I have currently 6 faulty BK3120 couplers, that behave all the same as described by the author. After connecting the supply voltage, only the I/O ERR diode is lit (and the green for power). The Master PLC doesn't see the device. When I connect new BK3120 to supply, it blinks all diodes first and the RUN stays on. You don't have to connect anything else, just 2 wires. I can not see any visibly damaged parts inside, when dismantled. So I suppose it is a controller fault, but I don't understand, why we have so many faulty devices recently.