Communication between Modbus and Siemens PLC

Looking for help in trying to communicated with a foxboro DCS using Modbus, to communicated with a siemens plc? Tried making up a lookup table, but no luck.



Thanks
John

Hi John,

As far as I know Siemens plc can only be set as modbus slave. Is the DCS master or the plc master?

Hello John:

First which S7?

S7-200 can be a modbus slave on a EN231 modem module, and can be set as Modbus master or slave through a code library that you can add to the programming software.

S7-300/400 CPUs can be set as Modbus master or Slave through a serial processor (CP340/CP441-2), with a (expensive) "lodable driver" package and integrated function blocks.



Then you have to match the addresses used by the Modbus functions you are starting/responding to. Say your S7 is a slave, and the Foxboro requests 4 data values through Function 4 starting at address 10; where in the S7 memory do you match these addresses?

The different manuals will help you calculate this. In a S7-300 slave, you would start at DB900.DBW0 (the specified start DB for Function 4 data) and then use a math formula provided to calculate the offset from that address for you to place the data requested by the master.
There are different start DBs and offset calculations for other Modbus functions.
Sounds complicated? Not really, that is what setting up a lookup table involves. And once you have studied the address calculation algorithm they provide, it is quite straight mapping.

Hope this helps,

Daniel Chartier

Hi John,


dchartier is right on this one.

There is a somewhat less expensive variant for S7-300/400.
By using the ET200S Profibus i/o blocks, 1SI modules with Modbus protocol can be used (look for 6ES7138-4DF11-0AB0).

Apart from that, I am sure that the Foxboro DCS can connect directly to an S7 PLC, maybe via OPC. This is probably simpler, less expensive, and most performant.