S5 - OP37 problem

Good day to you all, I hope you can help me here. A new customer recently phoned me because his OP37 was displaying the mesage "PLC cannot be found". He said when this error began he checked the Profibus DP connection and found one broken wire. He repaired it, checked all the terminations, and still no success. Briefly, the setup is as follows: S5 115 CPU communicating with IM308 C in the same rack. The IM308 is master on the bus communicating with ET200 and then to OP37 on Profibus.
There are no Bus faults on the IM308 or ET200, the IM308 Run light is solid green.
Up front, I know little about the S5 range of PLC's but while researching this problem, I've read that you need certain DB's in the program to enable communications. This brings me to my first question, when I uploaded the Step 5 program from the S5 115 CPU, there were no DB51, DB52 for the communication setup. Are these DB's absulutely necessary in this type of setup?
I have taken the OP37 to Siemens and they tested it, Siemens reckon there is nothing wrong with it, all the tests were satisfactory.
I would really appreciate it if someone could assist me with this and possible give me some guidance on what to check or do.
Thanks in advance.
Ray

If this setup was already operational, it's very unlikely that the PLC software has anything to do with the problem. Unless of course someone recently felt the need to mess around with it.

Maybe someone tampered with the communication parameters in the OP37?

I understand the Profibus cable runs from the PLC to the ET200 and continues from there to the OP, where it is terminated? Does the ET200 still work properly?

If so, there is a slight chance it's just the polarity that's inverted (A & B in the wiring between ET200 and OP), or maybe the bus is not terminated correctly at the OP.

...when I uploaded the Step 5 program from the S5 115 CPU, there were no DB51, DB52 for the communication setup. Are these DB's absulutely necessary in this type of setup?

Click to expand...

The programmer needs to specify which DB's are used for PB communications. They do not necessarily have to be DB51, DB52, these are default values.

Hope this helps...