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.

  • \$\begingroup\$ What's the power consumption while up and running? \$\endgroup\$
    – winny
    May 30 '18 at 14:29
  • \$\begingroup\$ I have no idea since I have no documentation for the board (it was bought as-is on ebay and I need to have a customer support number to get the docs). Since I don't have the pinout, I cannot power it up and measure it myself either. \$\endgroup\$ May 30 '18 at 16:27
  • \$\begingroup\$ Right. That complicates things. Do you know the Vcc voltage? \$\endgroup\$
    – winny
    May 30 '18 at 16:45
  • \$\begingroup\$ 18 to 32 VDC input, 24Vdc/1A outputs (8 channel output module) \$\endgroup\$ May 31 '18 at 13:03
  • \$\begingroup\$ That's not the consumption of the board. \$\endgroup\$
    – winny
    May 31 '18 at 13:36

With an unpowered populated board, you may get all sorts of odd resistance readings between pins. Each IC will have protection diodes which could conduct at resistance-reading voltages, and may have internal resistors connected to pins which conduct at any voltage. Try altering the measurement conditions to distinguish these. The first simple test is to reverse the meter probes. A 515ohm resistor will behave according to Ohm's law if you add another resistor in parallel and then one in series, whereas diodes tend not to.

The implication being, as Chris Knudsen has pointed out explicitly, do it without components on the board if possible.

  • 2
    \$\begingroup\$ If possible, un-populate, or get an unpopulated version of the board, and ring out the nets on the bare pcb. \$\endgroup\$ May 30 '18 at 13:25

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