Measurement Computing Analyzer488 - Manual (Page 279)

Appendix G IEEE 488 Primer
G.11
8.1 Serial Poll
Suppose the Controller receives a service request. For this example,
let's assume there are several devices which could assert SRQ. The
Controller issues an SPE (Serial Poll enable) command to each device
sequentially. If any device responds with DIO7 asserted it indicates to the
Controller that it was the device that asserted SRQ. Often times the other
bits will indicate why the device wanted service. This Serial Polling
sequence, and any resulting action, is under control of the software
designer.
8.2 Parallel Poll
The Parallel Poll is another way the Controller can determine which
device requested service. It provides the who but not necessarily the why.
When bus devices are configured for Parallel Poll, they are assigned one bit
on the data bus for their response. By using the Status bit, the logic level of
the response can be programmed to allow logical OR/AND conditions on
one data line by more than one device. When SRQ is asserted, the
Controller (under user's software) conducts a Parallel Poll. The
Controller must then analyze the eight bits of data received to determine
the source of the request. Once the source is determined, a Serial Poll
might be used to determine the why.
Of the two polling types, the Serial Poll is the most popular due to its ability to
determine the who and why. In addition, most devices support Serial Poll only.