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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.
5.8.1 S
ERIAL
P
OLL
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 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.
5.8.2 P
ARALLEL
P
OLL
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.
CHAPTER 5: IEEE 488 Primer
