AB Quality Allen-Bradley 1771-IJ, User Manual

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5–9 Single Transfer Programming
Publication 1771ĆUM006B-EN-P - June 2002
Table 5.A
Scan Count Sequencing
At Count:
[1]
Program Executes Command To: I/O Scan:
1 PUT Control word into output word Sends output word
2 PUT Preset word #1 into output word Sends output word
3 PUT Preset word #2 into output word Sends output word
[1]
This count is the scan counter Accumulated value for PLCĆ2, PLCĆ2/15, PLCĆ2/20, and
MiniĆPLCĆ2 Controllers. For the PLC Controller, these values would be doubled: 2, 4,
and 6 respectively.
The scan counter must indicate that both an I/O scan and a program
scan have occurred. For PLC-2, PLC-2/15, PLC-2/20 and PLC-2/30
Processors and the Mini-Processor Module, the I/O scan and
program scan, because they are sequential, are synchronous: one
program scan occurs for each I/O scan. This means the scan counter
can simply be incremented each program scan. Section titled
Scan
Counter - PLC-2 Family Processors
describes a suitable scan
counter for these processors.
With the PLC processor, however, the I/O scan and program scan are
asynchronous; this means that the scan counter must be programmed
to increment based on the longer of the two scans. Section titled
Scan Counter - PLC Processor
describes a suitable scan counter
for these processors.
With the sample program following, note that the scan counter is
programmed to run continuously; it resets itself when the
accumulated value equals the preset value, and begins to count again.
This arrangement is normally preferred because it allows subsequent
programmed changes in bits of the output control word to be sent to
the module as soon as possible after the change is made.
Scan Counter Ć PLCĆ2 Family Processors
Figure 5.6 shows an example scan counter that can be used for any
PLC-2 family processor. This type of scan counter increments with
each program scan. Because the I/O and program scans of these
processors are synchronous, each increment of this scan counter
indicates that both an I/O scan and a program scan have occurred.