Measurement-computing Micro 488/EX rev.2.1 Manual do Utilizador descarregar pdf (Página 192)

Section 8 IEEE 488 Primer

8.10

8.7.14 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. (

&H05

)

8.7.15 Parallel Poll Unconfigure (PPU)

This disables all devices from responding to a Parallel Poll. (

&H15

)

8.8 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.

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Measurement-computing Micro 488/EX rev.2.1 Manual do Utilizador Página 192