comodore-iec-emu/docs/wiring.md

Wiring — Pi Zero 2 W ↔ Commodore IEC bus

PoC listener (printer) interface. Full rationale: PLAN.md §3 and _research/commodore-iec-serial-bus-2026-06-18.md §1.

⚠️ The Pi GPIO is 3.3 V and NOT 5 V tolerant. The IEC bus idles at 5 V. A level shifter is mandatory — never wire a bus line straight to a Pi pin.

Topology

A single 4-channel bidirectional MOSFET level shifter (BSS138 + pull-ups, the "logic level converter" board) carries all four lines:

• low side → Pi 3.3 V rail
• high side → IEC bus 5 V rail
• DATA is the one bidirectional, open-drain-emulated line.
• ATN / CLK / RESET are input-only (Pi always reads them).
• 100 nF bypass cap across the shifter VCC pins.
• Common ground between Pi GND and IEC pin 2.
• SRQ (IEC pin 1) left unconnected.

The shifter is non-inverting: bus low (asserted) ⇒ Pi reads LOW.

Pin map

IEC signal	IEC DIN pin	Pi GPIO (BCM)	Header pin	Direction
ATN	3	GPIO 2	3	input (via shifter)
CLK	4	GPIO 17	11	input (via shifter)
DATA	5	GPIO 18	12	bidirectional (via shifter)
RESET	6	GPIO 3	5	input (via shifter)
GND	2	GND	6 (or any)	—
SRQ	1	—	—	not connected

DATA open-drain emulation

GPIO 18 is never a push-pull driver of the bus. The module flips its direction:

• assert (pull bus low): GPIO 18 = output LOW → shifter pulls bus to 0 V.
• release / read: GPIO 18 = input (Hi-Z) → bus pull-ups float it to 5 V, and GPIO 18 reads the level the talker (C64) is driving — this is how bits are sampled during receive.

Logical mapping (handled in kernel/iec_lines.h): asserted ⇔ Pi reads LOW.

Bring-up checklist (before connecting the C64)

1. Load the module: sudo insmod kernel/iec_listener.ko.
2. Run the wiring selftest (drives DATA low, releases, reads back; reads sense lines) via the IEC_IOC_SELFTEST ioctl — expect:
   • DATA reads low while driven, high after release (Hi-Z floats up);
   • ATN / CLK / RESET all released (high) with the C64 powered off.
3. Meter/LED-check each line for the expected idle-high.
4. Verify common ground continuity Pi↔IEC pin 2.
5. Only then connect the C64 and try OPEN 1,4 : CLOSE 1.