C-2
IEEE-488 Bus Overview
Model 6220/6221 Reference Manual
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Introduction
The IEEE-488 bus is a communication system between two or more electronic
devices. A device can be either an instrument or a computer. When a computer is
used on the bus, it serves as a supervisor of the communication exchange
between all the devices and is known as the controller. Supervision by the control-
ler consists of determining which device will talk and which device will listen. As a
talker, a device will output information; as a listener, a device will receive informa-
tion. To simplify the task of keeping track of the devices, a unique address number
is assigned to each.
On the bus, only one device can talk at a time and is addressed to talk by the con-
troller. The device that is talking is known as the active talker. The devices that
need to listen to the talker are addressed to listen by the controller. Each listener
is then referred to as an active listener. Devices that do not need to listen are
instructed to unlisten. The reason for the unlisten instruction is to optimize the
speed of bus information transfer since the task of listening takes up bus time.
Through the use of control lines, a handshake sequence takes place in the trans-
fer process of information from a talker to a listener. This handshake sequence
helps ensure 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 ready 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, accepts 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.
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