I have a grblshield v5 and I'm looking to hook up an external microcontroller to handle the serial connection instead of driving it over USB. I soldered some headers onto the tx/rx pin breakouts but I can't seem to get a serial connection when the shield is plugged into the arduino. Things work fine when I wire it up directly on the arduino, but not passed through the grblshield.
Is this something that should work (it seems reasonable to me since I thought those pins were the same as the tx/rx pair coming from the USB)? Is there something else I'm missing?