Dear Experts,

The following difference is observed in Profibus DP communication to a third party communication device when using S7-400 and S7-300 CPU.

Case when S7-400 CPU : With 400 CPU for Read/Write data exchange with the third party (TP) device, there is continuous write request from 400 CPU, means if we try to write the same data from TP device (write facility locally on TP device is provided) which is being written from 400 CPU through profibus network, it is not possible due to continuous write request from 400 CPU, hence the locally entered value from the TP device gets overwrite by the value which is there in the write DB address value in S7-400 CPU.

Case when S7-300 CPU : The only difference is that, while writing locally at TP device, it accepts the locally entered value, irrespective of the value from S7-300 CPU, and when we change the value from 300 CPU, at that time only the write command triggers, means write request is not continuous. The customer's demand is that any value must no be writable from local device, and it should accept the value form profibus communication with S7-300 CPU only.

For both the case we are using SFC-14 & 15 for W/R data consistently, and the communication network is from integrated DP ports only in both the cases.

Experts comment is awaited please.

Reagrds,

Thank you Fritz for your prompt response,

As you asked I have attached herewith the cfg files and DP write program snaps for both 300 and 400 CPU. 

Here TP devices are feeder system controllers. 

PLC programming and DP communication settings for both to CPUs are identical as i attached.

Kindly have a look and let me know what is the limitation with 300 CPU for the issue.

Thanks and Regards,

Hello Fritz,

Thanks you

Sorry for late reply,

In the mean time I have also taken trails for get rid of the issue but it is still the same.

My comments on your points are as below,

1) Yes I am using only the same physical TP devices in both the case. I have also tried with directly interchange the physical TP devices directly between 300 & 400 PLCs. But observed same results. Hence sure that TP are exactly identical including gsd files.

2) Yes, the PLC is really a 315-2AH14 as per config file.

3) I have also tried with program it with direct L PIW and T PQW instructions (without SFC 15) but in that case also problem is same.

Now, really wondering what may be the issue with S7-300 CPU.

Please help.

Regards,

Hello Fritz..

Please help.

The problem is still as it is. Awaiting your comment if any clue to resolve the issue.

Regards.