from the Mitsubishi Modbus problem department...
Mitsubishi modbus
Communications systems and equipment. topic
Posted by Igor Velimirovic on 4 February, 2003 - 10:58 am
I use GX IEC Developer and I have a A1SJ71UC24-R4-S2 card for modbus comunication. Do you have maybe some manuals for programming modbus functions. I want to know how can I transfer data from master station to my card and what do i have to do, to get these data from card to cpu memory. i have some manuals but they are without any explanations. can you send me a simple example how to read holding registers with cpu. I use Q2aS(H)-S1 CPU.

Thank you!

Posted by Patrick Beckett on 6 February, 2003 - 10:54 am
The setup of the modbus card creates a series of 'windows' or mapping between the registers in the PLC and the Modbus addresses. e.g. Modbus address 00001-00100 could be mapped to M1-M100. Any modbus communication to and from the Modbus card from the outside world (either Reads or Writes) would then be carried out on M1-M100
automatically by the card.

The only code required is the code to initialise the Modbus Card on a first scan, or shortly afterwards.

The setup of the card is done by loading the 'window' start addresses into the buffer memory of the card on startup. There are 10 'windows' for registers and 10 more for

It is important to realise that not all modbus functions are supported by the Mitsubishi, only a commonly used subset are available. In addition, all the Modbus cards I have seen from Mitsubishi have been slave only, and unable to act as a master.

An Acrobat file showing a sample configuration (installed and working) can be sent to you if you contact me directly. The configuration of the master station is dependant on the master station you are using, but if you try and read/write to modbus addresses in the 'windows' defined in your modbus card setup, you are usually successful.

Patrick Beckett
Project Manager
Merlin Systems Limited

Tel: +44 (0)1978 313911
Fax: +44 (0)1978 313811

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