acorn_system_6502A_CPU_kicad

Acorn System 6502A clone card

This is a clone of the Acorn 6502A 1/2/3 MHz card for the System Microcomputer range.

It is not a perfect copy:

• some routing (especially on the front copper plane) is not a perfect copy
• some "extra wiggles" on the Acorn card have not been copied
• some fixes have been added (see below)

It is intended that as it stands this is a drop-in replacement for the original card. If the fix jumpers are left alone then it should function identically.

Fixes

Two extra decoupling capacitors and some extra routing has been added. C10 is added close to the CPU. Another C11 is close to IC12, for this to be effective an extra wire may need to be added on the underside of the board to connect VCC direct to the capacitor. It is not clear whether either of these are necessary but some pictures show an extra capacitor tacked onto IC12.

CPU card always to Generate data stobes

On the original card the NRDS and NWDS data strobe signals are not always generated by the CPU card but are set to tri-state when the bus is granted to another card. This would require any DMA card to contains identical circuitry to the CPU card. An optional link LKD1 can be cut and re-made to have the CPU card always generate these signals.

Data buffer direction on bus grant

There is seemingly a bug in the original card in that the direction is not reversed when the bus is granted and local (to the CPU card) resources are being accessed by another bus master. (The buffer enable signal is however correctly reversed)

LKD2 if broken and remade will enable the buffer direction to be reversed. This should allow local resources (ROM, 6116, VIA) to be accessed by alternative bus masters.