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. 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.
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 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.
Summary of Contents for Isolator488
Page 13: ...Section 2 Getting Started 2 3...
