Our PCB drilling machine provides a link between a Windows PC and the motors and solenoids of the machine itself: a microcontroller integrated with a high current drive circuit.In the previous instalment (March 2001) we described in detail the basic principles of the CNC drilling machine. The controller is just as unconventional: the machine is not driven directly from a PC, but indirectly via a special controller circuit. For trouble-free communication between the Windows PC and the controller over the Centronics interface we need to make use of several tricks that take us beyond the standard Centronics protocol.Let’s start by taking a look at the rather straightforward hardware of the controller board. The basic circuit consists on one side of a microcontroller with a PC interface and on the other the output drive circuits, connected to I/O-port pins. Of course there is rather more to the design than that.