Measurement Computing Parallel488 - Manual (Page 42)

Section 5 IEEE 488 Primer
5.10
5.7.14 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)
5.7.15 Parallel Poll Unconfigure (PPU)
This disables all devices from responding to a Parallel Poll. ($15)
5.8 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.