The AT&T PC 6300 is a rebadged Olivetti M24, and its video output uses a 25-pin D-sub (DB-25) connector rather than the standard IBM CGA DE-9. The signals are TTL-level digital RGB — not analogue — and the horizontal scan frequency is 25.9 kHz, which differs from the IBM CGA standard of 15.75 kHz, making the two systems incompatible without an adapter. The Olivetti M24 Theory of Operation (1984) confirms that the display controller board carries this 25-pin D-type connector directly, and that a second monitor may be connected via the same interface. Monitor-type identification is provided by two ID pins, whilst a dedicated degauss line and dual +15 V power rails round out the interface — features absent from the IBM CGA specification.
25 PIN D-SUB Connector at the computer.
| Pin | Name | Description |
|---|---|---|
| 1 | HSYNC | Horizontal Sync |
| 2 | ID0 | Monitor ID 0 |
| 3 | VSYNC | Vertical Sync |
| 4 | R | Red |
| 5 | G | Green |
| 6 | B | Blue |
| 8 | n/c | Not connected |
| 9 | n/c | Not connected |
| 10 | ID1 | Monitor ID 1 |
| 11 | MODE0 | Mode 0 |
| 12 | n/c | Not connected |
| 13 | /DEGAUSS | Degauss |
| 14 | GND | Ground |
| 15 | GND | Ground |
| 16 | GND | Ground |
| 17 | GND | Ground |
| 18 | GND | Ground |
| 19 | GND | Ground |
| 20 | GND | Ground |
| 21 | GND | Ground |
| 22 | n/c | Not connected |
| 23 | n/c | Not connected |
| 24 | +15V | +15 VDC |
| 25 | +15V | +15 VDC |
Notes
- The AT&T PC 6300 / Olivetti M24 DB-25 video connector is physically and electrically incompatible with the standard IBM CGA DE-9; a dedicated adapter (such as the Taxan Gold Card) is required to drive a CGA-standard monitor.
- Horizontal scan frequency is 25.9 kHz; IBM CGA operates at 15.75 kHz.
- Monochrome monitor: ID0 and ID1 are open (not driven).
- Colour monitor: ID0 is low (0 V), ID1 is high — the voltage level for the ID lines is likely TTL (5 V), not the +15 V supply rails on pins 24–25.
- The +15 V supply on pins 24–25 is unusual for a video connector and is likely intended for monitor power; full-service detail is contained in the AT&T Personal Computer 6300 Reference Manual (not reproduced here).
- Pin 7 carries the Intensity output per Olivetti schematics, but it does not appear in all connector lists; treat as implementation-dependent.
Signals
- HSYNC — TTL-level horizontal synchronisation pulse at 25.9 kHz.
- VSYNC — TTL-level vertical synchronisation pulse.
- R / G / B — TTL-level digital Red, Green, Blue video signals (not analogue).
- ID0 / ID1 — Monitor-identification bits; both open for monochrome, ID1 driven for colour.
- MODE0 — Display mode select signal; function varies by monitor configuration.
- /DEGAUSS — Active-low degauss control, not present on standard IBM CGA.
- GND — Signal and/or chassis ground; multiple pins (14–21) provide a common return path.
- +15V — Supply voltage on pins 24–25; likely monitor power, not a logic signal.
References
- AT&T PC 6300 Service Manual — AT&T Information Systems / Bitsavers, archive.org
- Olivetti M21 M24 Theory of Operation (November 1984) — Olivetti / Bitsavers, archive.org
- Colour Graphics Adapter Notes — John Elliott, seasip.info