Activate Bit every time there is a power outage

Hello all!

We have a plant in an area that is relatively prone to power outages. I am using RS Logix 5000 and there is no backup UPS that keeps the PLC running in the event of a power outage. I would like to know how to activate a bit every time there is a power outage and the PLC loses power so we can ensure that they are able to start up the plant safely every time.

Any thoughts would be greatly appreciated!

Thanks

You can store the status/phase of the equipment in some tag.
When the plc starts up, check if it was in a safe state (like standby/off) or if it was in production.

Copy that. Speaking more generally is there any way I could create a tag that latches every time the PLC loses power? Then make the operators have to manually unlatch it on the HMI. Thanks

http://www.plctalk.net/qanda/showthread.php?t=6464

I would just use the first scan bit.
The link above may help you out.

Regards,

PLC_n00b_1018 said:

Speaking more generally is there any way I could create a tag that latches every time the PLC loses power?

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how should the plc "know" that it was an intentional power cut or not? The plc will no know, so the tag would always be latched. So, you can just show the message every time the plc boots up.
edit: gclshortt was faster.

Thanks for the quick reply. Sorry for a n00b question but where do I find the first scan bit? I have RS Logix 5000 v 17.01

check the help for "power-up handler". That does exactly what you're looking for.

An XIC given the address "S:FS" in 5000 will be on for the first scan after powerup iirc.

probably more than you wanted to think about ...

I would like to know how to activate a bit every time there is a power outage and the PLC loses power

Click to expand...

are you sure that you phrased that request correctly? ... maybe (just maybe) you meant to say this instead:

I would like to know how to activate a bit every time the power goes off – or even just "flickers" off - even if only for just a fraction of a second or so ...

Click to expand...

here's the tricky part that many (most?) programmers don't consider in attacking the potential problems involved in a power "flicker" event ...

the ControlLogix system has a "hold up time" – which basically means that its chassis power supply will keep the processor scanning happily along for about a second or two even AFTER the 120VAC supply feeding the chassis power supply fails ... during that short (but important) period of time, the processor goes right on its merry way – doing its regularly assigned scan stuff ...

so ...

where this MIGHT become critically important is when the plant's power supply happens to drop out just for a fraction of a second or so (at least for anything LESS THAN the processor's "hold up" time) ... in that case, then the PLC processor does NOT see that particular event as a "power failure" ...

in simple terms, the processor might not even really "notice" that the plant's power service "flickered" off ...

in that sort of case, relying on some of the RSLogix/Studio 5000 built-in "start up" features might not provide quite the operation that you MIGHT be expecting ... the simple rung shown below might be more in line with what you need ...

just to nail down the idea I'm describing - in some "flicker" events, neither the First Scan bit - nor the Power-Up Handler - will come into play - since the ControlLogix processor never really "stops" - and so therefore it doesn't experience a "start up" event ... oops! ...

so ...

whatever scheme you finally come up with – I highly recommend that you test the results of its operation by trying several events where the power supply merely "flickers" – and doesn't really go "off" for several seconds or more ...

for some types of systems the unexpected response to a power "flicker" – as opposed to a power "failure" – can be quite surprising – and may fall squarely into the "bad" column of life's experiences ...

NOTE: if you're considering monitoring the voltage from a 24VDC power supply – instead of from a 120VAC supply line – then post again and mention that fact ... hint: the DC power supply invariably has capacitors to filter/smooth its output – and the effect of the capacitors can lead to more complexity than I've covered here ...

be safe ...
.