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70 Series 90-70 Hot Standby CPU Redundancy User’s Guide – December 1993
GFK-0827
Where the Background Window Time is the time in milliseconds that you should set the
background window timer. The other elements in the formula are described above in
the calculation for words per sweep. The constants (F and C) are for the CPU 780.
Background Window Time =
448 0 x (100 + 176 x 0.0008 x 2)
(120000 – 4480)
Background Window Time =
3.9 rounded up to 4 ms
Miscellaneous Operation Information
Timer and PID Function Blocks
These function blocks will remain in lock step between the two synchronized units
provided:
A. Enabling logic is identical on both units. This includes power flow, frequency of
calling sub-block, etc..
B. The sub-block in which the function block occurs has the same name in both
units. Note that __MAIN is always common.
C. Reference registers (3 for timers, 40 for PID) and reset references are copied
from the unit that was in stand-alone to the unit that just transitioned to RUN
for each timer and PID function block. This copy does not need to be performed
if both units transition to RUN in the same sweep.
For example, if the following ladder logic is identical in sub-blocks on both units,
then %M100, %R250, %R251, and %R252 must all be transferred on
resynchronization to keep both units running timers synchronously:
––––] / [––––––––––– –––––––––––––––––––( )
%M100
%M100
TMR
1.00s
PV CV%L10 –
– %L20
%R250
Timed Contacts
When both systems are synchronized, timed contacts will have exactly the same value in
both units. That is, whenever T_SEC is on in one unit, it will also be on in the other unit
as long as both units are synchronized.
OVR_PRE %S Reference
The OVR_PRE %S reference which indicates whether one or more overrides is active is
not supported by the CPU 780. This reference should not be used in a redundancy
system.