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Wang 600 ROM Emulator


Microcode ROM emulated with UVEPROM

The original wire ROM requires challenging repairs, but accurate dumps of 600 series ROMs are available and later numbers of the 600 series had IC ROMs rather than wire braid.

This suggests the possibility of designing a modern replacement ROM. Such a replacement could hold multiple ROM images that could be hot-swapped to serve up ROMs for different models of machine.

The main design challenge was similar to the ROM reader - providing 42 bits of ROM output. The 600 machine employs simple S-R latches to capture and hold the ROM output for the duration of a microcode cycle so it is important that an emulator provides clean output without glitches that could be incorrectly captured by the latches. This led to a fairly simple design where six 27C256 UVEPROMs provided a total of 48 bits of ROM output. Address updating occurs early in the microcode cycle and the UVEPROM outputs can be tri-stated during this period to avoid glitches. Once the address is updated, the UVEPROMs are enabled and the ROM data appears cleanly, without glitches.

An emulator of this kind offers the possibility of restoring 600 and 700 machines with faulty wire ROMs and also allows different ROM microcodes to be loaded so as to emulate different machine models.

Once again, it was quick and easy to have the PCB made at one of the online fab houses, and for very little money.