S7 CP341 RS485 ASCII Protocol - FB8 P_SND_RK

Hello,

Currently we have a problem with an installation that runs already for years.

Description of the installation:
A Siemens CPU318 with a CP341 is communicating to several partners with a RS485 connection (ASCII configured). In the software, the siemens FB8 ("P_SND_RK" is used to send messages (the PLC only sends and does not receive).

Description of the problem:
Occasionaly (sometimes 1 time a day) the communication stops working. There is a small sequence programmed that is sending out a message and it is waiting for the "done" signal of FB8

The only way to solve the problem is make a warm restart of the CPU! Then the sequence starts working again, as the done bit of this FB8 comes in again.
We already tried several things, like resetting the sequence, make a reset puls at the "R" input VAR of FB8 but nothing helps!

Also there is nothing to see in the diagnostic buffers of both CPU and CP card.
Also we programmed a memory bit on a rising edge detection of the "ERROR" output parameter.
But when the fault happens, there is no error detected!

Does anyone know why a warm restart is the only solution and why the FB8 does not give an error?
And what a reason could be that the communication suddenly hangs?

Because the installation already works for many years in this way, I'm thinking more of an elektrical problem.
But the problem with this protocol is also that you don't have any wire-break detection.

If somebody had a similliar expierience or has an idea pls let me know!

Kind regards,
Jan

Just to clarify, are you saying this problem has been present from day 1?

Hello,

No this problem occured during the last month.
Before there were no problems

Maybe you can try to simulate the situation which cause the "done" not send back - as your description stop the communication of RS485 device manually.
You can also take a look status information when error code is 0.
0 11 Warning:- New job cannot take effect since previous job is not yet completed.- The job is now being processed in a priority class having lower priority.
0 25 Communication has started. The job is being processed.

The problem is that we can not simulate it because we dont't know exactly when it goes wrong. Now it is running for 4 days without any problems... but sooner or later it will come back again

Jan,

Have you found the cause of this issue?

I am having the very same issue, but I am programming a new system.
I have not been able to get a done signal from the CP 341 ever...

I need to use the done signal to move the state machine on to the next state.

Any suggestions would be helpful.

Regards

Sandi