The Atari 7800 ProSystem (1984/1986 retail) includes an 18-pin expansion port intended for a keyboard module that would have extended the console into a home computer. The port carries video signals from the MARIA graphics chip (luminance, colour phase, composite sync, and blanking), the composite video output, audio, the phase-2 clock from the Sally (6502C) CPU, and control lines for the master oscillator. No expansion module was ever commercially released, and the port is populated only on early NTSC production units; later board revisions retain the footprint but leave it unpopulated.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | +5V | +5 VDC |
| 3 | CVIDEO | Input to RF modulator (Video+Audio) |
| 4 | MLUM0 | Maria Luminance Bit 0 |
| 5 | MLUM3 | Maria Luminance Bit 3 |
| 6 | BLANK | Blanking output |
| 7 | OSCDIS | Disable 14.31818 MHz Master Clock |
| 8 | EXTMEN | External Maria Enable Input |
| 9 | GND | Ground |
| 10 | EXTOSC | External clock to replace Master Clock |
| 11 | CLK2 | Phase 2 Clock from the 6502 |
| 12 | MSYNC | Maria Composite Sync |
| 13 | MLUM1 | Maria Luminance Bit 1 |
| 14 | MLUM2 | Maria Luminance Bit 2 |
| 15 | MCOL | Maria Color Phase Angle |
| 16 | RDY | Input to the 6502 |
| 17 | AUDIO | Audio |
| 18 | GND | Ground |
Notes
The connector is an 18-pin edge connector with pins 1–9 on one side and pins 10–18 on the other, as shown in the diagram above (connector at the Atari). The port was populated only on early NTSC production units; the footprint is present on most board revisions but components are absent on the majority of units sold. CVIDEO (pin 3) carries the combined composite video and audio signal fed to the RF modulator; it is distinct from the separate AUDIO (pin 17) line. OSCDIS (pin 7) and EXTOSC (pin 10) together allow an external module to substitute its own clock in place of the 7800’s 14.31818 MHz master oscillator.
Signals
- GND — Signal and power ground (pins 1, 9, 18)
- +5V — +5 VDC power supply to expansion module
- CVIDEO — Composite video output (video and audio combined) feeding the RF modulator
- MLUM0–MLUM3 — MARIA graphics chip luminance output bits 0–3; together they encode the luminance component of the video signal
- BLANK — Active blanking output from MARIA; indicates horizontal or vertical blanking interval
- OSCDIS — Disables the 7800’s internal 14.31818 MHz master clock oscillator when asserted
- EXTMEN — External MARIA enable input; allows an expansion module to assert control over the MARIA chip
- EXTOSC — External oscillator input; supplies a replacement master clock when OSCDIS is asserted
- CLK2 — Phase-2 clock output from the Sally (6502C) CPU
- MSYNC — MARIA composite sync output
- MLUM1, MLUM2 — Additional MARIA luminance bits (see MLUM0–MLUM3)
- MCOL — MARIA colour phase angle output; encodes the chroma phase for NTSC colour generation
- RDY — Ready input to the 6502C CPU; holding this low pauses the CPU between cycles
- AUDIO — Analogue audio output