Simple and compact PCB to replace the C64 Kernal (this is how Commodore spell Kernel) ROM with a 27C256 with the original ROM dump and 3 additional alternative ROMs.
The project is constructed in Kicad. The output files are generated in Gerber format.
The PCB replaces the Kernal ROM. The PCB plugs in to the Kernal ROM socket. Two switches or a rotary switch may be used to switch between the alternative ROMs.
The EPROM is segmented as follows:
- 0k-8k Original Kernal ROM (bank 1) Address line 13 LOW; Address line 14 LOW
- 8k-16k Jiffy DOS (bank 2) Address line 13 HIGH; Address line 14 LOW
- 16k-24k Cockroach Turbo ROM [nostalgia reasons] (bank 3) Address line 13 LOW; Address line 14 HIGH
- 24k-32k Dolphin DOS (bank 4) Address line 13 HIGH; Address line 14 HIGH
The ROM configuration I have used is as follows:
- 0k-8k kernal.901227-03
- 8k-16k JiffyDOS_C64
- 16k-24k cockroach_turbo_rom
- 24k-32k dolphin_dos-30
A switch is connected between +5v and address lines 13 adn 14.
2 x 20 contact (only need 12 contact by 20 is more common) Board-to-Board through hole straight pin headers. For example https://au.element14.com/amp-te-connectivity/2-826629-0/btb-conn-header-20pos-1row-2-54mm/dp/1580047 These are soldered to the 2 rows of 12 through-holes on the underside of the board and are used to plug in to the Kernal ROM socket. Solder these first.
1 x 28 contact DIP 2.5mm x 15.24mm socket. The socket should slightly stand off the board to not interfere with the pin headers. For example https://au.element14.com/multicomp/2227mc-28-06-07-f1/socket-ic-dil-0-6-28way/dp/1103852 This soldered to the topside of the PCB for mounting the 27C256.
1 x 4 position, minimum 2 pole rotary switch or 2 x single throw double miniature toggle switches For example https://au.mouser.com/ProductDetail/Alpha-Taiwan/SR2511F-0304-19R0B-E9-N-W-159 OR https://au.element14.com/e-switch/100sp1t1b1m1qeh/toggle-switch-spdt-5a-120vac-28vdc/dp/2522278
1 x 27C256 EPROM (or compatible - eg AT28C256 will also work) I've used a write once compatible EPROM here.
If a 4 position rotary switch is used, it is wired as follows:
- Pole A: connected to address line 13
- Pole B: connected to address line 14
- Pins 1, 3, 5 and 6 connected to GND
- Pins 2, 4, 7 and 8 connected to +5V
This should result in the following truth table:
| Pos | A13 | A14 | Why |
|---|---|---|---|
| 1 | L | L | Pin 1 (Pole A Way 1) Low, Pin 5 (Pole B Way 1) Low |
| 2 | H | L | Pin 2 (Pole A Way 2) High, Pin 6 (Pole B Way 2) Low |
| 3 | L | H | Pin 3 (Pole A Way 3) Low, Pin 7 (Pole B Way 3) High |
| 4 | H | H | Pin 4 (Pole A Way 4) High, Pin 8 (Pole B Way 4) High |
If using two single throw double pole switches, wire as follows:
- Switch 1: Center pin to address line 13, top pin to GND, bottom pin to +5V.
- Switch 2: Center pin to address line 14, top pin to GND, bottom pin to +5V.