This is the 6502 second processor circuit board. The main features are the CMOS 6502 CPU (as opposed to the original NMOS Beeb ones) which runs at 3MHz, a 50% increase on the BBC B processor, the Ferranti Tube ULA, and a Tube boot ROM.
In addition, there is 64K of RAM and various logic circuitry.
This is an Issue 4 board. In fact, all of my 6502 cheese wedges are issue 4s so I wonder if any earlier or later issues were in common use.
There are various links which can be set on the board. One of these allows the second processor to take its power from the BBC Micro via the Tube ribbon cable, instead of its own power supply. Another link allows a 4MHz 6502 processor to be used correctly.
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