Modbus RTU communication issue between TM172OBM18R and Weintek MT6051iP over RS-485

[Lautaro25-02-98]

Hi everyone,

I’m trying to communicate a TM172OBM18R PLC with a Weintek MT6051iP HMI using Modbus RTU over RS-485, but I always get a communication error on the HMI.

Hardware / software

- PLC: TM172OBM18R
- Software: EcoStruxure Machine Expert – HVAC
- HMI: Weintek MT6051iP
- Software: EasyBuilder Pro
- Link: RS-485 1-wire (D+ / D– with common GND)

PLC configuration (Machine Expert – HVAC)

Using RS485-1 On Board:

- Proto_RS485_1 = 3 – Modbus RTU
- Baud_RS485_1= 38400
- DataBit_RS485_1 = 8
- StopBit_RS485_1 = 1
- Parity_RS485_1 = Even
- Addr_RS485_1 = 1 (slave ID)

In Configuration → Modbus objects I created some Modbus objects mapped to %MWx words and assigned them to interface RS485-1 so they can be accessed by an external Modbus master.

The PLC program compiles and runs fine.

HMI configuration (EasyBuilder Pro)

- Device: tested both “MODBUS RTU (Adjustable)” and “Schneider MODBUS RTU”
- COM Port: COM1 – RS485 1-wire (physical port wired to the PLC)
- Baudrate: 38400
- Data bits: 8
- Parity: Even
- Stop bits: 1
- Station No.: 1 (same as PLC)

I’m trying to read one simple word (for example %MW0 mapped in the Modbus objects table) using the corresponding Modbus address in the HMI tag.

Wiring

- PLC D+ → HMI Data+
- PLC D– → HMI Data–
- PLC GND → HMI GND/COM

Polarity and continuity were checked; there are no wiring faults.

Symptom

The HMI always indicates a communication timeout with the PLC: the device status shows a communication error and no valid data is received from the PLC, even though baudrate, parity, ID, etc. match on both sides.

What I’ve already tried

- Switched between RS485-1 On Board and RS485-2 On Board (updating the Modbus objects interface accordingly).
- Tried different baudrates and parity settings.
- Tested both HMI drivers (generic Modbus RTU and Schneider Modbus RTU).
- Rechecked cabling and commissioning parameters multiple times.

Questions

1. For this kind of external HMI connection, what is the recommended interface on the TM172: the RS485 on-board ports or the RS485 plugin passive interface?

2. Are there any official examples or application notes showing Modbus RTU communication between M171/M172 controllers and third-party HMIs?

Any help or hints would be greatly appreciated.

Best regards,
Lautaro Tapia

[ismailozinan]

Hi,

Is it possible check cabling and TM172 modbus settings? Please make sure that the TM172 RS-485 settings are correctly configured as attached.

BR.