Measurement Computing Analyzer488 - Manual (Page 278)

Appendix G IEEE 488 Primer
G.10
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)
7.15 Parallel Poll Unconfigure (PPU)
This disables all devices from responding to a Parallel Poll. ($15)
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.