I have a DRO-PROS unit with glass scales that I hooked to Mach3. I had to use a logic chip to drive the signals without taking much current. You can show the DRO coordinates or even the error between them. There are lots of ways to create quasi-feedback. You can use the macro pump to go through a reset routine 5 times a second or you can call out an M-code at points in the Gcode. The macro can keep adjusting the Mach3 DRO to match the encoders if the error is above a certain limit. The problem is you kind of lose track of zero. You can also get information from less expensive DROs with scales like calipers have. I have these scales on another mill but haven't tried connecting to Mach3. If your machine uses the 3-lug drives on the handles, I have all kinds of parts for driving these with stepper motors, including complete CNC packages. If interested contact me at
pthomps2@gmail.com