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Building a Digital I/O Controller for Atari 2600+

This document walks you through building a digital I/O controller module that connects to the Atari 2600 joystick port and simulates joystick inputs (up/down/left/right/fire). It uses a digital I/O device such as the MCC USB-1024LS, wired to a DB9 Atari joystick cable to send digital signals directly to the Atari console.

The controller works by pulling specific pins on the DB9 joystick port low (to ground), mimicking how the real joystick functions. This setup is useful for experimenting with Atari hardware from modern control systems.

Hardware Overview

Digital I/O Devices

You will need a USB digital I/O module with TTL-level (5V) outputs. Recommended options:

Device Description Notes
MCC USB-1024LS 24 digital I/O lines, 5V TTL Well-documented and supported
Arduino Nano + Screw Terminal Shield 14 digital I/O lines Requires custom sketch

Atari Joystick Port Pinout (DB9)

Refer to the Atari joystick port pinout, which shows the connector as seen from the front (on the Atari console):

Pin Function Notes
1 Up Active LOW
2 Down Active LOW
3 Left Active LOW
4 Right Active LOW
5 Paddle B Not used
6 Fire Button Active LOW
7 +5V Power Unused
8 Ground (GND) Shared return path
9 Paddle A Not used

If you're using a male DB9-to-bare wire cable, the pinout may appear mirrored. Use continuity testing to confirm pin mapping.

Identifying Wires via Continuity Testing

To build the controller, you'll need to match each wire in the DB9 cable to its pin. Wire colors are often non-standard.

Steps:

  1. Insert paper clips into the front of the DB9 connector to make contact with the pins.
  2. Set your multimeter to continuity mode (beep or ohm check).
  3. Touch one multimeter probe to a paper clip in Pin 1 (Up).
  4. Touch the other probe to each wire end until you hear a beep.
  5. Repeat for Pins 2–4, 6, and 8 (GND).

Write down the wire color associated with each pin for reference during wiring.

Avoid touching adjacent paper clips at the same time — you may short pins during testing.

Wiring the Controller Module

Once you have mapped the wires, connect them to the I/O device.

Signal Logic

  • The Atari expects active LOW signals: pulling the signal to GND = pressed.
  • Set digital output pins LOW (0V) to press, HIGH (5V) to release.

MCC USB-1024LS: Port A Wiring

If using the MCC USB-1024LS, connect the joystick wires to Port A, using pins 24–28 for control, and 29 for GND:

Function DB9 Pin DAQ Port Terminal Pin # Bit Notes
Up 1 Port A 24 P0.0 Press = LOW
Down 2 Port A 25 P0.1 Press = LOW
Left 3 Port A 26 P0.2 Press = LOW
Right 4 Port A 27 P0.3 Press = LOW
Fire 6 Port A 28 P0.4 Press = LOW
GND 8 GND 29 – Required for signal return

Important: Connect the GND wire (pin 8) from the joystick cable to terminal pin 29 (GND) on the DAQ to complete the circuit.

Cable Prep & Strain Relief

When handling joystick cables:

  1. Strip each wire ~5mm from the end. Be careful with cables that have nylon/tinsel insulation inside - trim or singe with a lighter.
  2. Insert stripped wires into screw terminals on the I/O module.
  3. Double-check wire mapping before applying power.

Strain Relief

Use a cable tie and cable tie gun to secure the joystick cable to the screw terminal block. This prevents stress on the connections and improves durability.