The "Opto" Project

The "Opto" project addresses the challenge of connecting wireless 
joysticks to the Atari MultiJoy adapter. It can also successfully 
serve as an interface between Atari and Sega adapters/controllers 
(which correctly wires power supply), it supports second button 
using the Joy2Bplus standard (second button or UP direction), 
or it is possible to use it with the 4play adapter on the Atari ST, 
which also lacks power on its parallel port.

Technical Description

The MultiJoy interface allows up to 16 joysticks to be connected 
to an 8-bit Atari computer. However, its design does not support 
controllers that require a power supply for operation. These are 
mostly wireless controllers, such as the 8BitDo M30 2.4G, the Retro 
Turbo 2000 Super Wireless Gamepad from Commotron, and modern 
controllers from Atari's + series, as well as XBox gamepads with 
a BlueRetro Atari adapter.

We considered how to connect such devices to the MultiJoy, which 
led to the creation of the “Opto” device.

MultiJoy has no VCC support. It selects the active joystick using 
a multiplexer that pulls pin 6 of the port low (logic 0), simulating 
standard port behaviour (GND). However, when a different joystick 
port is selected on the MultiJoy, the power ground pin is disconnected.

This is why galvanic isolation is required to power the joysticks 
— this means isolating both the power supply and the signal lines.

The power supply section uses an isolated DC/DC converter, which draws 
power from a USB power bank or power adapter connected via a USB-C 
port. The DC/DC converter requires a constant power load to function 
correctly, which is consumed by a LED. The converter’s output is 
connected to the joystick power pins.  Switch S2 allows you to select 
either Atari mode (powering pin 7) or Sega mode (powering pin 5).

Signal isolation is provided by optocouplers, powered by a 3.3V regulator. 
The current is set to ensure reliable signal transmission without 
excessive power consumption.

Switch S1 allows you to configure the second button on the controller 
to act either as a second fire button or as the UP direction, which 
can be useful in certain games. This function is only available when 
the controller is connected directly to the computer and not via MultiJoy.

Robert Petružela  (robert[dot]petruzela[at]volny[dot]cz)
Jan Křupka	(krupkaj[at]centrum[dot]cz)

