Modbus with modicon Momentum


Thread Starter



Sorry if this sounds so noob, but it's my first time dealing with network programmation. I need to access registers in a water measuring device using a Modicon Momentum PLC, but cannot find an easy instruction to do it. can someone give me some pointers, I searched online a bit and read a few thing about the XMIT instruction in Concept is that it? If someone could right me a short tutorial I would appreciate.

Robert Willis

First thing you need to provide is the specific protocol being used by the target device. The Modicon Momentum can support the following protocols.

Modbus RTU RS232 or RS485
Modbus ASCII RS232 or RS485
Modbus Plus
Modbus TCP/IP Ethernet

Since you specifically mentioned the XXMIT EFB it appears that you may be attempting to use either the Modbus RTU or Modbus ASCII protoocl.

The MSTR, READ_REG and WRITE_REG would be the EFB's that would be used with Modbus Plus and Modbus TCP/IP
Yes the protocol would be Modbus RTU 232. I am using the serial port on the CPU with a serial cable directly connected to the device...any pointers on that?


Robert Willis

Take a look at the programming cable that you use with the Modicon Momentum and set up the XXMIT function block according to the documentation. The Concept XXMIT EFB allows you to define all of the parameters that are needed for successful communication. I would start by reading a single 4x register from the target device to determine the offset required for the correct address. If possible pick a register that will display a variable that can be verified. Once you have established communication you can then increase the message size to accommodate a range of addresses. The target devices documentation should provide you with a register map and indication of the largest message size supported.

Robert Willis

Just re-read your message and wanted to clarify the following.

XMIT --- EFB that is used with the 984LL editor in Concept.

XXMIT ---- EFB that is used with the IEC61131-3 editors in Concept.

They both will perform the same function, however the XXMIT is simplier and easier to configure
Alright thx a lot, I have read on the XXMIT instruction and I think I understand it, will try it today and let you know how it turns out.

thx a lot