Preserving test program set (TPS) performance after legacy automated test equipment (ATE) upgrade

Legacy ATE typically requires economical obsolescence upgrades ranging from minor to major at various points during its life cycle, due to availability and viability of repair assets. System level replacement costs are generally prohibitive. For mature systems where significant investments in TPS software and hardware have been made, there is substantial incentive to maintain compatibility with legacy operation in order to preserve the cumulative investment in Test Program Set (TPS) assets. The ATE assets may be obsolete for many reasons, but shifts in microcircuit level technology seem to be a recurrent theme. Therefore, the replacement asset may operate significantly different internally than the legacy asset even though it may be advertised as a plug and play replacement. We will follow a major obsolescence upgrade for the Avionics Intermediate Shop (AIS), where three automated stations are being upgraded after more than thirty years in the field. While TPS changes are allowed where absolutely required, we determined that the lowest risk and lowest cost approach was to preserve legacy performance at the asset level where possible. Assets to be replaced include the computer, display, printer, digital multimeter, frequency counter, waveform digitizer, spectrum analyzer, waveform generators, synchro generator, synchro receiver, digitizer calibrator, pneumatic controller and pneumatic pump. The legacy computer precedes the microprocessor era and does not employ standardized word sizes, which presents some interesting design challenges. The computer communicates via custom designed buses, mostly clocked serial although parallel direct memory address (DMA) buses were used for critical high throughput instruments. The test programs are written in F-15 Adapted PLACE ATLAS (FAPA), which the offline compiler converts into assembly language then machine code for the legacy computer. Aspects of the test programs are timing critical, so an approach using indeterminate software emulation of the legacy computer was not completely compatible with the existing TPSs due to instabilities. The original AIS instruments were largely custom designed for military usage before commercial instruments with extended operating ranges were available. These legacy instruments preceded many of the modern standard building block performance envelopes, so unusual features required innovative modern instrument selections. This paper describes the Boeing determinate hardware based design approach which preserves the legacy computer timing. Modern instrument selection examples to match legacy instrument performance are discussed as space permits.