12 PIN IDC MALE at the computer.
The AT&T UNIX PC — sold under the model designations 3B1 and PC7300 — was a 1985 Unix workstation built around the Motorola 68010. Unlike contemporaneous IBM-compatible machines, its video output is not a D-sub connector but a proprietary 12-pin IDC (Insulation Displacement Connector) male header on the logic board, as shown in Figure 1-6 of the original AT&T UNIX PC Reference Manual (document 999-300-1891S, 1986). The display is an integrated 12-inch monochrome CRT running at 720×348 pixels with a 20 MHz pixel clock, so the connector carries only a single composite-video signal together with separate horizontal and vertical sync, dual +12 V power rails for the display circuitry, a two-wire speaker connection, and several ground returns. The function of pin 12 is not publicly documented.
| Pin | Name | Description |
|---|---|---|
| 1 | VSYNC | Vertical Sync |
| 2 | GND | Ground |
| 3 | HSYNC | Horizontal Sync |
| 4 | GND | Ground |
| 5 | VIDEO | Video |
| 6 | GND | Ground |
| 7 | +12V | +12 VDC |
| 8 | GND | Ground |
| 9 | +12V | +12 VDC |
| 10 | SPK | Speaker |
| 11 | SPK | Speaker |
| 12 | ? | ? |
Signals
- VSYNC — Vertical synchronisation pulse for the monochrome CRT; pin 1.
- GND — Signal ground returns on pins 2, 4, 6, and 8.
- HSYNC — Horizontal synchronisation pulse; pin 3.
- VIDEO — Monochrome composite video signal; pin 5.
- +12V — Two separate +12 VDC supply rails on pins 7 and 9, providing power to the display assembly.
- SPK — Two-wire speaker connection on pins 10 and 11; the machine’s speaker is driven directly through these lines.
- ? — Pin 12 — function unknown.
Notes
- The connector is a 12-pin IDC (0.1 inch pitch) male header, not a D-sub of any kind.
- The 3B1/7300 uses a monochrome-only display; there are no colour or intensity signals.
- The two +12 V rails (pins 7 and 9) are believed to supply distinct sub-circuits within the display assembly; do not assume they are interchangeable without consulting the full schematic.
- Pin 12 has no publicly documented function.