Title: PFC300 + 750-652: Modbus RTU communication works only after manually disabling Continuous Send

CODESYS
1

Hello,

I am using a PFC300 with a serial card 750-652 as a Modbus RTU server. My program simulates an Eurotherm regulator to communicate with the Instron Temperature Configurator.

Communication works with Docteur Modbus on PC, but Instron displays:

Port Opened OK
250°C Lost communication with the device
Device and/or port not ready

I found a workaround that restores communication:

  1. I start the PLC application and the Modbus server.
  2. In WAGO-I/O-CHECK, I change Continuous Send Mode from enabled to disabled.
  3. Communication with Instron then works.
  4. After restarting the PLC, Continuous Send reverts to enabled.
  5. The issue recurs without a restart: I set xConnect to FALSE, disable Continuous Send in I/O-CHECK, and then set xConnect back to TRUE. Opening the server reactivates the mode.

Serial configuration:

  • RS485 half-duplex
  • 9600 bauds, 8 data bits, no parity, 1 stop bit
  • No flow control
  • Modbus RTU, slave address 1
  • Continuous Receive disabled
  • RS485 switching time: 100 µs
  • Version of the 750-652 displayed in I/O-CHECK: 01.02.25(06)

Library versions:

  • WagoAppPlcModbus: 1.1.5.13
  • WagoAppCom: 1.7.1.2
  • WagoTypesCom: 1.6.2.3
  • WagoSysKbusModule: 1.8.6.4
  • WagoAppScanKbus_FR: 1.0.7.0

I retrieve the serial interfaces using WagoAppScanKbus_FR.FbGet_I_Port_Serial. The server is called every PLC cycle:

mySimpleSerialServer(
    xConnect           := xAutorisationInstron,
    I_Port             := PRG_ES.aI_Port[3],
    axDiscreteInputs   := myDiscreteInputs,
    axCoils            := myCoils,
    awInputRegisters   := myInputRegisters,
    awHoldingRegisters := myHoldingRegisters
);

I reviewed the topic 8212 with 750-652 modbus trouble, where changing library versions resolved an emission issue. However, my setup uses a PFC300 as a Modbus server, so I am unsure if this solution applies.

Is this behavior a known library or firmware issue? Is there a supported method to disable Continuous Send programmatically, or a compatible update that would resolve the problem?

I am referring specifically to the Continuous Send configuration parameter, not the SC command bit in byte C0.

Thank you for your help.