PLC Retaining HMI setpoints on power cycle

harleymc100

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Join Date
Jan 2014
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london
Posts
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HI everyone.

I have a Micrologix 1400 paired with a PV Plus 7.
What is the "best" or "correct" way to change setpoints and have them retained when the plc power is cycled.

We have a program that on first scan writes a "default value" into an integer using a MOV instruction. When the integer (HMI address) is changed, the "default value" obviously doesn't change.

I hope this makes sense, i know the above is not right but just wondering what best practice is?
In my eyes it would be to get rid of the "default values" and MOV instruction altogether.

Cheers
 
Yup, by default, MicroLogix memory is retained during power cycles (non-volatile), so just stop writing over the values and they will stay.
 
Just to add to this - the reason those "set default values at power-up" MOV's were included to begin with, is most probably to allow for the fact that some users might get all excited changing parameters, and mess things up beyond the point of being able to revert their changes. If this happens, a simple power cycle will get them out of trouble.

As janner_10 suggests, whether or not this is the "best" feature for your machine in your facility is not for us to decide. If your facility has a lot of operators (or even worse, managers) who get excited and push buttons without much consideration and thought, this feature would be very useful. If your facility has more "considered" operators who have spent years fine-tuning the machine and know exactly what every setting does and how to get the best out of the machine, then this feature would likely be more annoying than helpful.

If you're confident that it's the latter, then yes, just remove the "first scan MOV's" and you're done. Although I might suggest that out of an abundance of caution, you could just put in a simple bit that you could toggle on or off to disable or enable the "set defaults at startup" feature. Put it in series with each of the MOV's, or even just once at the rung that calls the "set defaults" routine, if the program structure permits. That just means if Passion Fingers Percy does come along and mess things up beyond comprehension, you've got the option of toggling this bit on and power cycling the PLC, and you're back to "default" again. If you wanted to, you could even make your "set defaults at power up" tag settable on the HMI, so the operators could enable/disable the feature without calling a programmer to toggle a bit. Or, if you wanted to tinker a little more, you could have a button on the HMI for "press to set defaults" without the need to power cycle at all.

Horses for courses, whatever works best for your facility.
 

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