Issue
How do I set up RS-232 Serial Modbus Communication between HMISTO705 and Modicon M241?
What are the driver settings required and how do I wire it up?

Product
Harmony HMISTO705
Modicon M241 PLC

Environment
HMI to PLC communication setup

Resolution
The speed, Data Length, Parity Bit and Stop bits must match on both the HMI and PLC applications.
However, in the HMI, you must choose "NONE" for Flow Control because the PLC's RJ45 pinout does not have the physical ability for flow control (Request to Send and Clear to Send pins do not exist on it)

In EcoStruxure Operator Terminal Expert, the settings will look as follows:


In the M241 configuration software EcoStruxure Machine Expert (or the legacy SoMachine):
In the Devices Tree tab, under Serial_Line_1, Delete the Machine_Expert_Network_Manager node:




Add the "Modbus Manager" node under the Serial_Line_1 node:


Select the Serial_Line_1 node, and set the Physical Medium to RS232.
Also adjust the Baud rate, Parity, Data bits, Stop bits to match that of the HMI settings:


Double-Click on the Modbus_Manager_1 node to open the Configuration tab.
Review the settings.
And leave the settings as default:


The pinout for the M241 Serial line can be found in the Modicon M241 Logic Controller, Hardware Guide
The pinout of the HMISTO705 COM Interface Port can be found in the Harmony STO Color Model, User Manual

The wiring between the two devices should be made up as follows:

Note that a Loopback is required between the CTS and RTS pins of the HMISTO705.
Twist the the TXD and RXD wires together for improved external electromagnetic interference and cross-talk.
Use a shielded cable where possible.

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