Hallo, we are thinking about adding some physical buttons to the CNC to help with placing the head. Since there are no free DINs in the UNO maybe another UNO or NANO connected via serial TX/RX to the one hosting GBRL should do, can we just send Gcode via the serial ports?
On the other hand, since we are thinking of moving to TinyG/DUE there there may be available DINs.
Has anybody done something similar? Any idea, hint?
tx
Simone
You could connect to a 2nd serial port device and then watch inbound data. This is how the Laser Solder Widget works, or the Texterator workspace.
You could use an analog input and a few resistor values to detect one keypress out of a set of several, a bit like this http://www.instructables.com/id/How-to-access-5-buttons-through-1-Arduino-input/step3/The-theory-Multiple-buttons-on-one-pin/
Not endorsing that solution here, but resistor ladders are fun 
If you get one of the unos that breaks out analog 6 and 7, move probe to one of those, then use the i2c bus with an IO expander to get unlimited buttons.
(Or if you don’t need probe, you can use the i2c bus on a stock uno)