I recently programmed my FPGA with a logic analyzer core for the purpose of reverse engineering a crappy modem I have. I found on it what I think is either JTAG or TTL-serial pads. There are 4 blank holes and from what I can tell, 2 pins go to the processor and the other two pins go to power(3.3V) and ground.
The ground pin I'm for sure is ground however. It has near 0 resistance with one of the pins from the power supply. (I assume it's connected through a ground-plane). All of my measurements with a multi-meter seemed to say nothing exceeds 3.3V off of these pins
Now, my problem: I want to hook an FPGA up to this to figure out what this port actually does. The FPGA is hooked up to my computer and gets it's power over USB. I tried measuring with a multimeter what the ground difference is(GND from FPGA to GND of modem) and I got back a surprising result. There appears to be a difference of about 20-23V.. I sure don't want to hook my 3.3V FPGA up to this with that kind of voltage.
So what exactly should I do? I assume the difference is because the power supply of the modem is not grounded(no third prong)
How do I safely hook my FPGA up to this externally supplied modem without frying the modem or my FPGA?