Measurement-computing Digital488 Manuel d'utilisateur télécharger pdf (Page 70)

Section 4 IEEE 488 Primer

4.10

4.7.14 Parallel Poll Configure (PPC)

This configures devices capable of performing a

Parallel

Poll

as to

which data bit they are to assert in response to a

Parallel

Poll

. ($05)

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

determine which device requested the service without additional information. This

information is provided by the multiline commands for

Serial Poll

and

Parallel Poll

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Measurement-computing Digital488 Manuel d'utilisateur Page 70

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