24 pin DZM 12 DREH Connector at the Computer.
The Commodore Plus/4 user port is a 24-pin edge connector present on the Plus/4 only — the C16 and C116 members of the 264/TED family do not carry it. Eight bidirectional parallel data lines (P0–P7) are driven by the MOS 6529B Single Port Interface chip, whilst the serial signals (RxD, TxD, RTS, CTS, DTR, DSR, DCD) are provided by the MOS 6551 ACIA, which supports hardware RS-232 at rates up to 19,200 baud. The port also supplies +5 VDC, unregulated 9 VAC from the power brick’s secondary winding, a buffered reset output (/BRESET), and ground returns.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | +5V | +5 VDC |
| 3 | /BRESET | ? |
| 4 | P2/CSE | Data 2/Cassette Sense |
| 5 | P3 | Data 3 |
| 6 | P4 | Data 4 |
| 7 | P5 | Data 5 |
| 8 | RxC | Receive Clock |
| 9 | ATN | Attention? |
| 10 | +9V | +9 VAC |
| 11 | +9V | +9 VAC |
| 12 | GND | Ground |
| A | GND | Ground |
| B | P0 | Data 0 |
| C | RxD | Receive Data |
| D | RTS | Request to Send |
| E | DTR | Data Terminal Ready |
| F | P7 | Data 7 |
| H | DCD | Data Carrier Detect |
| J | P6 | Data 6 |
| K | CTS | Clear to Send |
| L | DSR | Data Set Ready |
| M | TxD | Transmit Data |
| N | GND | Ground |
Signals
- GND — Ground reference (pins 1, 12, A, N).
- +5V — +5 VDC logic supply (pin 2).
- /BRESET — Buffered reset output (pin 3); function details not fully documented in available service documentation.
- P0–P7 — Eight bidirectional parallel data lines implemented via the MOS 6529B Single Port Interface (pins B, 4–7, F, J; pin 4 doubles as Cassette Sense input).
- RxD — Receive Data from MOS 6551 ACIA (pin C).
- TxD — Transmit Data from MOS 6551 ACIA (pin M).
- RTS — Request to Send, MOS 6551 ACIA RS-232 handshaking (pin D).
- CTS — Clear to Send, MOS 6551 ACIA RS-232 handshaking (pin K).
- DTR — Data Terminal Ready, MOS 6551 ACIA (pin E).
- DSR — Data Set Ready, MOS 6551 ACIA (pin L).
- DCD — Data Carrier Detect, MOS 6551 ACIA (pin H).
- RxC — Receive Clock for the ACIA (pin 8).
- ATN — Attention line (pin 9); exact function not fully documented.
- +9V — Unregulated 9 VAC from the power brick secondary winding (pins 10–11).
Notes
The user port is physically present only on the Plus/4; the C16 and C116 do not include it. The MOS 6551 ACIA provides hardware-assisted RS-232 at up to 19,200 baud, unlike the software-only TTL-level RS-232 implementation of the Commodore 64 user port. The 9 VAC pins supply unregulated alternating current directly from the power supply transformer secondary.