This modular microprocessor system can be used to create a wide range of custom-tailored computer systems out of a standard set of active modules. New modules can be easily incorporated into the system as the requirements of the instrumentation change and as improved integrated circuits become available. A unique feature of this system is the existence of two sets of bus traces on the mother board. The second bus can be used as an extension of the system bus, an interprocessor but in a distributed processing system, a hardware driven peripheral bus, or the system bus of a second microprocessor. The second bus is shown to add substantially to the versatility and power of the system. The second bus also provides the means by which a single processor system can be upgraded into a distributed processor network without making the special interfaces designed for the single processor obsolete.