Williams Stargate R8948 and R8731 Rev B CPU Board BS.BA Inversion Modification
Stargate and early Robotron games originally shipped with the R8948 CPU board, later games shipped with the R8731 Rev B CPU board. Unmodified these boards will run Stargate without any issues but to run Robotron and later games the BS.BA Inversion Modification is required. R8731 Rev B CPU boards shipped with the modification already carried but the process is identical to that for the R8948 CPU board if it is missing from your board.
Why do later games not run on an R8948 CPU board? If you examine the schematics, the BS.BA signal is inverted on R8731 revision B and later CPU boards but not on the R8948 CPU board. Stargate ROM boards do not use the BS.BA signal so this modification will have no affect for Stargate.
Follow the steps below to apply this modification to your board. The full process and suggested cable routing is detailed in the R8731 Rev B Assembly Drawing.
Not sure what board revision you have?
Modification Step 1 - Cut PCB track carrying BS.BA signal:
The BS.BA signal that we want to invert is routed from pin-3 of IC 6F to pin-32 of connector 1J4 (the header of the ROM board ribbon cable). The first step is to cut the track carrying the signal. The easiest place to do this is at pin-3 of IC 6F.
The picture shows the track location on the reverse of the board, cut the track anywhere highlighted in red.
Modification Step 2 - Cut PCB track grounding IC 5A pin-3:
To invert the BS.BA signal we will be using IC 5A which is a 7404 hex inverter. We will be using pin-3 as the input and pin-4 as the output. The boards solder tracks grounds pin-3 so before we can use it we need to cut the track grounding it.
The picture shows the location, cut the track between the track going past and pin-3 of 5A.
Modification Step 3 - Connect the wires to IC 5A pin-3 and pin-4:
Solder 2 wires to pin-3 and pin-4 of IC 5A respectively, these will route the BS.BA signal to IC 5A and the inverted BS.BA signal from it. The Williams factory modification used 30AWG wire so there is no need for anything heavy duty here.
The picture shows IC 5A with the two wires attached.
Modification Step 4 - Connect the wire to IC 6F pin-3:
Solder the wire coming from IC 5A pin-3 to IC 6F pin-3, this will route the BS.BA signal to IC 5A for inversion.
The picture shows IC 6F with the wire attached.
Modification Step 5 - Connect the wire to connector 1J4 pin-32:
Solder the wire coming from IC 5A pin-4 to the plate through hole shown in the picture, this will route the /BS.BA signal to pin-32 of connector 1J4.
The picture shows the plate through hole location with the wire attached.
Finished
That's it, you are done! Now all you have to do is recheck your work and try the modified board with a Robotron, Joust, Sinistar or Splat! ROM board.
The image shows a modified board with the routing detailed in the R8731 Rev B Assembly Drawing.
