36 PIN CENTRONICS MALE at the Printer
The geoCable (originally a Berkeley Softworks product from the late 1980s, later sold by Creative Micro Designs as the CMD geoCable II) connects the Commodore 64’s 24-pin DZM 12 DREH card-edge User Port to a standard 36-pin Centronics parallel printer connector. It routes CIA-2 Port B lines PB0–PB7 (User Port pins C–L) and PA2 (pin M) directly to the printer port data and control lines, with /FLAG2 (pin B) providing the handshake input. The cable allows GEOS to drive parallel printers at full speed, bypassing the much slower Commodore serial bus. The User Port signals are provided by the MOS 6526 CIA #2 chip, with Port B programmable as input or output via the data direction register at address $DD03.
DZM 12 DREH to the C64 UserPort.
| C64 | Printer |
|---|---|
| Ground | A |
| Flag 2 | B |
| PB0 | C |
| PB1 | D |
| PB2 | E |
| PB3 | F |
| PB4 | H |
| PB5 | J |
| PB6 | K |
| PB7 | L |
| PA2 | M |
| Ground | N |
Signals
- Ground — Signal ground; User Port pins A and N are both ground returns.
- Flag 2 — /FLAG2: active-low handshake input to CIA-2; a falling edge sets the interrupt flag in the CIA-2 Interrupt Control Register. Corresponds to the printer /ACK signal.
- PB0–PB7 — Port B bits 0–7 of CIA #2 (MOS 6526); each line is individually programmable as input or output via the Data Direction Register B at $DD03. Carry the 8-bit data byte to the printer.
- PA2 — Bit 2 of Port A of CIA #2; used as an auxiliary control or strobe line by GEOS printer drivers.
Notes
- The C64 User Port uses a 24-pin DZM 12 DREH card-edge connector (3.96 mm pitch, 1.57 mm card thickness). Bottom-side pins are labelled A–N; top-side pins are numbered 1–12.
- The +5 V supply on User Port pin 2 is rated at 100 mA maximum; the 9 VAC lines (pins 10–11) are also 100 mA maximum. The geoCable carries only signal and ground lines.
- /PC2 (User Port pin 8) pulses low for one machine cycle after any read or write to CIA-2 Port B; some printer driver implementations use this as the data-strobe signal rather than PA2.