The design of a novel process computer for a scanning tunnelling microscope is described. Our stabilisation system of a tunnel junction is based on a digital signal processor DSP32C from AT&T, mounted on a commercially available PC AT-compatible plug-in card. The resolution of the homemade electronics (I/O-system, high-voltage amplifier) comes up to 18 bit for the z direction and 16 bit for the x/y direction. Hardware-offset adjustment and sample-tilt compensation are implemented. Test measurements confirm the high-resolution of our novel control unit. The concept could also be used for different scanning probe microscopes.