Ieee-488Busoverview - Keithley 486 Instruction Manual

APPENDIX E
IEEE-488 Bus Overview
Introduction
Basically, the IEEE-488 bus is simply a communication
system between two or more electronic devices. A device
can be either an insburnent or a computer. When a com-
puter is used on the bus, it serves to supervise the com-
munication exchange between alI the devices and is
known as the controller. Supervision by the controller
consists of dete mining which device will talk and which
device will listen. As a talker, a device will output infor-
mation and as a listener, a device will receive informa-
tion. To simplify the task of keeping track of the devices, a
umque address number is assigned to each one.
On the bus, only one device can talk at a time and is ad-
dressed to talkby the controller. The device that is talking
is known as the active talker. The devices that need to lis-
ten to the talker are addressed to listen by the controller.
Each listener is then referred to as an active listener. De-
vices that do not need to listen are instructed to m-&ten.
The reason for the unlisten instruction is to optimize the
speed of bus information transfer since the task of listen-
ing takes up bus time.
Through the use of control lines, a handshake sequence
takes place in the transfer process of information from a
talker to a listener. This handshake sequence helps en-
sure the credibility of the information transfer. The basic
handshake sequence between an active controller
(talker) and a listener is as follows:
1. The listener indicates that it is read to listen.
2. The talker places the byte of data on the bus and
indicates that the data is available to the listener.
3. The listener, aware that the data is available, ac-
cepts the data and then indicates that the data has
been accepted.
4. The talker, aware that the data has been accepted,
stops sending data and indicates that data is not
being, sent.
5. The listener, aware that there is no data on the bus,
indicates that it is ready for the next byte of data.
Bus Description
The IEEE-488 bus, which is also frequently referred to a
the GPIB (General purpose Interface Bus), was designed
as a parallel transfer medium to optimize data transfer
without using an excessive number of bus lines. In keep
ing with this goal, the bus has only eight data lines that
are used for both data and with most co mmands. Five
bus management lines and three handshake lines round
out the complement of bus signal lines.
A typical set up for controlled operation is shown in Fig-
ure E-l. Generally, a system will contain one controller
and a number of other instruments to which the com-
mands are given. Device operation is categorized into
three operators: controller, talker and listener. The con-
troller does what its name implies; it controls the instru-
ments on the bus. The talker sends data while a listener
receives data. Depending on the type of instrument, any
particular device can be a talker only, a listener only or
both a talker and listener.
There are two categories of controllers: system controller,
and basic controller. Both are able to control other instn-
merits, but only the system controller has the absolute
authority in the system. In a system with more than one
controller, only one controller may be active at any given
time. Certain protoco!. is used to pass control from one
controller to another.
The IEEE-488 bus is limbed to 15 devices, including the
controller. Thus, any number of taikers and %reners up
to that limit may be Frescat 01: ti.e ous at one time. Al-
though several devices may be conunan ded to listen si-
multaneously, the bus CL have only one active talker, or
communications would be scrambled.
A device is placed in the talk or listen state by sending an
appropriate talk or listen command. These talk and listen
commandsarederivedfromaninstnxnent'sprimaryad-
dress. The primary address may have any value between
0 and 31, and is generally set by rear panel DIP switches
or programmed in from the front panel of the instrument.
The actual listen address value sent out over the bus is ob-
tained by ORing the primary address with $20. For exam-
E-l