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Chapter 7 NI-488.2 Programming Techniques
© National Instruments Corporation 7-17 NI-488.2 User Manual for Windows
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Parallel Polling
Although parallel polling is not widely used, it is a useful method for
obtaining the status of more than one device at the same time. The
advantage of parallel polling is that a single parallel poll can easily check
up to eight individual devices at once. In comparison, eight separate serial
polls would be required to check eight devices for their serial poll response
bytes. The value of the individual status bit (
ist
) determines the parallel
poll response.
Implementing a Parallel Poll
You can implement parallel polling with either the traditional or
multi-device NI-488.2 calls. If you use multi-device NI-488.2 calls to
execute parallel polls, you do not need extensive knowledge of the parallel
polling messages. However, you should use the traditional NI-488.2 calls
for parallel polling when the GPIB interface is not the Controller, and the
interface must configure itself for a parallel poll and set its own individual
status bit (
ist
).
Parallel Polling with Traditional NI-488.2 Calls
Complete the following steps to implement parallel polling using
traditional NI-488.2 calls. Each step contains example code.
1. Configure the device for parallel polling using the
ibppc
function,
unless the device can configure itself for parallel polling.
ibppc
requires an 8-bit value to designate the data line number, the
ist
sense, and whether the function configures the device for the
parallel poll. The bit pattern is as follows:
0 1 1 E S D2 D1 D0
E is 1 to disable parallel polling and 0 to enable parallel polling for that
particular device.
S is 1 if the device is to assert the assigned data line when
ist
is 1, and
0 if the device is to assert the assigned data line when
ist
is 0.
D2 through D0 determine the number of the assigned data line. The
physical line number is the binary line number plus one. For example,
DIO3 has a binary bit pattern of 010.