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In order to keep track of both I/O and program scans, the PLC scan
counter rungs manipulate a particular type of memory bit. This must
be a bit in the input image table which is not wired to a
corresponding input device, so that the bit is turned off each I/O
scan. Choose a bit which satisfies all of the following conditions:
•
is in a module group that is scanned immediately after the
encoder/counter module is scanned
•
is an actual terminal address on an installed input module
•
is off at all times
The first condition requires a brief knowledge of I/O scan
sequencing. The remote I/O PLC processor scans I/O modules in
order of module group no. It first scans module group 0 in all I/O
racks, then module group 1, then 2 and so on. Therefore, when
selecting an input image table bit for scan counter control, choose a
bit with a greater module group no. than the module group no. of the
encoder/counter module. If the encoder/counter module is placed in
module group 7, and unused input in module group 0 would be
appropriate for this purpose. Note that the I/O rack number is of no
consequence in selecting this bit.
The second condition requires that this input address correspond to
an actual input module terminal of the controller.
For the third condition, it is suggested that the input terminal at the
address be connected to the L2 (AC low) or DC COMMON terminal
on the input module wiring arm. This helps prevent the terminal
from being connected to an input device at some later time.
In Figure 5.7, it is assumed that bit 11111 satisfies all three of these
conditions.
The I/O scan and program scan both manipulate bit 11111. In rung
1, this bit is examined as an input condition for the scan counter.
When this bit is OFF, CTU 200 increments. When CTU 200
increments, bit 20017 is set on. In rung 2, bit 20017 latches bit
11111 on. Then, in rung 3, bit 20017 is turned off. Bit 11111 remains
on until the next I/O scan. With this arrangement, CTU 0 cannot
increment until both an I/O scan and program scan occur.
Note that, for the PLC processor, two scan counts are used between
each step in single transfer, as indicated in NO TAG.
Single Transfer Programming
Publication 1771 UM006B-EN-P - June 2002
5–11
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