Applies To:
  • CitectSCADA 6.xx, 7.xx
  • Vijeo Citect 6.xx, 7.xx 

If you use the MELSFX2N driver to communicate with a Melsec FX3U PLC via Serial, you will not be able to read/write to Internal Relays M3072 and above. The MELSFX2N driver was originally developed to support communications with the Melsec FX2N PLCs, which only provide 3072 Internal relays (M0 to M3071). The Melsec FX3U PLCs, on the other hand, have had their Internal Relay address range extended to 7680 (M0 -> M7679). However, the underlying protocol that the MELSFX2N driver uses does not support read/write requests to the extended FX3U addresses.


To be able to read the extended FX3U addresses, you have the following 3 options:

1. Install an OPC Server which communicates with the FX3U via Serial and use the Citect OPC driver
2. Install the MX Component software by Mitsubishi and use the Citect MXCOMP driver
3. Attach an Ethernet Module (FX3U-ENET) to the FX3U and use the Citect MELSCNET driver

Melsec, FX3U, FX2N, MELSFX2N, Internal Relays