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Theory of 984 HSBY Operation
840 USE 106 00 January 2003 35
Example This example shows the effect of a configuration change from baseline as shown in
the Scan Time Increase Table in
PLC Scan Times, p. 34
.
A particular HSBY application has a standalone scan time of 36 ms in a PLC of type
CPU 424 02. The state RAM to be transferred consists of 3000 coils (0x), 2500
discrete inputs (1x), 2500 input registers (3x) and 8000 holding registers (4x).
The state RAM difference to the reference configuration is shown in the Effects of
a Configuration Change from Baseline table below:
This application therefore would have an overall scan time in Hot Standby:
40 ms (reference with CPU 424 02 0x) added by HSBY
+ 36 ms (standalone scan time)
+ 28 ms (offset through configuration increase)
=104 ms
0x3000 - 1563 =
1464
1464*3/8 =549 Bytes
1x2500 - 512 =
1988
1988*3/8 = 746 Bytes
3x2500 - 1212 =
1288
1288*2 = 2576 Bytes
4x8000 - 1872 =
6128
6128*2 + (6128*2/8) = 13788 Bytes
Total: 17659 bytes = scan time offset = 17659 * 1.6ms ~ 28ms
Note: No matter how long your transfer takes, it does not cause a watchdog
timeout.