Experienced repair technicians have identified recurring failure points in the FX-ISB-8X-3 design:
On the isolated side, the optocoupler's phototransistor will pull a line low when active. Look for a pull-up resistor (e.g., 4.7kΩ) connected to the isolated VCC. If the input LED flashes but the phototransistor does not toggle, suspect a failed optocoupler. fx-isb-8x-3 schematic