Last year I wrote an article about the Williams System 3/4 sound board. That board was used in the early solid state Williams pinball machines. Later machines (System 6/7) used what could be considered a more advanced version of this board. The second generation board is very similar to the original board with some changes to the address decoding and the addition of an extension header which was utilized for speech in some games.
The System 6/7 sound board was also used in Williams arcade games (Robotron, Joust, Stargate, Defender, etc…) so there were WAY more of these boards manufactured and these boards tend to be cheaper and easier to find than the older, larger sound boards. The later sound boards were intended to be backward compatible with the earlier games but recently I learned that this is not the case when a reader commented on my System 3/4 sound board article pointing me to a problem that had been discussed a few times on RGP recently. That problem being that the ROMs originally used on the older sound boards don’t work in the newer boards with their ROM socket strapped for the same type of EPROM.
The reason the original ROMs don’t work in the newer board is because the address decoding scheme is less strict on the original board mirroring the ROM into several locations. This wouldn’t be a problem in itself if the programmers had not “cheated” and relied on this fact in their programming. Ultimately it boils down to inconsiderate programming.
There are 2 options to solve this problem. One would be to modify the ROM to work within the address decoding of the newer sound board. The other option is to modify the board to “convert” the newer board to act like an older board by re-routing a couple of pins. The latter is the conversion that I found to be easiest to figure out and that’s the conversion I will cover in this article. Once the board is converted you will be able to use an unmodified System 3/4 sound board ROM.
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