PLC Low Battery Alarm

Hello all, I have a question for the experts. What I want to do is when the plc battery is low, I would like to send an alarm to my HMI panel. I am using an M221 Schneider Electric plc with Ecostruxure Machine Expert Basic software. I looked at the online manual and found this %s75 system bit, 0=normal and 1 = low battery. I am still learning plc programming and not sure how to use this system bit to write a simple ladder logic program to alarm the HMI panel. Thanks for any help.

One thing I forgot to mention I know how to link plc to HMI, I just not sure how to program this. When the battery is low I would use the system bit to turn on an output using a memory bit. I hope I am explaining this right.

Can the HMI read tags from the Schneider? If yes, then can you tell the HMI to read that %s75 bit?


If yes but the HMI cannot read the %s75 bit directly, then a adding simple rung in the PLC like this:


might do the trick


If you do not understand that notation, or want to do the equivalent in ST or FBD, then you should run through the tutorial on this site or elsewhere.

Drbitboy is correct, however, it may be possible that the HMI you use could read that bit directly, if you are going to use an HMI or there is already one fitted then you could make this an alarm bit so it displays on the alarm banner or alarm screen, if already configured and they use a range of bits already for alarms then do as post above to map it into a variable bit. You could also create a pop or other flashing banner up on the HMI to make it a bit more visual.
One thing I would suggest is have a regular maintenance schedule to replace the batteries in all PLC's regardless of their state. We used to do this every 12 months even though some PLC's (especially if rarely turned off) may last up to 10 years. Most of the PLC's I have used you can turn the power off to replace the battery as there is normally a capacitor that will hold the backup voltage for a few minutes at least (typically 10-20 mins), however, even though some PLC manufacturers do not recommend replacing the battery while powered I used to do this and never had one loose a program.