Ok this is my first time trying some kind of reverse engineering and I have few experiences in PCB testing so it may be a basic question.
Basically I am trying to get the schematics for an 8 channel output circuit for which I only have the board, with no documentation whatsoever. I have mapped out the VCC and the GND on the input pins by using a ohmmeter to measure resistance between the VCC input of an DAC converter and the input pins. I have mapped the pins for which I get 0 ohms as VCC inputs. I have mapped out the GNDs in a similar manner.
Another thing to know is that the board is not powered up while I am doing these tests. There are no voltages or currents applied anywhere but for the voltmeter I am using.
My question is this:
When I measure resistance between VCC input of the DAC and the pins I have mapped as GNDs, I get 515 ohms of resistance. The same thing happens when I measure resistance between the GND input of the DAC and the VCC pins.
What could this resistance represent? I would have tought that since ground planes and power planes are separate, I would get an overload reading on the ohmmeter?
I am thinking I do not understand power and ground planes correctly.