Wang 600 Wire Braid ROM
The 600 series microcode is one bit narrower than the 700 series, being 42 bits wide. The ROM shown here is fully populated for addressing, resulting in 2048 words of 42 bits, 86016 bits or 10.7k bytes.
The sense circuit and outputs are different to the 700 series. The 600 series uses integrated circuit latches connected directly to the sense coil windings. This eliminates the amplifier transistors, provides output latching on the ROM board and also permits a more sophisticated ‘strobed’ form of gating on the sense latches that helps to improve noise immunity.
This ROM had a dead bit due to a failed latch IC, which was repaired with a NOS replacement.
Readout with the ROM Reader then showed three addresses with all zeros and continuity testing showed that the address wires were open circuit for each of these addresses. Careful inspection suggests that it would be difficult to replace these wires, with a high risk of doing more damage. Therefore, this ROM has been left in situ but replaced electrically with a ROM emulator. This has allowed the rest of the machine to be restored to full function and also provides alternative microcodes including diagnostic functions.