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Theory of IEC HSBY Operation
50
840 USE 106 00 January 2003
Example This example shows the effect of a configuration change from baseline as shown in
the IEC Scan Time Increase Table (See
Overall PLC Scan Time, p. 49
).
A particular application has a standalone scan time of 25 ms in a PLC of type CPU
434 12. The state RAM to be transferred consists of 200 coils (0x), 300 discrete
inputs (1x), 150 input registers (3x), 400 holding registers (4x) and 14000 IEC HSBY
registers (3x).
The state RAM difference to the reference configuration is:
This application therefore would have an overall scan time in Hot Standby:
40 ms (reference with CPU 434 12/ 534 14)
+ 25 ms (logic solve)
+ 44 ms (offset through memory increase)
= 109 ms
Effects of a Configuration Change from Baseline
0x
200 - 1536 = - 1336 -1336*3/8 = - 501 Bytes
1x
300 - 512 = - 212 - 213*3/8 = - 80 Bytes
3x
150 - 512 = - 362 - 362*2 = - 724 Bytes
4x
400 - 1872 = - 1472 -1472*2 + ( - 1472*2/8)| = - 3312 Bytes
IEC Hot Standby regs 14000(3x) = 14000*2 = 28000 bytes Total = 28000 - 501 - 80 - 724
- 3312 = 23383 bytes Scan time offset = 23383*1.9ms ~ 44ms