A Microprocessor Based Fault Finder

A microprocessor based "first out" fault annunciator that uses the MC6802 is discussed in this paper. The design objectives are outlined. A cost effective unit was needed to replace an existing hand-wired relay design. The hardware description includes a discussion of the input isolation and filtering circuitry. The software discussion covers special software control features. Flow charts of the programs are included, and discussed. Extensive use of the MC6802 interrupt structure permitted a minimum device count system. Use of the dual direction interrupt port on the MC6821 has permitted slaving up to 5 units on a "single wire". The unit has proven to be a reliable cost effective replacement for the old relay design.