TM172 System Fault

[otrotabi]

I was reprogramming a TM172PDG28R a while ago. after changing the polarity of an input. So I did as always and I was not paying much attention to the whole process. 

However after restarting I got this:

System Fault 

WDT : 3

I had run into this issue only once before and last time I was able to restore the PLC by inserting a blank program, so I did it once again in the few seconds that the PLC responds at start up. So basically the PLC program has been cleared, but the HMI program is still there and it does start.

It seems I was not lucky this time, because the issue is still there.

Only thing slightly unusual about what I did is that I could not find the cable I usually work with and used a backup one instead. As the communication was succesfull I assumed everything was fine.

So the questions are: what are my choices ? WDT points to a watchdog timer issue, can I somehow try to get an instruction to ignore WDT ? Anyway to reset the PLC ?

The other even more important question is: what the ... may I have done to disrupt a PLC ? I swear I will throw away the other cable but since the communication established fine and the program seemed to have been downloaded, I doubt the cable is to blame.

[Bastian_Schmitz · Accepted solution]

WDT:3 says "The lowest priority task not called ( User interface or Backgound is in stack for more the 30 sec.)".

Maybe something goes wrong during the download. Maybe on the Display part.

Because you mention that the display did not work correct.

You could donwload the display alone via the download button in the display section.

Sometimes it happens that the display part of the project is corrupt. In that case you could connect via Programming but display works not correct. Plese check this point via compile all.

If you want to recover the PLC generate a empty program via ESME HVAC and upload everything via USB Pen drive. 

In that case you could be sure that the Application and the Display part will be erased.

[otrotabi]

Thank you for your help. What kind of pen drive is needed ? I actually never used one but I have always connected to a PC through USB cable instead. Please let me know if there is a tutorial somewhere.

This happened yesterday evening. This morning I came to the office and was able to download a complete empty through comissioning project into the PLC and it seems back to normal.

I was able to get a Hello ! back on the screen afterwards... Are there any tests that can be used like in a PC when you use the Bios memory routines ?

After reading your message, I went to the hardware guide to read about the USB Pen drive, and I run into something that probably caused the issue and I was completely ignorant of:

NOTE: Do not apply voltage via 24 Vac/dc while the equipment is already connected to a PC via Mini-B USB cable (TM172P•••••• only).
Before applying power via 24 Vac/dc power supply connection:

• Disconnect the Mini-B USB cable.
• Supply the M172 controller via its 24 Vac/dc power supply connector.
• Reconnect the Mini-B USB cable.

So, certainly it was my fault, I never knew this was necessary and may have been lucky so far.

[otrotabi]

As a follow up to this, I have not so far discovered why the PLC hangs. It is hard to know because at simulation I get no clues, but it looks like there is something wrong at the HMI program.

However, although the manual explicitily does warn about disconnecting USB before reconnecting power, I have talked to other people and this should probably cause any major issues although following this practice should be recommended.