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Program Synchronization
D-3
Adjusted Maximum Program Scan Time
The Maximum Program Scan Time parameter you enter for
Synchronous mode gets adjusted before being used by the ControlNet
PLC-5 Backup system. The Adjusted Maximum Program Scan Time
is equal to the smallest integer number of NUT’s whose time value is
greater than or equal to the Maximum Program Scan Time parameter.
For example, if the configured Maximum Program Scan Time
parameter is 65ms and the ControlNet channel NUT value is 10ms,
the Adjusted Maximum Program Scan Time would be 70ms (i.e.
10ms x 7). In this example, each program scan time would be fixed at
70ms.
Program Scan Overrun
For Synchronous mode operation, it is possible for the actual program
scan time to exceed the Adjusted Maximum Program Scan Time.
When this overrun condition occurs, the Sync LED turns solid red
and the Maximum Program Scan Time Exceed Counter (word 48 in
the ControlNet Backup Integer File) is incremented. You must zero
out the Maximum Program Scan Time Exceed Counter for the Sync
LED to turn from solid red to solid green. The Maximum Program
Scan Counter is cleared by writing a non-zero value into the Reset
Diagnostic Counters word (word 32 of the ControlNet Hot Backup
Integer file).
When a program scan overrun occurs, the two processors temporarily
go out of sync, however they do not stay out of sync. They go back
into sync at the earliest time possible. They should be back into sync
at the next housekeeping. Even when the processor are
resynchronized, the Sync LED remains solid red until you clear the
Maximum Program Scan Time Exceed Counter.
Should frequent program scan overruns occur, it may be necessary for
you to increase the Maximum Program Scan Time parameter. The
Maximum Program Scan Time parameter can be changed
"on-the-fly" to the primary processor while the processor is in RUN
mode. Both processors will automatically begin using the new
Adjusted Maximum Program Scan Time parameter.
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