Some DRO's simply read out position, with the ability to set the reference, or zero, at some arbitrary location. Other DRO's implement very complex functions, such that they make the user a part of a semi-automated numeric control. Nearly all DRO's use linear quadrature encoders to pick up position information from the machine. These are by far the most expensive part of the system, and the part that would be hard to make by hand. Low resolution Mylar strip scales are now being made by U. S. Digital, with up to 360 lines/inch. These will give 1440 counts/inch, when counting all quadrature transitions.
A PC computer can be used as the quadrature counter of a DRO. A number of public-domain and low-cost programs have been written to read several channels of quadrature encoders through a PC's parallel port, and display position on the screen. There is a possibility that since these signals are only sampled, not watched continuously by dedicated hardware, that some counts could be lost. You can calculate how many times a second you need to sample the encoders to prevent lost countsm and ensure your hardware is adequate to the task for your intended machine tool. In general, an average 80286 system should be overkill, but if the CPU is interrupted from the user program (emulating a DRO) for any extended period, by operating system tasks at a higher priority, then counts could be lost. So, it would be good to carefullly evaluate power management software and any utilities that leave TSR (Terminate and Stay Resident) routines in memory.
A glossary of related terms -