Tom Radom, owner of Jaguar XJ220 chassis #50, has reverse-engineered and rebuilt the SPD-1039 diagnostic interface for the Zytek ECU used in the XJ220, XJ220S, and XJR-15. The original hardware is effectively unobtainable. Without it, owners of roughly 275 XJ220s ever produced cannot diagnose their own cars.
When a check engine light forced the issue, Radom tried opening the original SPD-1039 and failed: the internals were potted in silicone. He pivoted to a logic analyzer, capturing serial traffic between the interface and the ECU. That recorded data was enough to recreate the hardware using an FT232RNL USB-to-serial bridge. The original DOS diagnostic software runs untouched inside an emulator. Only the physical interface layer needed rebuilding, and that is exactly what he rebuilt.
Radom is distributing the replacement modules to XJ220 and XJR-15 owners at no charge, reasoning that diagnostic capability adds more value to these cars than his development costs. The full protocol analysis, hardware choices, and the decision not to rewrite the DOS software are worth reading at Port220.com, especially if you maintain any vehicle running a 1990s ECU with a dead proprietary tool chain.
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