Scan Counter Plc 2 Family Processors - AB Quality Allen-Bradley 1771-IJ User Manual

Encoder/counter modules

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Table Of Contents

Table of Contents

Table 5.A

Scan Count Sequencing

[1]

At Count:

Program Executes Command To:

PUT Control word into output word

PUT Preset word #1 into output word Sends output word

PUT Preset word #2 into output word Sends output word

[1]

This count is the scan counter Accumulated value for PLC 2, PLC 2/15, PLC 2/20, and

Mini PLC 2 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.