Mophead (Customer) asked a question.
I am using a P2-550.
After a power Cycle, which has priority on the state of a tag, retentive or initial value?
After a power Cycle, which has priority on the state of a tag, retentive or initial value?
Related Questions
Nothing found
Loading
Retentive will have priority when the PLC turns on again, as long as you have battery power keeping the retentive data.
You can either use the low battery system bit to alert you if battery is low, or just replace the battery at the beginning of each year to make sure it is always good. When replacing a battery, make sure the PLC is powered on first so retentive data is loaded and still there. If you remove battery power while the PLC is off then nothing holds the retentive data.
If you are 300 miles from the customer that is when some glitch will cause it to decide to use initial values instead of retentive values. This is the main reason that I am still hesitant to use P-series PLCs. Too many options for data and you cannot be sure 100% of the time what the numbers will be when it goes through a power cycle. This happened to one of mine, and I finally ended up writing constants into some values to make it trustworthy. Now, if I need to change those (scale parameters) I have to edit the PLC code, but that is better than a random surprise.
Siemens S7 controllers are like this as well. I like the concept, but practically, it seems to cause issues. Siemens has both Default and Start values and depending on if you have created a UDT the Start supersedes the Default and if no Start is specified the retentive supersedes the Start. It's probably better to be verbose, put assignments in the initial scan for critical parameters and don't worry about it.
Thanks.
I think an HMI on the network that does not normally get powered down with the rest of the system might be changing values when the PLC boots.
I'll look into that.
It was the HMI.
I added a minimum press time to keep the toggle buttons from changing the state after boot.