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Offline Marco W.  
#1 Posted : Monday, February 20, 2017 1:51:14 AM(UTC)
Marco W.

Rank: Newbie

Joined: 2/20/2017(UTC)
Posts: 1

I have a general question about the way Matrikon OPC Server for Modbus writes data to a Node.

If I have a PLC that constantly (for example once per second) writes values to a group of Aliases in my OPC Server, will the Server:
A) Constantly write those data values to the target node; OR
B) Only perform the Modbus Write commands IF the data values are different to the current values in the target node?

(in my case I am hoping that the answer is (B), but either way I would like to know, as it will determine the way I program the PLC)


Offline Alexander Hein  
#2 Posted : Monday, February 20, 2017 7:57:46 AM(UTC)
Alexander Hein

Rank: Advanced Member

Joined: 12/9/2016(UTC)
Posts: 5

Was thanked: 1 time(s) in 1 post(s)
Hello Marco,

as what I know, the OPC server does not support any "on data change"/"report by exception" concepts on the device communication side.
Furthermore all "OPC writes" are also supposed to be a "device writes" (there is no concept of a "cache writes" in the OPC Spec. as well).
Which means that each write (from an OPC client) to an OPC server item, will also force a write down to the device.

I'm sorry that the actual answer is answerA and not as answerB, but this is the way how the MaatrikonOPC server for Modbus is supposed to work.

I hope you find this information helpful.

Best Regards

Alexander Hein
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