I was wondering if there is a Communication Mode under which the master just polls slaves while the slaves are transmitting data only if some piece of data has changed. I have the impression that S7-400s work like this. Am I wrong?
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I've checked this with a CPU 416/2-DP, a CP 443 and the OPC-Server of SimaticNET IE on on side and the same hardware an my own OPC-Server (based on libnodave) on the other side just before writing this answer.
I've found no difference in the communication between the Siemens OPC-Server and libnodave, except of one more packet sended by Siemens from the PC to the PLC (which seems to be empty).
This means that the S7-400 is always sending all requested data, not only the changed part of the requested data.
Is your impression based on the performance-difference between different PLCs or between different PC-programs ?
Axel
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I was wondering if there is a Communication Mode under which the master just polls slaves while the slaves are transmitting data only if some piece of data has changed. I have the impression that S7-400s work like this. Am I wrong?
I've checked this with a CPU 416/2-DP, a CP 443 and the OPC-Server of SimaticNET IE on on side and the same hardware an my own OPC-Server (based on libnodave) on the other side just before writing this answer.
I've found no difference in the communication between the Siemens OPC-Server and libnodave, except of one more packet sended by Siemens from the PC to the PLC (which seems to be empty).
This means that the S7-400 is always sending all requested data, not only the changed part of the requested data.
Is your impression based on the performance-difference between different PLCs or between different PC-programs ?
Axel