Because of a different project of mine, I started designing a new daughterboard for the Fiero ECM, featuring a single 32K Flash-PROM instead of the built-in ROMs and Calibration PROM, plus 2K of RAM and a J1850 transceiver. With this daughterboard this will probably be the only Fiero with an original engine, ECM and calibration featuring a J1850 diagnostic bus.
The new daughterboard is the same size as the original board, and I unsoldered some parts (mainly the CPU, CALPAK socket and ribbon connectors) from the old daughterboard. The PROM chip was dropped and replaced by the 32K Flash-PROM. This PROM is large enough to hold the 8K original code contained in two 4K ROM chips on the main board as well. Now the flash can be programmed either off circuit in a programmer, or even in-circuit by the ECM's CPU! This shouldn't be done while the engine is running though, because 64 bytes of ROM aren't accessible while the programming is in progress.
To allow development of a J1850 handler RAM was desparately needed, so a 2K RAM chip was included in the design as well.
Not visible in the picture is the Motorola MC68HC58 J1850 (OBD2) Data Link Controller. I will (probably) not implement a full J1850 handler though, but just a few report messages like RPM, throttle position and current speed. While there is more than enough space to implement a full J1850 handler, I don't have the time to do it, and I don't know if the old CPU has enough processing power left to handle all those messages.