Measurement Computing Isolator488 - Manual (Page 31)

Section 3 IEEE 488 Primer
3-10
3.7.14 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)
3.7.15 Parallel Poll Unconfigure (PPU)
This disables all devices from responding to a Parallel Poll. ($15)
3.8 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.