I am having serious problems getting a Modicon Micro PLC (Master) to communicate with a Twido (Slave) using Modbus. The micro has a TSXSCP114 card. The Twido has the RS485 terminal block connection option card plugged in. In software I am using the READ_VAR command to read a memory word in the Twido and copy it to a the Micro.

All the obvious things have been tried. Comms setup/protocols on each unit etc. The two were connected via a TSC SCA50 connection box. I have removed this and am connected directly. Each unit, Micro and Twido, also have a Magelis XBTN401 connected.

Does anyone have any thoughts on what may be amiss? Thanks in advance.

 

Do you have a single Modbus communications network setup with three devices: a Micro, Twido, and a Magelis?

If yes, this is your problem. Modbus networks can only have a single master (initiator of communications), in your case this is the Magelis. Your only solution is to create two networks, one the Micro and Twido and the other the Magelis and one of the PLCS.