The Atari Serial Input/Output (SIO) bus is a fully proprietary 13-pin daisy-chain interface used on all Atari 8-bit computers, including the 400, 800, and the later XL and XE series. It operates at 19,200 baud using TTL-compatible voltage levels (not RS-232) and connects the computer to peripherals such as disk drives, printers, and the cassette recorder using identical connectors at each end, allowing peripherals to be chained in sequence. All bus transactions are computer-initiated; peripherals respond only when commanded, using a structured five-byte command frame with checksum and defined acknowledgement codes. The Atari 850 Interface Module was Atari’s official accessory for bridging the SIO bus to four RS-232 ports and a parallel port, enabling connection to non-SIO external devices.
| Pin | Signal | Description |
|---|---|---|
| 1 | Clock In | Bidirectional clock, reserved for synchronous transfer |
| 2 | Clock Out | Clock output from the computer |
| 3 | Data In | Asynchronous serial data, peripheral → computer |
| 4 | GND | Signal ground |
| 5 | Data Out | Asynchronous serial data, computer → peripheral |
| 6 | GND | Signal ground |
| 7 | Command | Active-low command-frame strobe from the computer |
| 8 | Motor Control | Cassette recorder motor control |
| 9 | Proceed | Active-low input to the computer (PIA CA1) |
| 10 | +5V / Ready | +5 V supply to peripherals / ready line |
| 11 | Audio In | Analogue audio input from the cassette recorder |
| 12 | +12V | +12 V supply (Atari 400/800 only) |
| 13 | Interrupt | Active-low input to the computer (PIA CB1) |
Notes
The SIO connector is a 13-pin proprietary Atari design; it is not electrically compatible with RS-232. Signal levels are TTL-compatible: output high ≥3.0 V, output low ≤0.4 V; input high threshold ≥2.0 V, input low threshold ≤0.8 V. Both the computer and each peripheral carry two identical SIO sockets, allowing peripherals to be daisy-chained in any order. Pin 12 (+12 V, 500 mA regulated) is present on the Atari 400/800 only; it is absent on XL and XE series models. Pin 10 serves a dual purpose: it supplies up to 50 mA of +5 V power to peripherals and simultaneously acts as a computer-ready signal. The Command line (pin 7) is held active low during transmission of a command frame, distinguishing command frames from data frames on the shared bus. The Proceed and Interrupt lines are active-low signals routed to the PIA chip and can generate hardware interrupts (IRQ); they were not used by any contemporary Atari peripheral.
Signals
- Clock In — bidirectional clock line, reserved for synchronous transfer; goes high at the leading edge of each data bit and low at the midpoint; not used by the Atari OS.
- Clock Out — clock output from the computer; goes high at the start of each Data Out bit; not used by any known peripheral.
- Data In — asynchronous serial data received by the computer from a peripheral; 8 data bits, LSB first, 1 start bit, 1 stop bit.
- GND — signal ground (pins 4 and 6).
- Data Out — asynchronous serial data transmitted by the computer to peripherals; same framing as Data In.
- Command — active-low line asserted by the computer during every command frame; peripherals use this to distinguish command frames from data frames.
- Motor Control — controls the cassette recorder motor; asserted by the computer to start tape transport.
- Proceed — active-low input to the computer; connected to PIA CA1 (pin 40); can assert an IRQ; not used by standard Atari peripherals.
- +5V/Ready — dual-function line: supplies up to 50 mA of regulated +5 V to peripherals, and signals that the computer is ready.
- Audio In — analogue audio input from the cassette recorder, routed to the computer for data loading via tone decoding.
- +12V — 500 mA regulated +12 V supply; present on Atari 400/800 only, absent on XL/XE models.
- Interrupt — active-low input to the computer; connected to PIA CB1 (pin 18); can assert an IRQ; not used by standard Atari peripherals.