I have replaced the 80s EMCO TM-02 controller in my VMC100 with a MACH3 PC based control system including new electronics. There is a servo controller, stepper controllers, programmable logic controller (PLC) and some power supplies and minor things. Interface to PC is via parallel port, using break-out-board (BOB) from Granite Devices. The original connectors are cut away and cables are connected to new electronics located in a metal cabinet bolted to the left side of the mill. There is also a large transformer for changing 230VAC voltage to 110VAC for the Vexta stepper controllers - this voltage is also handy for the servo power supply, and offers isolation from mains.
The servo controller is a Granite Devices VSD-E 160. It has a simple 150VDC high-voltage power supply based on a bridge rectifier and storage capacitor. PLC controls a relay that short-circuits a current inrush limiting resistor at the DC power supply circuit. There is a bleeder resistor to bleed the charge from capacitor when power is switched off -- the energy stored in the capacitor is easily enough to kill a person. Spindle control is with step/direction control from PC. The tool changer rotation is based on a spindle servo, so spindle had to be a true servo spindle instead of a more simple RPM control. In the servo controller there is scaling of the input pulses so that the full speed of spindle can be reached with the limited pulse rate of PC parallel port. This limits angular accuracy of spindle, but is a necessary compromise.
The stepper controllers are Vexta (Oriental Motor) UDK5128. They can drive the 5-phase penta-connected original Berger-Lahr steppers of the mill from step/direction inputs. I have also added X and Y axis limit switches, as EMCO originally relied on software limits only. The UDK5128 overheat output signal goes to PLC, which also controls the enable input of stepper controllers. The stepper input/output signals are optically isolated so interfacing is simple.
Below is a conceptual schematic of the system (click to enlarge):
