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6.8.1 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.
6.8.2 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 6: IEEE 488 Primer