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4.7.15 Parallel Poll Unconfigure (PPU)

This disables all devices from responding to a

Parallel Poll

. ($15)

4.8 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 and are 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 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 which a bus device can signal to the

Controller

that an

event has occurred. It is similar 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 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 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 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 provided by the multiline commands for

Serial Poll

and

Parallel Poll

4.8.1 Serial Poll

Suppose the

Controller

receives a service request. For this example, let's assume

there are several devices which could assert

SRQ

. The

Controller

issues an

SPE

(Serial Poll enable) command to each 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 control of the software

designer.

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Measurement-computing Digital HS Manuale Utente Pagina 49

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