25 PIN D-SUB MALE at the Amiga
DZM 12 DREH at the C64 UserPort
ParaLoad is a homebrew parallel cable that connects a Commodore 64 to an Amiga computer, allowing fast-loader cartridge software to transfer data at speeds well beyond the C64’s native serial bus. The C64 end attaches to the 24-pin DZM 12 DREH card-edge User Port, where CIA-2 (MOS 6526) drives Port B lines PB0–PB7 and Port A bit 2 (PA2), with /FLAG2 providing a handshake interrupt on falling edge. The Amiga end uses the 25-pin D-sub parallel port, driven by CIA-A Port B; the A1000 carries a male DB-25P connector, whilst A500, A2000, and A3000 models carry a female DB-25S.
| C64 | Amiga |
|---|---|
| Ground | A |
| FLAG2 | B |
| PB0 | C |
| PB1 | D |
| PB2 | E |
| PB3 | F |
| PB4 | H |
| PB5 | J |
| PB6 | K |
| PB7 | L |
| PA2 | M |
Signals
- Ground — Signal ground; User Port pin A (also pin N and pin 1 on the top row).
- FLAG2 — CIA-2 flag input (/FLAG2); sets an interrupt in the CIA-2 ICR on a falling edge; used as a handshake line from the Amiga to the C64.
- PB0–PB7 — Port B bits 0–7 of CIA-2 (6526); individually programmable as input or output via the Data Direction Register at $DD03; carry the 8-bit parallel data between machines.
- PA2 — Bit 2 of CIA-2 Port A; used as an auxiliary control or status line by fast-loader software.
Notes
- The C64 User Port is a 24-pin DZM 12 DREH card-edge connector with 3.96 mm pitch and 1.57 mm card thickness; pins are lettered A–N on the bottom edge and 1–12 on the top edge.
- The Amiga A1000 has a male DB-25P parallel connector with +5 V on pin 23 and ground on pins 14–22. The A500, A2000, and A3000 have a female DB-25S with ground on pin 23 and +5 V pull-up on pin 14. Using an A500-standard cable on an A1000 without an adapter risks damage due to this pin-23 voltage difference.
- PB0–PB7 on the C64 are driven by CIA-2 (U2, 6526) at I/O address $DD01; the data direction register is at $DD03. Each line is independently configurable as input or output.
- /PC2 on the C64 User Port (pin 8, top row) pulses low for one machine cycle after any read or write to CIA-2 Port B; this pin is not part of the ParaLoad cable but is relevant when adapting other parallel interfaces.
- The Amiga parallel port CIA-A Port B is at address $BFE001; the /DRDY* and /ACK* lines (pins 1 and 10) provide Centronics-style handshaking in printer mode but usage in ParaLoad is governed by the fast-loader software protocol.