An opto-isolator interface circuit designed for testability

The primary requirement called for the design of an interface to connect external binary sensors to a computer system whilst protecting the computer system from any erroneous high voltage levels that may occur, under fault conditions, at the sensor outputs. The sensors were to be used as part of a diagnostic system on a machine tool, to locate in the event of machine tool failure, the failed replaceable unit. The author summarizes the design of a simple line replaceable opto-isolator interface circuit incorporating on-line testability to improve its maintainability