25 PIN D-SUB MALE at the computer
25 PIN D-SUB FEMALE at the cable.
The Amiga A500, A2000, and A3000 provide a hardware RS-232-C serial port via a DB-25 (D-sub 25-pin) female connector on the rear panel, implemented through the Paula chip’s built-in UART. The interface follows the RS-232-C standard in DTE orientation and delivers true RS-232 voltage swings (output −13.2 V to −2.5 V / +8.0 V to +13.2 V, 10 mA max), supporting baud rates from 110 to approximately 250,000 bps. Whilst pins 1–8, 20, and 22 carry standard RS-232-C signals, many of the remaining pins are assigned to Amiga-proprietary functions such as audio I/O, supply voltages, and system clocks — these differ from the earlier A1000 model. Cables should only connect the RS-232 pins actually required for the application and must leave all other pins unconnected.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | SHIELD | - | Shield Ground |
| 2 | TXD | OUT | Transmit Data |
| 3 | RXD | IN | Receive Data |
| 4 | RTS | OUT | Request to Send |
| 5 | CTS | IN | Clear to Send |
| 6 | DSR | IN | Data Set Ready |
| 7 | GND | - | System Ground |
| 8 | CD | IN | Carrier Detect |
| 9 | +12V | OUT | +12 Volts DC (20 mA max) |
| 10 | -12V | OUT | -12 Volts DC (20 mA max) |
| 11 | AUDO | OUT | Amiga Audio Out (Left) |
| 12 | n/c | - | Speed Indicate |
| 13 | n/c | - | |
| 14 | n/c | - | |
| 15 | n/c | - | |
| 16 | n/c | - | |
| 17 | n/c | - | |
| 18 | AUDI | IN | Amiga Audio In (Right) |
| 19 | n/c | - | |
| 20 | DTR | OUT | Data Terminal Ready |
| 21 | n/c | - | |
| 22 | RI | IN | Ring Indicator |
| 23 | n/c | - | |
| 24 | n/c | - | |
| 25 | n/c | - |
Notes
- Direction is DTE (Computer) relative DCE (Modem). Note: Do not connect SHIELD(1) to GND(7)
- Pin 1 (SHIELD) is frame ground; it must not be tied to signal ground (pin 7).
- Pins 9 (+12V, 20 mA max) and 10 (−12V, 20 mA max) supply power to external RS-232 level-converter circuits; pin 21 supplies +5V at up to 50 mA on A500/A2000/A3000.
- Several pins marked n/c in the table carry Amiga-proprietary signals on A500/A2000/A3000: pin 11 (AUDO, audio out left), pin 17 (RXC, buffered 716 kHz port clock), pin 18 (AUDI, audio in), pin 21 (+5V), pin 24 (TXC1, 3.58 MHz clock), and pin 25 (RESB*, buffered system reset). The A1000 assigns different proprietary signals to many of these same pins.
- The Paula chip UART baud rate is set via the SERPER register: SERPER = (3,579,545 / baud) − 1 for NTSC systems.
- RS-232 input thresholds: positive logic ≥ +3.0 V, negative logic ≤ −0.5 V, with 1.0 V hysteresis.
- MIDI operation at 31.25 kHz is achievable via an external MIDI adapter on this port.
Signals
- SHIELD — Frame (chassis) ground; do not connect to signal ground (pin 7).
- TXD — Transmit Data; serial data output from the computer to the modem or DCE device.
- RXD — Receive Data; serial data input from the modem or DCE device to the computer.
- RTS — Request to Send; output from the computer asserting readiness to transmit.
- CTS — Clear to Send; input from the DCE device granting permission to transmit.
- DSR — Data Set Ready; input indicating the modem or DCE is ready and connected.
- GND — Signal ground reference for all RS-232 signals.
- CD — Carrier Detect; input indicating the modem has detected a carrier on the telephone line.
- +12V — Amiga-proprietary supply; +12 V DC at up to 20 mA (A500/A2000/A3000 only).
- -12V — Amiga-proprietary supply; −12 V DC at up to 20 mA (A500/A2000/A3000 only).
- AUDO — Amiga-proprietary; audio output (left channel) from the Amiga sound hardware.
- AUDI — Amiga-proprietary; audio input to the Amiga sound hardware.
- DTR — Data Terminal Ready; output from the computer indicating it is ready to communicate.
- RI — Ring Indicator; input that signals an incoming call; present on A500/A2000/A3000, not on A1000.
References
- Amiga Hardware Reference Manual, 3rd Edition — Commodore-Amiga Inc., 1991; Appendix E documents the complete DB-25 serial connector pinout for all Amiga models.