Isolator488

IEEE 488 Bus Isolator

USER’S MANUAL

IOtech, Inc. • 25971 Cannon Road • Cleveland, Ohio 44146 • (440) 439-4091

Warranty Information
Your IOtech warranty is as stated on the product warranty card. You may contact IOtech by phone,
fax machine, or e-mail in regard to warranty-related issues.
Phone: (440) 439-4091, fax: (440) 439-4093, e-mail: sales@iotech.com

Limitation of Liability
IOtech, Inc. cannot be held liable for any damages resulting from the use or misuse of this product.

Copyright, Trademark, and Licensing Notice
All IOtech documentation, software, and hardware are copyright with all rights reserved. No part of this product may becopied, reproduced or transmitted by any mechanical, photographic, electronic, or other method without IOtech’s prior writtenconsent. IOtech product names are trademarked; other product names, as applicable, are trademarks of their respective
holders. All supplied IOtech software (including miscellaneous support files, drivers, and sample programs) may only be usedon one installation. You may make archival backup copies.

FCC Statement
IOtech devices emit radio frequency energy in levels compliant with Federal Communications Commission rules (Part 15)
for Class A devices. If necessary, refer to the FCC booklet How To Identify and Resolve Radio-TV Interference Problems(stock # 004-000-00345-4) which is available from the U.S. Government Printing Office, Washington, D.C. 20402.

CE Notice
Many IOtech products carry the CE marker indicating they comply with the safety and emissions standards of theEuropean Community. As applicable, we ship these products with a Declaration of Conformity stating which
specifications and operating conditions apply.

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 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.

Specifications and Calibration
Specifications are subject to change without notice. Significant changes will be addressed in an addendum or revision to themanual. As applicable, IOtech calibrates its hardware 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.

Quality Notice
IOtech has maintained ISO 9001 certification since 1996. Prior to shipment, we thoroughly test our products andreview our documentation to assure the highest quality in all aspects. In a spirit of continuous improvement, IOtech
welcomes your suggestions.

Isolator488
Instruction ManualPart No. Isolator488-901
Revision 1.2
December 1991

IOtech, Inc.  25971 Cannon Road  Cleveland, Ohio 44146  (440) 439-4091

IMPORTANT!
READ THIS BEFORE INSTALLATION!
WARNINGThe enclosure of the Isolator488 Bus Isolator isinternally connected to earth ground through thepower cord. However, the shield of the IEEE 488
cables attached to the Isolator488 and signal groundpins may be at a high voltage with respect to earth
ground. Be sure to remove power from all devices inyour system before handling IEEE 488 cables
connected to the Isolator488. Failure to follow thiswarning may result in personal injury or death.

IOtech, Inc. • 25971 Cannon Road • Cleveland, Ohio 44146 • (440) 439-4091

Table of Contents
Section 1
1.1
1.2
1.3
1.4

INTRODUCTION
General Description
Available Accessories
Specifications
Abbreviations

Page
1.1
1.1
1.2
1.3

Section 2
2.1
2.2
2.3
2.4
2.5
2.6

GETTING STARTED
Inspection
Configuration
Modifications
Line Voltage Selection
Hardware Installations
Operation
BusLoading
Cabling Length
Signal Line Direction
Parallel Polling
Passing Control

Page
2.1
2.1
2.4
2.7
2.9
2.10
2.11
2.11
2.12
2.13
2.13

IEEE 488 Primer
History
General Structure
Send It To My Address
Bus Management Lines
Attention (ATN)Interface Clear (IFC)
Remote Enable (REN)End Or Identify (EOI)
Service Request (SRQ)
Handshake Lines
Data Valid (DAV)
Not Ready For Data (NRFD)
Not Data Accepted (NDAC)
Data Lines
Multiline Commands

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

2.6.1
2.6.2
2.6.3
2.6.4
2.6.5
Section 3
3.1
3.2
3.3
3.4
3.4.
3.4.2
3.4.3
3.4.4
3.4.
3.5
3.5.1
3.5.2
3.5.3
3.6
3.7

Section 3 (cont.)
3.7.1
3.7.2
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

Go To Local (GTL)
Listen Address Group (LAG)
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
3.9
3.9
3.9
3.9
3.9
3.9
3.10
3.10
3.10
3.10
3.10
3.10
3.11
3.11
3.11
3.12
3.12

Section 4
4.1

SERVICE INFORMATION
Factory Service

Page
4.1

Appendix A

Character Codes and IEEE Multiline Messages

Section 1

Introduction

Introduction
1.1 GENERAL DESCRIPTION
The Isolator488 increases the number of IEEE instruments and devices allowable
on the bus from 15, including the controller, to 28 while also providing opticalisolation between the system controller and devices on the isolated bus. Since the
Isolator488 does not occupy a bus address, its operation is entirely transparent to theIEEE controller. The IEEE standard defines 31 valid primary bus addresses.
Therefore, no special software or bank switching is necessary to access the additional
devices allowed by the Isolator488. The Isolator488 and has no effect on bus datatransfer rates.
Two IEEE ports are provided. One connects directly to the controller's IEEE bus,
the other to an additional 14 instruments, printers, plotters and specialized peripherals
which may need to be optically isolated from the system controller.
1.2 AVAILABLE ACCESSORIES
Additional accessories that can be ordered for the Isolator488 include:123-0900
CA-7-1
CA-7-2CA-7-3CA-7-4
CN-20
CN-21
CN-22
CN-23

Instruction Manual
1.5 foot IEEE 488 cable
6 foot IEEE 488 cable
6 foot Shielded IEEE 488 cable
Reverse entry 6 foot IEEE 488 cable
Right Angle IEEE 488 adapter, male and female
IEEE 488 connector Extender, male and female
IEEE 488 multi-tap bus strip, four female
connectors wired in parallel.
IEEE 488 panel mount feed-through connector,
male and female

1.1Section 1

Introduction

1.3 SPECIFICATIONSPROPAGATION DELAYS:
Local to Remote Talker
Data Lines
Command Lines
Remote to Local Talker/ATN to Local Data
/ATN to Local DAV
Data Lines
Command Lines
Parallel Poll Response
SIGNAL LINE DIRECTIONS:
Data Lines:
DIO1 - DIO7
Handshake Lines:
DAV, NRFD, NDAC
Command Lines:
SRQ, ATN, REN, IFC
ATN
EOI
CONNECTORS:INDICATORS:POWER:ENVIRONMENT:DIMENSIONS:WEIGHT:
CONTROLS:SUPPLIED ACCESSORIES:ISOLATION:

85 nS typ.,
90 nS typ.,

120
125

nS Max.
nS Max.

355 nS typ.,155 nS typ.,85 nS typ.,
90 nS typ.,140 nS typ.,475 nS Max.
2075 nS Max.
120 nS Max.
125 nS Max.
205 nS Max.

Bidirectional
Bidirectional
To Local Bus
To Remote Bus
Bidirectional
Two IEEE 488 connectors with metric studs
LED for Power
105-125V or 210-250V,50, 60 Hz; 20 VA MAX.
0 to 50 °C; 0 to 70% RH
425 mm deep x 45 mm wide x 208 mm high.
(16.75 "x 1.75 "x 8.20 ")2.1 kg (4.6 lbs)
Power switch
Power cable and manual
IEEE 488 common on Controller Bus to IEEE 488
common on Instrument Bus:
1600 volts peak maximum @ sea level to 10,000 ft.
0 - 70° C, and 0 - 80% RH non-condensing OR
500 Vac maximum, 105V-Hz

Specifications are subject to change without notice.

1.2

Section 1

Introduction

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

IEEE bus address "n"Attention line
Controller Active
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
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

1.3

Section 2

Getting Started

GETTING STARTED
2.1 Inspection
The Isolator488 was carefully inspected, mechanically and electrically, prior to
shipment. When you receive the interface, carefully unpack all items from theshipping carton and check for any obvious signs of physical damage which may haveoccurred during shipment. Report any damage to the shipping agent immediately.
Remember to retain all shipping materials in the event that shipment back to thefactory becomes necessary.
Every Isolator488 is shipped with the following....
• Isolator488• 123-0900• 123-0800

IEEE Bus Isolator
Instruction Manual
Accessory Kit ( includes power cable,fuse, and rack mounting hardware)

2.2 Configuration
Before using the Isolator488, you should be aware of the connections
between the IEEE digital commons, the shield lines on each bus, and earth ground.
The following information is provided so that you can determine if the factory
configuration is appropriate for your application or if some changes need to bemade prior to use.
The following is a list of terms and their definitions as used in this manual:

Isolation

The ability of the Isolator488 to break the electricalconnection between the shields and digital commonsof its IEEE 488 ports while allowing the transfer of
data.

2.1

Section 2

Getting Started

Earth Ground

The Ground terminal on the AC power cable. This isinternally connected to the chassis of the Isolator488.

Controller Port

The IEE 488 port labeled CONTROLLER on the rear
panel of the Isolator488. The system
controller
must be connected to this port. Instruments may also beconnected to this port, but instruments placed in the
CONTROLLER port will not be isolated from thesystem controller.

Instrument Port

The IEEE488 port labeled INSTRUMENTS on therear panel of the Isolator488. Instruments may beconnected to this port. Instruments connected to theInstrument Port may be isolated from devices on theController Port depending on the internal configurationof the Isolator 488.

Controller Common The logic common connection on the Controller Port.
Pins 18, 19, 20, 21,22, 23, and 24 of the Controller
Port are all connected to Controller Common.
Instrument Common The logic common connection on the Instrument Port.
Pins 18, 19, 20, 21, 22, 23, and 24 of the Controller
Port are all connected to Instrument Common.
Controller Shield

The Shield connection (Pin 12) on the Controller Port.

Instrument Shield

The Shield connection (Pin 12) on the Instrument PortThe factory configuration of the Isolator488 is as follows:

2.2

Section 2

Getting Started

2.3

Section 2

Getting Started

As shown in the preceding diagram, the Isolator488 is shipped with a 'soft ground'consisting of a 1 MΩ resistor in parallel with a 0.01 µF capacitor between local
Common and Earth Ground. This configuration should be suitable for mostapplications. If you wish to make any changes to this configuration, follow thesteps given below.
2.3 Modifications
The Isolator488, as shipped from the factory, is configured for the typical useof the unit which is to isolate the instrument port from Earth ground and fromController common. The factory configuration should be used unless you have asignificant variation in your application beyond galvanic isolation devices on theInstrument port. If the factory configuration is not appropriate for yourapplication, the following procedures may be undertaken to modify the
Isolator488. It is necessary to open the enclosure to make any of the followingchanges. If yoou have any questions about a particular change, please contact theapplications department at (440) 439-4091.
1. Disconnect the power cord from the AC line and from the Isolator488.
Disconnect any IEEE cables prior to disassembly.
WARNING
Never open the Isolator488 case while it
is connected to the AC line. Internal
voltage potentials exist which could
cause personal injury or death.

2. Place the Isolator488 on a flat surface. Remove the six screws on top of thecase and remove the top cover. Located to the right of the Controller Port IEEE
488 connector are locations for C16, R4, and F2. Located to the right of theInstrument Port IEEE 488 connector are locations for C59, R5, and F3. Refer tothe following figure:

2.4

TO CONTROLLER

JPR1

F2

R4

C47

C48
C50

TO INSTRUMENTS

JPR2

C49

F3

Common Components

Common Components

C16

Instrument Port

Controller Port

Isolator488 Internal View

R5

C59

Section 2
Getting Started

2.5

Section 2

Getting Started

The following options are available on each port:
1. The Shield pin of the IEEE 488 connector (pin 12) may be connected to thelogic common of the IEEE 488 connector (pins 18, 19, 20, 21, 22, 23, and 24) orleft unconnected.
2. Logic common of IEEE 488 connector (pins 18, 19, 20, 21, 22, 23, and 24) may
be connected to the chassis of the Isolator488, connected to the chassis through asoft ground, or left unconnected.
The following changes may be made to the Controller Port:
1. To remove the Controller common from earth ground, remove R4, and C16 byeither desoldering the components (the bottom panel or the Isolator488 must be
removed in order to do this) or cutting the component leads so that they may beremoved. Make note of this change for later reference.
2. To connect Controller common to earth ground, insert a 1/2 amp fuse in the fuseclips labelled F2. The 1/2 amp fuse connects the Controller common to earth
ground and limits fault currents to 1/2 amp. Make note of this change for later
reference.
3. To connect pin 12 (shield) of the Controller Port to Controller Common, solder
a jumper wire in the location labeled JPR1. Make note of this change for later
reference.
The following changes may be made to the Instrument Port:
1. To add a soft ground between the Instrument Common and earth ground, insert
a 1 MΩ 1/4 watt resistor in the location labelled R5. Insert a 0.01 µF 2kv ceramic
capacitor in the location labelled C59. Solder these components in place from thesolder side of the circuit board using rosin core solder. The bottom panel or the
Isolator488 will need to be removed in order to do this. Make note of thesechanges for later reference.

2.6

Section 2

Getting Started

2. To connect the Instrument Common to earth ground, insert a 1/2 amp fuse in thefuse clips labelled F3. Make note of this change for later reference.
3. To connect pin 12 (shield) of the Instrument Port to Instrument Common, solder
a bare jumper wire in the location labeled JPR2. Make note of this change for later
reference.
After making any changes to the Isolator488 internal circuitry, be sure to
document your changes then carefully reassemble the unit.
2.4 Line Voltage Selection
The Isolator 488 is designed to accept either 105 to 125 volt or 210 to 250
volt, 50 or 60 Hz, AC power. Each unit, when shipped from the factory, is labeledwith its input voltage setting. If this setting is not appropriate, the internal voltage
selection witch must be changed. Failure to operate the Isolator488 fromappropriate power source may result in damage to the unit. To check or change thevoltage selection switch disconnect all cables from the Isolator488 and follow thesteps below.

Isolator Fuse and Line Voltage Switch Locations

115V

S2

WARNING :LETHAL VOLTAGES
MAY BE PRESENTS1

F1Power Connector

2.7Section 2

Getting Started

WARNING
Never open the Isolator488 case while it
is connected to the AC line. Internal
voltage potentials exist which could
cause personal injury or death.
1. Place the Isolator488 on a flat surface. Remove the six screws on top of thecase and remove the top cover. Located next to the rear panel power connector isswitch S2 and fuse F1.
2. Insert the tip of a small screwdriver into the slot of the switch and move theswitch to the left or right so that the desired line voltage selection appears on theswitch. Make note of the new setting for later reference.
CAUTION
Use the 1/2 amp fuse, FU-1-.5, for 115
volt operation. Use the 1/4 amp fuse,FU-1-.25, for 230 volt operation. Failure
to use the correct fuse could result indamage to the Isolator488.
3. Replace the 1/2 amp fuse, FU-1-.5 (for 115 volt operation), with the 1/4 Amp
fuse (for 230 volt operation), part number FU-1-.25, that is included in the
Isolator488 Accessory Kit.
4.

Carefully reassemble the unit.

WARNINGThe Isolator488 is intended for INDOORUSE ONLY. Failure to observe thiswarning could result in equipment
failure, personal injury or death.

2.8Section 2

Getting Started

2.5 Hardware Installation
Included with the Isolator488 Bus Isolator are accessories for rack or benchuse. If rack mount installation is required, install the two rack ears using theenclosed screws. These ears can be installed so either the front or the rear of theunit faces the front of the rack fixture.

View of Enclosure showing Rack Hardware Installation

Enclosure
Enclosure
Top or Bottom
Top View

Rack Ear
Screw (2 per ear)

If bench installation is required, the rubber feet can be installed on theunderside of the unit near each corner.
View of Enclosure Bottom Showing Feet Placement

Enclosure
EnclosureTop
View
BottomView

2.9

Section 2

Getting Started

2.6 Operation
To begin operating the Isolator488, plug the power supply into the rear panel
jack. Apply power to the Isolator488 by depressing the rear panel power switch.
The front panel POWER LED should turn on.
If the POWER indicator does not light, there may not be any power supplied tothe interface. In this event, check to make sure the AC power is supplied to the
Isolator488, and that the power cord is properly installed into the unit. If the problem
is unresolved, refer to the Service Information section (Section 4) of this manual.
If proper operation is obtained, turn off the Isolator488 and connect the otherIEEE 488 devices to each of the Isolator488 ports. Apply power to all devices in thesystem.
Once all IEEE devices have been connected and powered on, the Isolator488 willallow the system controller to command up to 13 IEEE devices on its Controller Port(in addition to itself and the Isolator488), and up to 14 IEEE devices on the Instrument
Port. The Isolator488 has no address of its own, and therefore will operate completelytransparent to the system. Be careful not to have two IEEE devices with the same
address connected to either ports. Failure to do so will result in the bus 'locking up'
when one of the devices is accessed.
2.6.1 Bus Loading
The IEEE 488 specification allows for a maximum of 15 bus loads.
The Isolator488 presents one bus load on each port. Therefore, anadditional 14 devices can be placed on each the CONTROLLER andINSTRUMENT ports. Because the IEEE controller presents one bus load,a total of 27 instruments, printers, plotters and other peripherals can beaccessed from a single IEEE controller.

2.10Section 2

Getting Started

2.6.2 Cabling Length
The IEEE 488 specification allows for cabling distance between bus
devices of 2 meters (approx. 7 feet). The total worst case distance from thetwo furthest devices is not allowed to exceed 20 meters (approx. 70 feet).
The addition of the Isolator488 does not increase this distance.

2.6.3 Signal Line Direction
The signal lines and their direction include....
Data Lines:
DIO1 - DIO7
Handshake Lines:
DAV
NRFD
NDAC

Bidirectional
Bidirectional
Bidirectional

Command Lines:
SRQ
ATN
EOI
REN
IFC

To Controller Bus
To Instrument Bus
Bidirectional
To Instrument Bus
To Instrument Bus

Bidirectional

The majority of the command lines have their signal direction fixed.
This forces bus control from the CONTROLLER bus.
When Attention (ATN) is asserted, the data lines, talker handshakeline (DAV) and the EOI line direction is forced from the CONTROLLER
to the INSTRUMENT port. The listener handshake lines (NRFD and
NDAC) are forced from the INSTRUMENT to the CONTROLLER port.
The commands are sent through the Isolator488 to the INSTRUMENT
port.

2.11

Section 2

Getting Started

When ATN is unasserted, the Isolator488 asserts NDAC on both ports,
for a minimum of 200 nanoseconds, while looking for an active talker. If
the talker is detected on the CONTROLLER port, the NDAC lines are
released and the data and handshake line directions remain unchanged. If
the active talker is detected on the INSTRUMENT port, the data line
direction is force to the CONTROLLER port and the handshake line
directions are reversed. The Isolator488 delays a minimum of 1200
nanoseconds to allow the data lines to settle before releasing the NDAC
lines. Once changed, data transfers to 1 megabyte per second are possible.
2.6.4 Parallel Polling
During data transfer operations, the data line transceivers are operated
in tri-state. When a parallel poll is detected, the CONTROLLER port
transceiver is changed to open-collector and the data line direction is forced
from the Instrument Port to the Controller Port. Parallel polling
propagation delay through the Isolator488 is typically 85 nanoseconds.
2.6.5 Passing Control
If another device is included in the system which can receive control,
it must be placed on the CONTROLLER port. IEEE 488 control can not bepassed through the Isolator488.

2.12

Section 3

IEEE 488 Primer

IEEE 488 Primer
3.1 HISTORYThe IEEE 488 bus is an instrumentation communication bus adopted
by the Institute of Electrical and Electronic Engineers in 1975 and revised in
1978. The Isolator488 conforms to this most recent revision designatedIEEE 488-1978.
Prior to the adoption of this 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 hemight 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 total specification of theelectrical and mechanical interface as well as the data transfer 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 software thatis 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.
Before any information transfer can take place, it is first necessary to specify
which will do the talking (send data) and which devices will be allowed tolisten (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 wellrun committee, only one person may speak at a time and the chairman is
responsible for recognizing members and allowing them to have their say.
On the bus, the device which is recognized to speak is the Active Talker.
There can only be one Talker at a time if the information transferred is to beclearly understood by all. The act of "giving the floor" to that device iscalled Addressing to Talk. If the committee chairman can not attend themeeting, or if other matters require his attention, he can appoint an acting
chairman to take control of the proceedings. For the GPIB, this device
becomes the Active Controller.
3-1Section 3

IEEE 488 Primer

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 being transmitted. A
device is instructed to listen by being Addressed to Listen. This device isthen referred to as 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 is presenting the day's lesson. Each student istold to raise their hand if the instructor has exceeded their ability to keep up
while taking notes. If a hand is raised, the instructor stops his discussion toallow the slower students the time to catch up. In this way, the instructor iscertain that each and every student receives all the information he is trying topresent. Since there are a lot of students in the classroom, this exchange of
information can be very slow. In fact, the rate of information transfer is nofaster than the rate at which the slowest note-taker can keep up. The
instructor, though, may have a message for one particular student. The
instructor tells the rest of the class to ignore this message (Unlisten) andtells it to that one student at a rate which he can understand. This
information transfer can then happen much quicker, because it need not wait
for the slowest student.
The GPIB transfers information in a similar way. This method of datatransfer is called handshaking. More on this later.
For data transfer on the IEEE 488, the Active Controller must …
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
Data Byte
Transfer
Control

Able to Talk
and Listen

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 andAddressed to Listen were used. These terms require some clarification.
The IEEE 488 standard permits up to 15 devices to be configured
within one system. Each of these devices must have a unique address to
avoid confusion. In a similar fashion, every building in town has a uniqueaddress to prevent one home from receiving another home's mail. Exactly
how each device's address is set is specific to the product's manufacturer.
Some are set by DIP switches in hardware, others by software. Consult the
manufacturer's instructions to determine how to set the address.
Addresses are sent with universal (multiline) commands from theActive Controller. These commands 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 used 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 voltage represents a logic"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 the information contained on the data lines is
to be interpreted as a multiline command. If it is not, then that
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 toplace 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 respond to remote operation. Generally, the REN commandshould 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 signal the last byte of a multibyte datatransfer. The device that is sending the data asserts EOI during thetransfer 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 bysimultaneously asserting EOI and ATN.
3.4.5 Service Request (SRQ)
When a device desires the immediate attention of the Active
Controller it asserts SRQ. It is then the Controller's responsibility todetermine which device requested service. This is accomplished 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 datatransfer, 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, like the other IEEE 488 lines, are activelow.
3.5.1 Data Valid (DAV)The DAV line is controlled by the Talker. The Talker verifies that
NDAC is asserted (active low) which indicates that all Listeners haveaccepted the previous data byte transferred. The Talker then outputs
data on the bus and waits until NRFD is unasserted (high) whichindicates that all Addressed Listeners are ready to accept the
information. When NRFD and NDAC are in the proper state, 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 used by the Listeners to inform the Talker when they
are ready to accept new data. The Talker must wait for each Listenerto unassert this line (high) which they will do at their own rate when
they are ready for more data. This assures that all devices that are toaccept 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 each device addressed to listen 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 thedata byte.

3-6Section 3

IEEE 488 Primer

1st Data Byte

2nd Data Byte

DIO1-8(composite)
DAV

Valid

Not
Valid

SourceNRFD
Acceptor

All
Ready

NDAC
Acceptor

None
Accept

None
Ready
All
Accept

Valid

All
Ready
None
Accept

Not
Valid

None
Ready
All
AcceptIEEE Bus Handshaking
3.6 DATA LINESThe GPIB provides eight data lines for a bit parallel/byte serial datatransfer. 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 thedata 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 manuallycontrolled. ($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 bits of the data bus are set to 001 while the 5 leastsignificant bits are the address of the device being told to listen.
3.7.3 Unlisten (UNL)
This command tells all bus devices to Unlisten.
Unaddressed to Listen. ($3F)

The same as

3.7.4 Talk Address Group (TAG)
There are 31 (0 to 30) talk addresses associated with this group.
The 3 most significant bits of the data bus are set to 010 while the 5 leastsignificant bits are the address of the device being told to talk.
3.7.5 Untalk (UNT)This command tells bus devices to Untalk.
Unaddressed to Talk. ($5F)

The same as

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 predefined 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 Pollstatus 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 tobegin simultaneously. ($08)
3.7.12 Take Control (TCT)This command passes bus control responsibilities from the current
Controller to another device which has the ability to control. ($09)
3.7.13 Secondary Command Group (SCG)
These are any one of the 32 possible commands (0 to 31) in thisgroup. They must immediately follow a talk or listen address. ($60 to
$7F)

3-9

Section 3

IEEE 488 Primer

3.7.14 Parallel Poll Configure (PPC)This configures devices capable of performing a Parallel Poll as towhich data bit they are to assert 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 theresponsibility of the Active Controller to send and the bus devices torecognize. Most of these happen routinely by the interface and are totallytransparent to the system programmer. Other commands are used directlyby the user to provide optimum system control. Of the uniline commands,
SRQ is very important to the test system and the software designer has easy
access to this line by most devices. Service Request is the method by whicha bus device can signal to the Controller that an event has occurred. It issimilar to an interrupt in a microprocessor based system.
Most intelligent bus peripherals have the ability to assert SRQ. A
DMM might assert it when its measurement is complete, if its input isoverloaded 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 thatremoves the burden from the 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 iswrong."
Since SRQ is a single line command, there is no way for theController to determine which device requested the service without
additional information. This information is provided 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 thisexample, let's assume there are several devices which could assert
SRQ. The Controller issues an SPE (Serial Poll enable) command toeach device sequentially. If any device responds with DIO7 asserted it
indicates to the Controller that it was the device that asserted SRQ.
Often times the other bits will indicate 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 notnecessarily the why. When bus devices are configured 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 toallow logical OR/AND conditions on one data line by more than one
device. When SRQ is asserted, the Controller (under user's software)
conducts a Parallel Poll. The Controller must then analyze the eightbits of data received to determine the source of the request. Once thesource is determined, a Serial Poll might be used to determine thewhy.
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

Service Information
4.1 FACTORY SERVICE
IOtech maintains a factory service center in Cleveland, Ohio. If problems areencountered in using the Expander488 you should first telephone the factory. Many
problems can be resolved by discussing the problems with our applications
department. If the problem cannot be solved by this method, you will be instructed as
to the proper return procedure.

Appendix A

Character Codes And IEEE Multiline Messages

$00

0 $10

NUL
$01

16 $20

SOH00
17 $21

$03

$04

EOT
SDC
$05

ENQ
PPC
$06

$07

BEL
$08

SPE
9 $19

HT
TCT
$0A

11 $1B$0C

12 $1C$0D

13 $1D$0E

14 $1E$0F,-

15 $1F

.

US

/
15

ACG

UCG

]

N

?
15

LAG

^

ACG = Addressed Command GroupUCG = Universal Command GroupLAG = Listen Address Group

A.1

l

|

m

}

n

~

SCG
SCG
95 $6F 111 $7F 127

_

o

UNT

SCG

TAG

{

k

SCG
SCG
94 $6E 110 $7E 126

30
79 $5F

O

UNL

z

SCG
SCG
93 $6D 109 $7D 125

29
78 $5E

1463 $4F

y

j

\

M

>

i

SCG
SCG
92 $6C 108 $7C 124

28
77 $5D13
62 $4E30
47 $3F14
31 $2F

[

L

=

x

SCG
SCG
91 $6B 107 $7B 123

27
76 $5C12
61 $4D29
46 $3E13
30 $2E

RS

SI

<

h

Z

K

w

SCG
SCG
90 $6A 106 $7A 122

26
75 $5B

11
60 $4C28
45 $3D12
29 $2D

GS

SO

;

g

Y

J

v

SCG
SCG
89 $69 105 $79 121

25
74 $5A10
59 $4B27
44 $3C11
28 $2C

FS

CR

:
26
43 $3B

X

I

f

SCG
SCG
88 $68 104 $78 120

24
73 $5909
58 $4A

+

ESC

FF

9

*

10
27 $2B

W

H

u

SCG
SCG
87 $67 103 $77 119

23
72 $580857 $4925
42 $3A09
26 $2A

SUB

VT

8
24
41 $39)EMSPD
10 $1A

LF
$0B

(

08
25 $29

e

V

G

t

SCG
SCG
86 $66 102 $76 118

22
71 $5707
56 $48

s

d

U

F

7

c

SCG
SCG
85 $65 101 $75 117

21
70 $5606
55 $4723
40 $3807
24 $28

CAN

GET
$09

6

'

ETB

T

E

r
SCG
99 $73 115

SCG
SCG
84 $64 100 $74 116

20
69 $5505
54 $4622
39 $3706
23 $27

8 $18

BS

&

SYN
7 $17

5

q
SCG
98 $72 114

b

S

D

p

a

SCG
83 $6319
68 $5404
53 $4521
38 $3605
22 $26C

4
20
37 $35

%

NAK
PPU
6 $16

ACK

$

04
21 $25R

112

SCG
97 $71 113

SCG
82 $6218
67 $5303
52 $44`SCG
81 $61

QB

3

96 $70P

17
66 $5202
51 $43

19
36 $34

80 $6016
65 $51

A

2

#

DC4
DCL
5 $15

@

01
50 $42

18
35 $3303
20 $2464 $5000
49 $41

1

"

DC3
4 $14

0

17
34 $3202
19 $23

3 $13

ETX

01
18 $22

DC2

48 $4016
33 $31

!

DC1
LLO
2 $12

STX

SP

DLE
1 $11

GTL
$02

32 $30

DEL
SCG

SCG

TAG = Talk Address GroupSCG = Secondary Command Group