Check with the logic vendor and make certain you don't violate any of their recommendations. JTAG circuits aren't hard to build but you MUST do it the way the mfg recommends, else you're on your own.

It's the same way with those infamous ISP circuits. You can build whatever you want to build, but don't expect it to work if you don't follow the mfg's recommendations, EXACTLY. Don't substitute a different part. Don't change the supply voltage. Do it the way the mfg says to do it!

I've had a number (12 or so) of cables, adapters, etc, and each one has worked because it's built to the mfg's spec's. Once you deviate from those spec's, you're at risk of random malfunction, and will be alone and helpless.

RE