Hello waypayk:
Have a look at the Cubloc PLC boards, and using Modbus.
www.cubloc.com The Modbus works through the Comm port.
I am using (2) USB > RS232 convertor cables on one computer that does not have a Comm port, and 1 Comm port plus 1 USB convertor cable on another back-up computer
I am using a "Cubase-32M board, and it has 16 inputs (24v), and 12 NPN transistor outputs. It has (6) analog inputs, which I am using for Feedrate over, Jog rate, and Rapid rate; plus a 20 key keypad. These I/O go through brains, which in turn activate OEM codes.
I have a bunch of panel pushbuttons on the inputs, which are using the Mach3 ports and pins, but with the ladder logic, many more outputs can be done, which in turn can send info back to Mach3.
The -64M board has 32 inputs (24v), and 32 NPN transistor outputs.
The -40M board has 24 inputs (24v), and 16 relay outputs.
They also have relay boards, of banks of 4, and 8 relays that will plug into the above boards.
There is a "learning curve" to get it working, but the Modbus adds considerable power.
Lots of possibilities.