Modbus readout with the "Serial com" plugin

Stefan Velghe

Posted on 24.09.2013 09:28
Hello, I try to read out MODbus with the Serial COM plugin.

If I create one request, everything goes fine.
Request: (ID,FC,data address,#registers,CRC)
=>
Response: (ID,FC,#databytes,crc)

The response is configured as followed:
prefix : (ID,FC,#databytes).
End definition: fixed length 2  (I expect 2 databytes after the prefix.)

This works perfect.
If I configure more requests, the response prefixes will remain the same for all the responses that I configured and the data is switched between the channels in the measure tab.
I can’t use the CRC because it is not optimized for Modbus.

Is anyone familiar with this issue or reading out Modbus with dewesoft over the PC’s Com port?

THX, Stefan
Martin Trummer
Software Development
Posted on 30.09.2013 09:39
hi,

setting up the Modbus protocol can be quite tricky in the SerialCom plugin.
So the first thing, I'd like to do is suggesting to use the Serial Modbus  plugin (article number: DEWESOFT-7-PLUGIN-MODBUS). In this plugin it will be much simpler to setup Modbus data. The plugin has been developed by DEWETRON - please contact [email protected] to get the plugin and support.
note: don't confuse this with the Modbus TCP/IP plugin (DEWESOFT-7-PLUGIN-MODBUS-TCPIP) which only works over TCP/IP - not over serial port

Back to the question:
I suppose, that the problem may be to correctly identify to which channel the return packet fit, right?
when you send several requests with the same ID and FC but for different registers, the responses that DEWESoft™ receives will all look the same: i.e. by only looking at the response, it's not possible to know which registers have been requested.
This is exactly the same problem that you have with EPAD modules - the problem and solution to this is explained in detail in chapter "10.3 EPAD modules" of the SerialCom plugin documentation V2.0.2 p.83.

If this is not your problem or you need more help with the SerailCom plugin, then please send a detailed description and your channel setup file to our support team: [email protected]

Stefan Velghe

Posted on 01.10.2013 05:55
Thx for your response.

I think your solution will slow down the communication.
At this moment I solved the problem by requesting a different amount of databytes for each request.
This works because the amount of databytes is a part of the Prefix.

I will ask for the "Serial Modbus plugin" so I can test it.
thx
Michael Kalmbach

Posted on 10.09.2019 20:05

@Stefan Velghe I realize this was many years ago now, but did you ever have any luck getting Modbus working?


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