Overview
The 6502-COB (Computer on a Backplane) is the full-featured, modular desktop member of A.C. Wright's AC6502 family. A real 65C02 CPU plugs into a passive backplane alongside a stack of swappable expansion cards — memory, video, sound, storage, serial, GPIO, real-time clock and more — so the machine can be configured and expanded just like the classic S-100 and bus-card computers of the 1970s and 80s.
History & Design
The COB is the "kitchen sink" of the AC6502 ecosystem: where the all-in-one ACE squeezes everything onto one board, the COB spreads each function across its own KiCad-designed card. Cards share a common 16-bit address bus, 8-bit data bus, and the standard 65C02 control signals, all defined by one ecosystem-wide memory map. Every AC6502 machine runs the same shared BIOS — kernel, machine-code monitor, and BASIC — so software written on the COB runs unchanged on its siblings.
System Specifications
| CPU | WDC 65C02 @ 1 MHz (65816 card optional) |
| RAM | 32KB base + up to 512KB banked (RAM Card) |
| ROM | 32KB EEPROM — 6502 BIOS (Memory Card) |
| Video | VGA 640×480 (Pico9918) or composite (TMS9918A) |
| Audio | 3-voice SID via ARMSID (Sound Card) |
| Storage | CompactFlash or SD (up to 128GB) |
| Serial | 65C51 ACIA + MAX232 (Serial Card) |
| GPIO | 65C22 VIA — keyboards, joysticks, timers |
| RTC | DS1511Y, battery-backed (RTC Card) |
| Expansion | Backplane — up to 12 card slots |
| Input | PS/2, 8×8 matrix keyboard, Atari joysticks |
At the machine
This COB has a small white reset button on the backplane board near the front. Press it to restart the machine back to the OK prompt — it leaves memory alone, so your program is still there afterwards. Take a look at the row of cards plugged into the backplane — each one adds a different capability to the computer.
The AC6502 Family
| System | Role |
|---|
| COB | Modular backplane desktop (this machine) |
| ACE | All-in-one single-board computer |
| KIM | Keypad + LCD add-on that turns an ACE into a KIM-1 |
| VCS | Cartridge-based game console |
| DEV | Emulation-based development system |
All five machines run the same shared 6502 BIOS, so a program written on one runs on them all.
Memory Map
| Range | Contents |
|---|
| $0000–$07FF | Zero page, stack, input & system variables |
| $0800–$7FFF | Program / BASIC RAM (30 KB) |
| $8000–$9FFF | I/O hardware — eight card slots |
| $A000–$B7FF | Kernal + jump table |
| $B800–$BFFF | Character set ROM |
| $C000–$EDFF | BASIC interpreter |
| $EE00–$FEFF | Machine-code Monitor |
| $FF00–$FFF9 | Wozmon (mini monitor) |
| $FFFA–$FFFF | CPU vectors (NMI, RESET, IRQ) |
Did You Know?
The backplane is completely passive — just wires. Every active part lives on a card you can pull out and swap.
Drop in the optional 65C816 card and the same machine gains a 16-bit CPU — the chip at the heart of the Apple IIGS and Super Nintendo.