9 pin D-SUB male connector
9 pin D-SUB female connector
The Apple 300/1200 modem connected to the Macintosh serial port using a 9-pin D-sub (DB-9) connector, carrying RS-422 balanced differential signals rather than the RS-232 voltages typically associated with that shell. This arrangement was used on the original Macintosh 128K and 512K, which equipped both the Modem and Printer ports with DB-9 sockets driven by the Zilog Z8530 Serial Communications Controller. From the Macintosh Plus (1986) onward, Apple replaced the DB-9 with the 8-pin mini-DIN (DIN-8) connector; the Apple Personal Modem and subsequent modems therefore connected via the System Peripheral-8 cable (M0187) rather than a direct DB-9 lead. Data signals are differential pairs (TxD+/TxD− and RxD+/RxD−), while the handshake line operates unbalanced in accordance with EIA-423-A.
| Pin | Name | Description |
|---|---|---|
| 1 | n/c | Not connected |
| 2 | DSR | Data Set Ready |
| 3 | GND | Ground |
| 4 | n/c | Not connected |
| 5 | RxD | Receive Data |
| 6 | DTR | Data Terminal Ready |
| 7 | DCD | Output from modem |
| 8 | GND | Ground |
| 9 | TxD | Transmit Data |
Notes
The DB-9 connector shell is the same physical form as a standard RS-232 DE-9, but the electrical signalling is RS-422 differential — the two standards are electrically incompatible. Pin 3 and Pin 8 both serve as ground references; Pin 1 and Pin 4 are unused in this modem application. Starting with the Macintosh Plus, Apple moved to the DIN-8 connector for all serial ports; connecting a modem to a Plus or later Mac required the Apple System Peripheral-8 cable (M0187, assembly 590-0340-A), which cross-wires the DIN-8 pins accordingly.
Signals
- n/c — not connected; pins 1 and 4 are unused in this configuration
- DSR — Data Set Ready; asserted by the modem to indicate it is powered and ready
- GND — signal ground reference; provided on both pins 3 and 8
- RxD — Receive Data; serial data received from the modem by the Macintosh
- DTR — Data Terminal Ready; asserted by the Macintosh to indicate readiness
- DCD — Data Carrier Detect; output from the modem indicating a carrier is detected
- TxD — Transmit Data; serial data sent from the Macintosh to the modem