I need help in identifying what could be the cause of an intermittent PLC communication loss? we are using siemens s7-200 plc and wonderware scada software.

there is a continous alarm in our HMI indicating PLC communication loss. its previous programmer is gone. what can be the cause of this communication loss? what feed from the PLC can be triggered so that you can signal a communication loss in the HMI.

thanks
newbie controller

 

We are experiencing the same problem. However, what we have are siemens s7-400 PLCs connected remotely to a DCS (siemens PCS7).

Here's the details:
We have 6 remote s7-400 PLCs located 100-500km away from the Central Control Room. The communication used was Ethernet but a Signal Extender was used in between the PLC and the DCS. The signal extender was a PATTON copperlink.

The problem occurs whenever it rains and usually, we just have to de-energized the signal extender to restore the communication. However, the problem seemed to occur frequently now even if it doesnt rain. We have no clue what really the problem is. We suspect that it has to do with the signal extender but we are not sure about it.

If someone knows what to do about this, it would be of great help to us.

 

Sounds to me like there is a cabling issue in the copper line you have. Perhaps a junction box is full of water and has now corroded the terminations.

If not, it could be that you are operating over the permissable distance at the baud rate you are trying to run at. Try reducing the baud rate on the extender setup to increase reliability on the data link, also 'set' the speed and duplex modes, try not to leave 'auto negotiate' selected as this can cause problems on bad quality cable runs.

Regards
Julian Kovacic
Onesteel Whyalla South Australia

 

Typically the communications errors are going to be posted if you are getting communications timeouts or if you are gettin unrequested traffic on the connection. The latter is usually in the form of an echo which would indicate a bad contact or crossed line somewhere.

It can also occur if you retry a request for data and the previous response finally arrives. This orphans the re-request and can result in a comm error. This is related to comm timeouts.

Comm timeouts mean that you are not waiting long enough for the device to respond.

Fred Loveless
Senior Application Engineer
Kepware Technologies
http://www.kepware.com

 

Dear,

This may be due to the improper panel earthing. Please check your all your device and panel earthing. Earth to neutral voltage should be around 5vAC. if it is more than 10 VAC this kind of problems may occure. I think this could be the problem.

regards
BCG Vaikom