RC6502-Apple-1-Replica/Bus.md

RC6502 Bus

The RC6502 system uses a 39-pin bus; the physical layout is a single row of 0.05" square header pins on 0.1" centers. Conventionally the boards have a male right-angle connector on one edge and these are plugged into a backplane with female connectors.

The pinout is as follows. The signal directions (input or output from the point of view of CPU and peripheral boards, respectively) are guidelines, and a specific board might have a different direction. For signals where both are marked in, both CPU and peripheral boards are usually capable of using the signal as an input, but it's expected that one board in the system will be generating the signal.

Signal  Pin  CPU Periph
  Name  No.  Dir. Dir.   Notes
--------------------------------------------------------------
   A15   1   out   in
     …  …
    A0  16   out   in
   GND  17   in    in
   Vcc  18   in    in
 Φ2out  19   in    in
/RESET  20   in    in    Must be actively controlled by one board
  Φ0in  21   in    in
  /IRQ  22   in    in    Pull-up req'd; usually provided by CPU board
 Φ1out  23   in    in    Sometimes called EX0
   R/W̅  24   out   in
   RDY  25   in    out
  SYNC  26   out   in
    D0  27   ↔     ↔
     …  …
    D7  34   ↔     ↔
    TX  35   ?     ?
    RX  36   ?     ?
  /NMI  37   in    out   Pull-up req'd; usually provided by CPU board
     ×  38               Also known as EX1
     ×  39               Also known as EX2