The Mitsubishi FX3U-80MT/DSS is a MELSEC-F-series main unit with 40 digital inputs at DC 24 V and 40 transistor source outputs, all integrated on a single base. The unit is rated for 256 local I/O points per the specification, expandable to 384 points with remote I/O. Program memory is 64K steps of RAM, backed by a buffer battery to retain the program during power loss.
Comparison with FX5U-80MT/ESS
A buyer evaluating the FX5U-80MT/ESS as a functional second source should note the primary circuit difference: the FX5U operates on 100 to 240 V AC, whereas the FX3U-80MT/DSS requires a dedicated 24 V DC supply. The output configuration also diverges — the FX3U is transistor/source, the FX5U is transistor/source but with AC power. A direct drop-in replacement without rewiring the power supply and verifying I/O compatibility is not supported by the specification. For a BOM line specified around the FX5U, a full panel power redesign would be needed.
Hardware Integration
The unit consumes 45 W at full load and is rated for an ambient temperature range of 0 to 55°C with an IP10 protection class, suitable for panel-mount applications without washdown exposure. RS-422 is provided as standard; USB is listed as an option and must be verified on the specific unit if required. The buffer battery ensures program retention during extended power-off intervals, though the RAM-based memory should be backed up externally for critical systems.