What this block does and what it demands from your FX3U rack
The FX3U-4LC is a dedicated 4-channel temperature controller that rides the FX3U backplane, consuming 8 I/O points from either input or output budget. It handles two-position, PID, heating/cooling PID, or cascade control with a control period of 250 ms per all four channels. Power comes from two rails: 160 mA at 5 V DC drawn from the main unit via the backplane, plus an external 24 V DC supply (rated +20% / -15%) at 50 mA for the loop interface driving the sensors and actuators on the terminal block. A built-in EEPROM stores parameters – rated for 100.000 write cycles – so setpoint and PID values survive power loss without a battery.
How it compares to the FX5-4LC
They are not drop-in interchangeable. The FX3U-4LC expects an FX3U base unit and its 5 V backplane power draw; the FX5 variant connects to the FX5 system bus and has different terminal layout. A panel specified for FX5-4LC would require adapter wiring and a programming migration.