S7 Timer issue

Liam Moran

Lifetime Supporting Member
Join Date
May 2011
Location
Carlow
Posts
395
A few days ago I got a callout to a customer that has a Mollers Stretch hooding machine. After the stretch hooder is a weigh check station after which is an Electrically operated door.
There is a switch on the conveyor, (connected to an input on the Stretch Hooder PLC) which is activated by the pallet, so when the switch is activated the PLC pulses an output to open the door.
A time after the switch is clear, along with a few more conditions, the PLC then gives a pulse to close the door.
The Customer reported that the door would neither open or close automatically. When I visited, I found the outputs to open and close the door were solid on.
 
Here is a screenshot of the logic.
I had no documentation files, so I was just looking at the raw logic in the PLC.

Faulty timer 1.jpg
 
The timers T126 and T127 are the pulse timers for the open and close outputs Q33.4 and Q33.5.
Note that both timers are always high, the time current values fluctuated between 2 and 10 seconds.
I put an always low flag into the timer T127 input, just in case one of the inputs I19.3 or I19.5 were being activated by noise or something, but the timer still remained high.
I substituted T10 and T11 for both these timers and everything worked as before, as in worked as it should.
I did a cross reference check for both T126 and T127 but they were not used anywhere else.
This machine is running since mid 2011, so it is there 2 years.
Has anyone seen anything like this before?
 
The CPU is a 315-2DP and there is no fancy stuff like motion control or special modules, just plain digital I/O. There are some drives connected on profibus to this CPU.
 
Yep

I had the same thing a while ago with a timer. The PV of the timer would start from a value way beyond that of the SP and when it eventually reached 0, the timer would just start again without affecting the output attached to it. Weird. I did the same thing as you. I changed the timer number which worked immediately then I changed it back to the original timer and it worked fine. Buggy Siemens..
 
I've seen the same thing with S5Timers behaving strangely like that. Acting as though they are used elsewhere in the program and they're not.

I did some research on it and if memory serves me correct it is a problem with the old 315's. I'm not exactly sure what the issue is but I seem to remember it is specific to that series.

It certainly caused me an hour or two of confusion.

;-)
 

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