Applies To:
  • CitectSCADA 3.x, 4.x

I get communication errors when I try to multi-drop Mitsubishi PLC's connected to a serial Com Port on the Citect computer. I get errors like bad response or garbled, unit not responding (response codes 0x1, 0x15) and error LED's on the link modules. It seems to work if I disconnect one unit. 

Check the cabling method as per;
  1. Use an RS232 to 485 converter (NOT RS232 to RS422):
    The method for connecting multiple Melsec units is by RS485 two wire bus as shown in the Melsec manual. This has been tested however the RS422 method has been known to give problems.
  2. Bus termination:
    The bus is terminated at each end only, if the converter has terminating resistors then disconnect the terminating resistor from the first PLC. Terminating resistors are supplied with the Melsec Computer/Multidrop link modules, eg C24 module and must be added by the user.
  3. Ensure correct grounding connections:
    The Melsec PLC's requires that SG (signal ground) be interconnected between the PLCs. Communications will be unreliable without this connection. They also require shielded cable with the shield connected to FG (frame ground). Thus a two pair cable is required. Consult the Melsec Computer/Multidrop link manual.


  • The impedance of the cable should match the terminating resistance value (110 ohms as recommended in the manual).
  • The protocol mode switch is set to 5.