micrologix 1500

Does anyone know if there is a bit that can be used in a program if the plc itself loses power on shut down?

Isn't it suppose to lose power on shutdown???

Are you looking for an external signal to something else or are you looking to latch something on internal to indicate that it has been powered down?

The 'first scan' (S:1/15) indicates the first scan after power-up, or transition to Run mode. I don't think ther is an easy way to tell the difference between a power down then back up and just a transition to Program then back to Run mode.

You could have an external relay circuit, the coil turned on by the PLC after start but then self-latching. Bring a contact from this relay in as an input. If on First Pass that input is on then it wasn't a power down just a Program to Run transition.

On the other hand if you are trying to catch the fact that a power down is taking place and do something before the PLC actually shuts down then a VERY FAST input might be able to catch the fact that the power is going down before the power supply hold-up time ends and the processor shuts down.

You mean "loses power" as in a dead backup battery not retaining the program? Or after a download with a program that doesn't have its data tables protected? As opposed to switching from Program to Run mode?

If you included a rung at the very bottom of your program that unconditionally passed power to a OTL (latching coil) rather than a OTE it will retain its last state (which will be the "ON" state of course) regardless of any Run/Program/Run transitions.

It will only be in the off state because the output table was cleared for some reason.

Then it's a matter of using Examine-Whatever (TM) instructions in your program whenever you want to detect that the data table has been cleared.

Which could be very useful (especially if Very Bad Things happen if the program isn't aware of those three carriers in a count zone and tries to stuff a few more in there).

bernie_carlton said:

The 'first scan' (S:1/15) indicates the first scan after power-up, or transition to Run mode. I don't think ther is an easy way to tell the difference between a power down then back up and just a transition to Program then back to Run mode.

You could have an external relay circuit, the coil turned on by the PLC after start but then self-latching. Bring a contact from this relay in as an input. If on First Pass that input is on then it wasn't a power down just a Program to Run transition...
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Click to expand...

This exact problem (distinguis between power up start and mode switch start) was the subject of my thread on MrPLC.com and was successfuly solved programatically on ML1500.

You could also write a fault routine, this would set a bit on the last scan before total power was lost or S2:20 is a power down/fault bit... not sure how you could use it or if it would work in what you need.