Allen Bradley SLC 504 shutdown routine (to save values when plc crashes)

kemppioy

Member
Join Date
Oct 2005
Location
Dallas
Posts
72
Hey,

How can I make sure some data in the plc does not change values during a plc stop situation or power down situation? Can I make some subroutine that saves some parameters and then makes sure they are the same when the plc powers back up? Problem is I have some integer values that strangely go to zero when the plc is turned off every once in a while. Then if the user does not reload the recipes (which are created also in the plc), the zero causes some problems on the machine. I want to make sure the values never change. Is there some retentive feature?

Thanks
Paul Kemp
 
By default, all values in a 5/04 are retentive, through powerdown/powerup, restart, just about everything short of downloading a program.

Are there HMI's that may be writing to the registers?
 
kemppioy said:
Problem is I have some integer values that strangely go to zero when the plc is turned off every once in a while. Then if the user does not reload the recipes (which are created also in the plc), the zero causes some problems on the machine. I want to make sure the values never change. Is there some retentive feature?
I would be careful with how my "moves" of data were done.

For example, let's say you are always moving a value from an analog device (transmitter, etc.) into a register. Now power is lost...

The PLC power may be the last to go as the power supply voltage bleeds down. The transmitter may have shutdown faster or an MCR may have dropped out quick, yet you are still "moving" values into your register until the PLC dies.

I would check the timing of how my moves were being made and possibly "AND" them with the MCR input.

And for the record, I agree with "rdrast" about the values already being retentive.

Lastly, you could program a compare instruction such that if your register is zero, still move in some kind of default "low" value. However, if you find the source of what mysteriously moves in zeroes (I suspect timing of your moves), this is unnecessary.
 
Check for "First Scan" bit S:15/0 (I think, not at work right now). This bit may tigger MOV commands to zero out these register values. This would have been done to return the machine to a safe state before startup. Just an idea.


Sportster
 

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