What you're looking at
The FX3U-16MT/ESS is a Mitsubishi MELSEC-F base unit with 8 DC 24 V inputs and 8 transistor source outputs, running on 100-240 VAC. It's part of the FX3U family, a mature platform that's been around long enough to have good documentation and a steady support base. The program memory is 64k steps in RAM, with a buffer battery to hold it, and the unit draws 30 W at the terminals. Cycle times are 65 ns for LD and 642 ns for MOV. Local I/O capacity is 256 points; total with remote is 384 points.
The FX5 series (like the FX5-32ET/ES or FX5U-64MT/DS) is the current generation, but don't expect a straight swap. The FX5-32ET/ES has 16 inputs and 16 transistor sink outputs, while the FX3U-16MT/ESS has only 8 of each and uses source outputs. The FX5U-64MT/DS is a different form factor — connector-type terminals versus screw terminals on the FX3U. The mounting footprint and backplane are not compatible, so swapping would mean a full rewire and possibly a new panel layout.
Wiring and mounting notes
The unit is IP10 — open style, requires cabinet installation. AC supply range is 100-240 V; it has 8 transistor source outputs. The RS-422 port is on board for programming or connecting to older HMI units. USB is optional — if you need it, you'll order the option module. The expansion bus lets you add analog, temperature, or additional I/O modules up to the 256-point local limit. The buffer battery keeps the RAM program safe during power downs, but if you leave it off for months, plan to reload the program.