poll status byte. ($19)
3.7.10 Serial Poll Enable (SPE)A device which is Addressed to Talk will output its Serial Poll status byte after SPE is sent and
ATN is unasserted. ($18)
3.7.11 Group Execute Trigger (GET)This command usually signals a group of devices to begin executing a triggered action. This
allows actions of different devices to begin simultaneously. ($08)
3.7.12 Take Control (TCT)This command passes bus control responsibilities from the current Controller to anotherdevice which has the ability to control. ($09)
3.7.13 Secondary Command Group (SCG)
These are any one of the 32 possible c ommands (0 t o 31) in this group.
immediately follow a talk or listen address. ($60 to $7F)

3-9

They must

Section 3

IEEE 488 Primer

3.7.14 Parallel Poll Configure (PPC)This configures devices capable of performing a Parallel Poll as to which data bit they are toassert in response to a Parallel Poll. ($05)
3.7.15 Parallel Poll Unconfigure (PPU)This disables all devices from responding to a Parallel Poll. ($15)
3.8 MORE ON SERVICE REQUESTS
Most of the commands covered, both uniline and multiline, are the responsibility of the Active
Controller to send and the bus devices to recognize. Most of these happen routinely by the interface andare totally transparent to the system programmer. Other commands are used directly by the user to provide
optimum system control. Of the uniline commands, SRQ is v ery important to the test sy stem and thesoftware designer has easy access to this line by most devices. Service Request is the method b y which abus device can signal to the Controller that an event has occurred. I t is sim ilar to an interrupt in a
microprocessor based system.
Most intelligent bus peripherals have the ability to assert SRQ. A DMM m ight assert it when its
measurement is complete, if its input is overloaded or for any of an assortment of reasons. A power supply
might SRQ if its output has current limited. This is a powerful bus feature that removes the burden fromthe System Controller to periodically inquire, "Are you done yet?". Instead, the Controller says, "Do
what I told you to do and let me know when you're done" or "Tell me when something is wrong."
Since SRQ is a single line command, there is no way for the Controller to determine which device
requested the service without additional information. This information is p rovided by the multiline
commands for Serial Poll and Parallel Poll.

3-10

Section 3

IEEE 488 Primer

3.8.1 Serial Poll
Suppose the Controller receives a service request. For this example, let's assume there areseveral devices which could assert SRQ. T he Controller issues an SPE (Serial Poll enable)command to each device sequentially. If any device responds with DIO7 asserted it indicates to theController that it was the device that asserted SRQ. Often times the other bits will in dicate why
the device wanted service. This Serial Polling sequence, and any resulting action, is under controlof the software designer.
3.8.2 Parallel Poll
The Parallel Poll is another way the Controller can determine which device requested
service. It provides the who but not necessarily the why. When bus devices are c onfigured for
Parallel Poll, they are assigned one bit on the data bus for their response. By using the Status bit,the logic level of the response can be programmed to allow logical OR/AND conditions on one dataline by mo re than one device. When SRQ is asserted, the Controller (under user's software)
conducts a Parallel Poll. The Controller must then a nalyze the ei ght bits of data received todetermine the source of the request. Once the source is determined, a Serial Poll might be used todetermine the why.
Of the two polling types, the Serial Poll is the most popular due to its ability to determine the who
and why. In addition, most devices support Serial Poll only.

3-11

Section 4

Service Information

SERVICE INFORMATION
4.1 Factory Service
Measurement Computing maintains a factory service center in Norton, Massachusetts. If problems
are encountered in using the Extender488, you should first telephone the factory. Many problems can be resolved
by discussing the problem with our applications department. If the problem cannot be solved by this method,
you will be instructed as to the proper return procedure.
4.2 Theory of Operation
At the heart of t he Extender488 is a 6809 microprocessor [U101] supported by 8K bytes offirmware EPROM [U102 (2764)] and 8K bytes of static RAM [U103 (6264)]. A counter [U105 (65C22)]
contained in the VIA is used to generate real time interrupts for the firmware operating system. The front
panel annunciators are driven by the VIA through an inverter/driver [U113 (74LS04)].
Serial communication to the mating Extender488 is accomplished by a UART [U105] thru the RS422 receiver [U112] and driver [U111]. For the Extender488/F it is accomplished by the UART and fiber
optic transmitter [U202] and receiver [U203].
The IEEE 488 bus is monitored by a 9914A IEEE bus device [U106]. Circuitry surrounding the9914A (U115 and U116) enables the device to monitor all bus activity, and to initiate bus activity whenrequired.
Decoding of the microprocessor address space is accomplished with a pr ogrammed PAL [U11 0(16L8)]. Below is the memory space allocation.
Address$6000-$7FFF$A000-$A007$A800-$A803$B000-$B00F$E000-$FFFF

Device
U103
U106
U105
U104
U102

4.1

Part Number
6264
9914A
6551
65C22
2764

Function
Static RAM
IEEE Controller
UART
VIA
Programmed EPROM

Section 4

Service Information

4.5 Extender488 and Extender488/F PARTS LIST
Designation
Part Numbers
C101
C-5-.1
C102
C-5-.1
C103
C-5-.1
C104
C-5-.1
C105
C-5-.1
C106
C-5-.1
C107
C-5-.1
C108
C-5-.1
C110
C-5-.1
C111*
C-5-.1
C112*
C-5-.1
C113
C-5-.1
C115
C-5-.1
C116
C-5-.1
C117
C-5-15p
C118
C-5-15p
C123
C-5-1
C124
C-2-10C201**
C-2-10C202**
C-5-.1
C203**
C-5-.1
D101
RF-1
D102
DD-2
D103
DD-2
D104
DD-2
D105
DD-2
D106
DD-2
J101
CN-2
J102*
CN-7-9J103
CN-11
J104**
CN-5-13
J201**
CA-6

4.4

Description
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
15pF Ceramic, 25v
15pF Ceramic, 25v
1µF Ceramic, 25v
10µF Electrolytic, 25v
10µF Electrolytic, 25v
0.1µF Ceramic, 25v
0.1µF Ceramic, 25v
Small Signal Diode
Red PC Mount
Red PC Mount
Red PC Mount
Red PC Mount
Red PC Mount
IEEE 488 Connector
PC Mount Male DB-9
9 Volt Power Jack13 x 2 0.1" Header
Cable AssemblySection 4

Service Information
Designation
U101
U102*
U102**
U103
U104
U105
U106
U107
U108
U110
U111*
U112*
U113

Part Numbers
IC-1
Extender488-600
Extender488/F-600
IC-41
IC-23
IC-16
IC-3
IC-4
IC-5
Macro488-601
IC-38
IC-36
IC-33

Description
MC68B09P Microprocessor
Programmed EPROM - 2764
Programmed EPROM - 2764
8k x 8 CMOS SRAM
65B22 Versitile Interface Adapter
R6551AP UARTTMS9914ANL IEEE Controller
SN75160BN IEEE Driver
SN75162BN IEEE Driver
Programmed 16L8A PAL
26LS30 RS-423 Driver
26LS33 RS-422 Receiver
74LS04 Hex Inverter

U115
U116
R101
R102
R103*

IC-51
Extender488-601
R-1-68K
RN-4-4.7K
RN-1-10K

74LS74 Dual D Flip Flop
Programmed 16L8A PAL
68K½, 1/4w carbon
4.7K½ x 7 SIP
10K½ x 9 SIP

R104
R107*
R201**
R202**
S101
S102*
Y101

RN-2-470
R-1-100
R-1-47R-1-1K
SW-8
SW-6-4CR-4* Used on Extender488 only** Used on Extender488/F only

4.5

470½ x 5 SIP
100½, 1/4w carbon
47½, 1/4w carbon
1K½, 1/4w carbon
DPST Rocker - power switch
4 Pole DIP
7.3728 MHz Crystal

Appendix A

Character Codes And IEEE Multiline Messages

A.1