Interactive Connector Diagram
Click or tap any pin in the diagram below to inspect its signal assignment, electrical specification, and wire colour standard.
Click any pin to view its signal assignment, electrical spec, and wire colour.
Supported Systems / Found On — Retro Machine Hubs
This connector is used on the following retro computer systems. Click any colour-coded badge below to open its full board-level repair, diagnostic, and schematic Machine Hub:
[Amiga Hub — Amiga (A500 / A600 / A1200)]
[Big-Box Amiga Hub — Big-Box Amiga (A1000–A4000)]
[Atari ST Hub — Atari ST (520ST / 1040STF / STE)]
[Osborne/Kaypro Hub — Osborne 1 & Kaypro II CP/M Luggables]
[TRS-80 & CoCo Hub — TRS-80 (Model I/III/4) & CoCo (1/2/3)]
[Apple IIe Hub — Apple II / II+ / IIe]
Connector Specifications
DB25 DTE Standard
Pin 2 = TXD Out, Pin 3 = RXD In, Pin 4 = RTS, Pin 5 = CTS, Pin 6 = DSR, Pin 7 = GND, Pin 20 = DTR
DE-9 DTE Standard
Pin 3 = TXD Out, Pin 2 = RXD In, Pin 7 = RTS, Pin 8 = CTS, Pin 6 = DSR, Pin 5 = GND, Pin 4 = DTR
Voltage Swing
±5 V to ±12 V EIA RS-232 (Requires MAX232 or 1488/1489 level shifters — never connect directly to 3.3V/5V TTL!)
Full Pin Assignment & Wiring Reference
| Pin | Signal Name | Direction | Electrical Spec / Role | Wire Colour / SCART / Adapter Target |
|---|---|---|---|---|
| DB25 Pin 2 / DE-9 Pin 3 | TXD (Transmit Data) | Out | Serial Data Output from Computer (Cross to RXD on Null-Modem!) | Connect to other end RXD |
| DB25 Pin 3 / DE-9 Pin 2 | RXD (Receive Data) | In | Serial Data Input to Computer (Cross to TXD on Null-Modem!) | Connect to other end TXD |
| DB25 Pin 4 / DE-9 Pin 7 | RTS (Request To Send) | Out | Hardware Flow-Control Request To Send Output (Cross to CTS!) | Connect to other end CTS |
| DB25 Pin 5 / DE-9 Pin 8 | CTS (Clear To Send) | In | Hardware Flow-Control Clear To Send Input (Cross to RTS!) | Connect to other end RTS |
| DB25 Pin 6 / DE-9 Pin 6 | DSR (Data Set Ready) | In | Data Set Ready Input (Cross to DTR!) | Connect to other end DTR |
| DB25 Pin 7 / DE-9 Pin 5 | GND (Signal Ground) | GND | 0 V Signal Ground Reference (DB25 Pin 7 <-> DE-9 Pin 5) | Black (Signal GND) |
| DB25 Pin 8 / DE-9 Pin 1 | DCD (Carrier Detect) | In | Data Carrier Detect Input (Often tied with DSR to remote DTR) | Grey |
| DB25 Pin 20 / DE-9 Pin 4 | DTR (Terminal Ready) | Out | Data Terminal Ready Output (Cross to DSR + DCD!) | Connect to other end DSR |
| DB25 Pin 9 & 10 (Amiga!) | +12V (9) & -12V (10) | Pwr | Amiga ONLY: Pin 9 = +12V DC, Pin 10 = -12V DC (LEAVE UNCONNECTED!) | DO NOT CONNECT! |
DB25 Female (Amiga / Atari ST / Kaypro) to DE-9 Female (PC USB-Serial) Null-Modem Wiring
Note how Pin 2 (TXD) and Pin 3 (RXD) swap numbers between a 25-pin DB25 DTE connector (Pin 2 = TXD Out, Pin 3 = RXD In, Pin 7 = GND) and a 9-pin DE-9 PC AT DTE connector (Pin 3 = TXD Out, Pin 2 = RXD In, Pin 5 = GND)! Building a hardware-handshaking Null-Modem cable lets you transfer .ADF, .ST, and CP/M disk images directly from a modern PC USB-to-RS232 adapter at 19,200 to 115,200 baud.
- Signal Ground: Wire DB25 Pin 7 to DE-9 Pin 5 (GND).
- Data Cross (TX <-> RX): Wire DB25 Pin 2 (TXD) to DE-9 Pin 2 (RXD), and DB25 Pin 3 (RXD) to DE-9 Pin 3 (TXD).
- RTS/CTS Flow Control Cross: Wire DB25 Pin 4 (RTS) to DE-9 Pin 8 (CTS), and DB25 Pin 5 (CTS) to DE-9 Pin 7 (RTS).
- DTR/DSR Handshake Cross: Wire DB25 Pin 20 (DTR) to DE-9 Pin 6 (DSR) + Pin 1 (DCD), and DB25 Pin 6 (DSR) + Pin 8 (DCD) to DE-9 Pin 4 (DTR). Leave DB25 Pins 9–19 and 21–25 completely disconnected!