External stepper drivers were chosen, so an UNO Shield with Screw terminals was wished for.
A shield was found (on the internet), with screw terminals (so it would be easy to connect the Arduino pins to the opto-coupler input ports on the external stepper drivers). But alas, it did not seem to be available for purchase any more.
So I have made an attempt at designing a simple UNO Shield, that has screw terminals, and opto-coupler input on the XYZ-limits as well as the three input buttons, that are used with GRBL.
Turns out that default GRBL 1.1 has Spindel PWM enabled by default, and therefore pins 11 and 12 are swapped. Some cutting traces, and soldering jumpers, and we are up and running.
Forgot to do a GND trace to one of the screw terminal pins. Easily fixed.
Here are fully updated schematics: cnc-grbl-uno-ver.1.pdf
When I went to implement the two changes for the PCB, I got caught up in improving other aspects as well, and now have a version 2 almost ready for PCB production.
However, about ver.1 and driving 3 Enable ports! Could potentially mean 60mA draw from one Arduino pin. That is 50% above specs. So now there is a driver Mosfet in place. And the schematics file has been updated.
Also the DC-DC module has been trimmed down to give only 7V on Vin
The front labels have also been updated, to accommodate that 'Probe' is actually connected to A5 in GRBL 1.1