Thread Starter

Gerry Hagberg

First of all, I think all user memory should be in battery-backed RAM. If the PLC is running on a desktop PC, then it should have a UPS. If the
application can't tolerate a UPS failure then I need to state the obvious? I don't think it should be necessary to record the data table
every scan on a desktop implementation as this should be inherentlynon-critical.

How does A-B do it? My understanding: Initially all data table states are as they were at power loss (or switch from run mode) because they are in battery-backed RAM. On entering run mode, the processor does a pre-scan of the program, looking for any non-retentive outputs and
resetting them. It then begins normal program execution.


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That is absolutely correct. One note here, it scans the whole program, even conditional sections, also it just looks at outputs, and does not calculate for instance pointer values.

I once had a PLC that would major fault in the prescan, because the program had never been run, and thus some coils that were addressed
with pointers, pointed to invalid locations. Took a while to find that one a 3 AM!

Stan Brown [email protected] 843-745-3154
Charleston SC.
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