Tone tracing PCB

[nicklinn]

Hello,

I have a PCB (lots of layers) with some large value FPGAs that I would really love to play around with (powers on, voltages good, clocks tick).  The problem is that I am having a hard time finding where the JTAG lines go.  I have managed to find TDI, TDO and TMS on one device using continuity check on multimeter but TCL is proving elusive (and haven't worked out the other chip at all).  I have access to the balls of the device through vias on the underside of the board so I could bodge some wire to it, but would much rather find out what is actually going on.

So I have one of those telecom tone tracers, but I am kinda leary using it directly since the output is around 9V and that high of voltage is liable to cause damage, but building an injector circuit that uses a lower voltage (while using the more complex detector) doesn't seem that hard.

Anyway before I spend a ton of time on this thought would seek some feedback, has anyone ever done this successfully (or unsuccessfully for that matter), or any thoughts of if this will even work?

Thanks for your time guys,
Nick

[nicklinn]

...
You can buy a useful FPGA development kit for $40-$150 range.  What is your time worth in terms of reverse engineering this?

Guess I should have expected this.... I will say that I do know what I am doing and I have the tools to do it.  The board isn't a consumer/telecom type board it's a project dev board that outgrew the project.  I have several excellent FPGA boards (dsp and uC too) and love them.  Is it worth it?  lol of course not but I have it and it sounds like fun so what the hell.

If you want to trace things with logic safe low voltages you should probably stick to around 0.15V or so.  Not enough to turn on even Schottky diodes very much so it shouldn't hurt anything assuming it is also applied in an ESD safe manner.

Thanks for your reply.