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PSI BOX Reference
Talk Only And Listen Only Devices.
_______________________________
The only exception to addressed devices are Talk Only or
Listen Only devices. There may be only one Talk Only device
on an interface bus. When the controller is in the idle state,
ATN low, the Talk Only device will always be ready to send its
data. A Listen Only device will receive any data from any talker
no matter what other devices have also been commanded to
listen.
One of the simplest IEEE systems is with a Talk Only
digital meter and a Listen Only printer. The printer will log
every reading that the meter sends. This type of system does not
require the presence of an IEEE Controller.
In addition to the basic function of instructing devices to
talk or listen the IEEE Standard provides interface messages that
allow the controller to accomplish the following four functions.
1)
Serial Polls.
2)
Parallel Polls.
3)
Device Clear and Device Trigger.
4)
Remote/ Local functions.
_________ _
Serial Poll.
When a device requires the attention of the controller it
asserts SRQ true. The Serial Poll is the reply that the Controller
gives to a device asserting SRQ, service request. It is the IEEE
device that causes the serial poll to be performed because it
asserts SRQ when it needs servicing by the controller.
The SERIAL POLL is used to determine which device has
requested service from the controller and why. Serial poll is the
only method that can correctly cause the device to return the
SRQ line false. In general, you will know which device
requested the service and so will know which to poll. However,
if it could be one of several devices that is requesting service,
you should perform successive serial polls on the different
devices until the SRQ line returns to the false state.
Page 20
IEEE 488 Introduction
Chapter 2
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