Measurement Computing Extender 488HS Rev.2.0 - Manual (Page 27)

Section 3 IEEE 488 Primer
3.9
3.7.13 Secondary Command Group (SCG)
These are any one of the 32 possible commands (0 to 31) in this group.
They must immediately follow a talk or listen address. ($60 to $7F)
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.