Measurement Computing Extender488 - Manual (Page 31)

Section 3 IEEE 488 Primer
3-11
3.8.1 Serial Poll
Suppose the Controller receives a service request. For this example, let's assume there are
several devices which could assert SRQ. T he 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.
3.8.2 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 c onfigured 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 mo re than one device. When SRQ is asserted, the Controller (under user's software)
conducts a Parallel Poll. The Controller must then a nalyze the ei ght 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.