Macintosh Modem Cable 590-0197
| Macintosh Connector (DB-9) | Apple Modem Connector (DB-9) |
| Comments | Signal Name | Pin | Pin | Signal Name | Comments | NOTES |
| Chassis/frame Ground | GND | *1 | 8 | GND | Chassis/Frame Ground | There are many variations on connecting pins 1, 3 and 8, but the basic intent is to bring all three of those pins to ground on the Macintosh side. Connect Signal Ground to Chassis Ground if you want a slightly better noise margin. |
| Signal Ground | SGND | *3 | 3 | SGND | Signal Ground |
| | *8 | 1 |
| Output; Transmit Data | TXD- | 5 | 9 | RCD | Input; Receive Data |
| +12-Volt Line | | 6 | 6 | DTR | Input; Data Terminal Ready |
| Input; Handshake | HSK | 7 | 7 | DCD | Output; Data Carrier Detect |
| Input; Receive Data | RCD- | 9 | 5 | TXD | Output; Transmit Data |
If you wish to connect the DB-9 Modem port connector on the Macintosh 128K/512K to a DB-25 connector on a non-Apple modem, a Macintosh Imagewriter cable connected to the Modem Eliminator cable works in most cases. Here are the pinouts for that configuration:
| DB-9 Macintosh DTE | DB-25 Modem DCE |
| Signal | Pin | Pin | Signal |
| Chassis Ground | 1 | 1 | Chassis Ground |
| Signal Ground | 3 | 7 | Signal Ground |
| Output; Transmit Data | 5 | 2 | Input; Transmit Data |
| Input; Handshake | 7 | 6 | Output; Data Set Ready |
| Input; Receive Data | 9 | 3 | Output; Receive Data |
If this configuration doesn't work, try these additional cable connections:
| DB-9 Macintosh DTE | DB-25 Modem DCE |
| Signal | Pin | Pin | Signal |
| +12-Volt Line | 6 | 20 | Input; Data Terminal Ready |
| Output; Handshake | 2 | 20 | Input; Data Terminal Ready |
| Output; Handshake | 2 | 4 | Input; Request to Send |
| Input; Handshake | 7 | 8 | Output; Data Carrier Detect |
You will have to make a cable to try these additional connections one by one or in combination. If you have the pinouts of your modem, make a cable that matches up the Apple Modem signals with the appropriate pins on the other modem's connector, e.g., connect pin 1 on the DB-9 connector to pin for Chassis/Frame Ground on the modem's DB-25 connector, and so on.
Refer to the RS-232 standard for the connection signals and corresponding pin numbers.