Found something: no 5 V on the I/O board

The fuses all passed a continuity test in August, but to rule them out properly I swapped every one for a new fuse. Same relay click, same two beeps, dark keypad, no homing.

Then I bridged the interlock loop at terminal block X111, pin 1 to pin 6, which bypasses the emergency stop and all eight door switches. No change.

The 48 V supply arrives at the I/O board: 47.5 V on the X2 wires. Then I measured all 16 pins of the keypad connector X22 against ground with the machine on. No pin shows 5 V: one is at 3.33 V, three at 1.05 V, four at 0.48 V, the rest at 0 V. If the keypad is supposed to get 5 V from here, it isn’t. So I measured the supply pins of the logic chips directly. CPU board: the 74AHCT574s get 5.04 V and the 74AHC574s get 3.33 V, both rails fine. I/O board: the 74AHC541s get 3.3 V, but a 74AHCT541 gets 0.18 V. The 5 V rail on the I/O board is dead. That would explain a dark keypad and dead motors while the CPU happily boots and beeps. Power off, the same supply pin reads 2.2 Ω to ground. The rail is shorted. I unplugged every cable from the I/O board except the 48 V input and pulled the CPU board: still shorted. So the fault is a part on the I/O board. It’s on my bench now, photos in the gallery, including an annotated one with the parts I’m checking. The back had white crusty residue around the serial and round connector pins, probably old flux. I cleaned it off with isopropyl alcohol. Next: find the shorted part.