Applies To:
  • CitectSCADA 5.xx

How do I communicate to Mitsubishi FX2N PLCs in a multidrop configuration? 

In Citect you can use the MELSEC protocol to address the multidropped FX2N PLCs.  The standard MELSECF protocol only allows a point-to-point connection.

On the FX2N you can install a small board on the communication port that gives you either a RS232 port (via the FX2N-232BD board) or a RS485 port (via the FX2N-485BD board). If you choose the RS232 board you will have to use an external RS232/RS485 converter.

(For the FX and FX0N there are similar modules: FX-485ADP or FX0N-485ADP. These work in a similar way and that can be set up in same way as FX2N-485BD. They are older and a little bit bigger modules than the BD and they are connected via port on left side on the FX or FX0N PLC.)

The register D8120 in the FX2N sets the communication format. I set this to a value of 6090 (0110 0000 1001 0000 for bits b15...b00). This means:

baudrate: 19200

data bits: 7

stop bits: 1

parity: none

format 1 protocol.

The register D8121 in the FX2N sets the station address. It is a good idea to set this register to read only.

The melsec.dbf file is by default set to the ranges for the melsecA series. I changed the values for the M and D ranges to 0..8255, this allows me to address the full range in the FX2N PLC's.

In Citect I used the following settings in my communication forms:

Boards form:
Type: COMx
Address: 0
Ports form:
Baud Rate: 19200
Data Bits: 7
Stop Bits: 1
Parity: None
IODevices form:
Protocol: Melsec
Address: fill in station address from address D8121

An inconvenience is that the FX2N uses octal addressing and the A-series (MELSEC driver) uses hexadecimal addressing for the inputs and outputs. If you read/write single inputs and outputs it works fine, but if you try to read multiple inputs/outputs in one block (for example X6 and X10), you will run into problems.

A workaround for this is to copy the in- and outputs (X and Y variables) to internal markers (M variables). Both the FX-series and the A-series use decimal addressing for the M markers.