First Scan & Read Latched Bit

Bering C Sparky

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Mar 2014
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Hello,

I am working on a problem I knew would come and bit me and was hoping someone could clue me in on a couple of things so I can try to solve this problem.

1) What are we using for first scan bit in Logix5000?

2) If I have a bit latched in Logix5000 and the machine (PLC) is then powered down, is this bit still latched when the machine (PLC) is powered back up?

Thanks in advance for any help.

BCS
 
just to complicate things - remember that you can have more than one PROGRAM in your ControlLogix APPLICATION/PROJECT ... each separate PROGRAM will have its own separate S:FS bit ...

that might not affect what you're doing - but since it COULD cause problems in some situations, it's best to at least be aware of the fact up front ...

a possible issue can arise if the processor "goes to run" - but depending on how you have your individual TASKS scheduled to execute the various PROGRAMS, then ONE of your S:FS bits could potentially "fire" quite a long time before - or after - ANOTHER one of your S:FS bits ...

that's USUALLY not a problem - but just something that you might want to be aware of - IF (big IF) you're counting on a particular S:FS bit to "fire" AS SOON AS the system "wakes up" ...

going deeper ...

suppose that you LATCH a bit to a status of ONE - by using an OTL (Latch) instruction ... in normal situations, that bit will still have a status of ONE after the processor has a "go to run" experience ...

but ...

suppose that you also have an OTE instruction addressed to that same bit ... when the processor has a "go to run" experience the PRESCAN operation of the OTE will "go write a ZERO" into the bit - which will defeat the intended "retentive" status of the latching operation ...

but ...

the question then arises - is the OTE located in a routine that has NO WAY for it to be scanned (or even PRE-SCANNED) ? ... we can discuss that aspect if you like - but it gets somewhat more complicated - and the operation can be different from the same situation when using a ControlLogix/CompactLogix platform when compared to an Allen-Bradley PLC-5, SLC-500, or MicroLogix platform ...
 
Last edited:
Thanks Ron, your cup always runith over with very helpful information.

I appreciate the insite and is something to keep on the back burner to think about, I do not belive that this will affect my present situation.

I was typing a large wall about a time issue I was having, trying to make true a bit between say 2330 and 0530 if certain conditions were not met.

The code was getting really ugly trying to do this, but I just had mangement change the time shift numbers report, to on the hour instead of on the 1/2 hour so now it is between 0000 - 0600 etc and makes my life much easier.

Thanks for all the help and info.
 

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