FOR LOOP and WDT TM172

[otrotabi]

So, I´ve been trying to develop a way to save alarms on EEPROM for the TM171/TM172 series. The idea is to save both the alarm code and the alarm date (without seconds). I am using a very simple protocol to store both data on a single UDINT variable.

Following this video (https://www.youtube.com/watch?v=h5dQ8tfF2dM&ab_channel=SquishyBrained) I reserved 20 UDINT EEPROM variables and I figured I would save the first alarm at the first reserved memory position and then when a new alarm comes, move the n-1 alarm to the n position, n-2 to n ... first to second and save the newest alarm at the first position again. Since EEPROM memories last 100,000 cycles I think there´s a very good chance that I will not run into EEPROM failure soon.

In the video, they use a built-in FB that takes care of this move, it is called memmove in Twincat but it does not seem to exist in Ecostruxure HVAC libraries. So I do this by using a FOR-LOOP block which works well in simulation but crashes in a real PLC if the number of reserved EEPROM variables grows.

So my questions are:

1) How can I troubleshoot this ? How do I know how many variables I can move and write ?

2) Generally speaking: can a FOR LOOP that takes too long (but not an infinite loop according to simulation) crash the PLC ?

3) Is it ok to use systimer to calculate timing on the PLC ?

4) In these rare cases when it is easier to use a FOR LOOP to complete a long but "rare" task, is it safe to use either syswd_Timed or sysWD_Backgroud to allow the for loop to complete? Is there an example somewhere on how to use these functions ? I found a very rare animal fishing trying to google for sysWD_timed 🙂

[Bastian_Schmitz]

For your purpose is a ready to use sample project available. You will found this here. 

[otrotabi]

Thank you Bastian. I will give it a try. 

Regards