Remote terminal fault detection

The fault detection, location and isolation methods used in the Multiplexer/Demultiplexer (MDM) for the Space Shuttle are discussed. The MDM is a remote terminal with a duplex structure having interchangeable input/output interface modules. An isolated serial bus structure is used internally to prevent failures in any given subassembly from faulting communications or operation of any other subassembly. Redundant control functions are cross-strapped, as are the power supplies. The test philosophy of the MDM involves the injection of faults to activate detection. All fault injection and detection circuit paths are analyzed to prevent the disguise of latent failure conditions. Fault detection is reported to the bus control computer which also participates in specific tests involving data. All inputs/outputs are testable to the limits of normal operating range, and the MDM structure allows all single-point failures to be bypassed for continued operation. The fault detection system's impact is felt in hardware, single-string reliability, and test time.