Identifying the 4 pins on a 4-pin connector
Is it possible (and safe for the electronics with the power OFF) to identify the 2 signal pins on a 4-pin connector, located on a mother board, by checking resistance between the suspected signal pins and the ground pin? I have already checked the pins with a voltmeter [while there was power to the mother board] and identified: Pin 1: +5VDC, Pin 2: Ground, Pin 3: +5VDC, Pin 4: +5VDC. Logically, the signal pins will have pull-up resistors, so I’m guessing that the two signal pins will have equal resistance to ground.
Extending this logic, the odd +5VDC pin will be the power pin to the infrared LED on the encoder board. Naturally, from the order of the pins it would be logical that the signal pins are Pin 3 and Pin 4, but I want to confirm this as the pin order on the encoder board is DEFINITELY different (i.e. Pin 1: Ground, Pin 2: signal, Pin 3: signal, Pin 4: Power). I’ve already worked through the encoder board and verified pin order and proper function with the help of this forum (link to that question below, but again the encoder board has been verified for both function and pinout).
So again, is there any reason that simply checking the pins on the motherboard with an Ohmmeter and declaring that the two pins with [near] equal resistance to ground are the signal pins? This may be a dumb question, but I’m trying to be extremely careful not to damage anything because the replacement of the custom mother board is very expensive, and I’m a hobbyist with limited practical experience.
Thanks in advance and great appreciation for the help so far!
Additional Detail (only if you’re interested)
Mother Board: Controls a piece of industrial equipment; board is located on a control pedestal
Optical Encoder Board: Located on the machine. Optical Encoder board consists of two TCST2300 components, each of which comprises an infrared LED shining onto a photoresistor. Here is a link to the question and answer related to understanding and testing the optical encoder board: TCST2300 Optical Sensor w/ Phototransistor Output - Rotary Encoder: Trying to Understand & Test
The machine I’m repairing is an Ercolina CE50H3 Ring Roller equipped with an NC Controller. Mine appears to be over 20 years old, so it’s not exactly like the one in picture: different colour scheme, not nearly as clean as a new unit, BUT it appears to have the same controls including the "RC-100" designation on the pedestal controller (i.e. https://ercolina-usa.com/ercolina-machines/angle-rolls-section-benders/ce50-angle-roll/)
