Adapt a Sega Mega Drive controller to the 9-pin Atari joystick standard using a Microchip PIC 16F1847 microcontroller.
The schematics and parts list for the adapter are freely available. You also need some way to program the PIC microcontroller.
First, some history.
- In 1977, Atari released the 2600 game console.
- In 1981, Commodore released the VIC-20 home computer.
- In 1984, Sega released the SG-1000 II console with one main improvement from the SG-1000: detachable controllers.
These machines all used the same physical connector with an almost identical pinout for their controllers, and their manufacturers stuck with the standard through later generations, tempting people to swap controllers and joysticks between the systems.
Most of the time, this more or less worked. Buttons were not always completely compatible, but most Atari and Commodore games only supported one button so it didn't really matter as long as one button worked. One important caveat: Sega chose to put power on pin 5 instead of pin 7, so mixing and matching had the potential to cause damage to both machines and controllers.
Initially each signal was carried by an individual pin: four pins for directions, one for the button, two pins reserved for power and ground, leaving two pins free for other functions.
For the Mega Drive, released in 1988, Sega wanted to have more buttons (four) than there were available pins on the controller port (three). They solved this problem by using one pin (7) as an output from the console to the controller, allowing two pins (6 and 9) to multiplex the A, B, C, and Start buttons.
Fast forward 20 years, to January 2019. A poster on an Atari 8-bit forum introduced a design for a three-button joystick for the 8-bit Ataris, and -- crucially -- included patched versions of existing games that supported the extra buttons. This was the Joy2B+ project.
Suddenly, there was a reason to connect a joystick with more than one button to an Atari.
Finally, in February 2019, a Hong Kong-based company called 8BitDo released a device that allowed Bluetooth game controllers to be used with Sega consoles, the 8BitDo Retro Receiver for SEGA... tantalisingly close to being compatible with Atari joystick ports.
The adapter uses a microcontroller to talk to the Mega Drive controller and map its four buttons to the three inputs available on an 8-bit Atari.
The B button on the controller is mapped to the normal joystick button. Buttons A and C are connected to pins 5 and 9 on the joystick port; these can be read as extra joystick buttons by the computer. Most games that only support one extra button use the C button (pin 9).
The start button has no natural mapping since the Atari can only read three buttons, but another poster in the aforementioned AtariAge thread suggested mapping a button to Up to use it as a jump button in games that use Up to jump.
The Start button is not usually in a good location for a jump button on a controller, so the adapter uses it to support this feature indirectly: holding the Start button down for a second maps the A button to Up.
Since the joystick button often starts games, a short press of the Start button is mapped to joystick button 1.
Because of the way the paddle circuits are used to read the second and third button inputs, 8-Bit Ataris read an unconnected second or third button input as if the button is being held down.
This allows* games running on the computer to detect the presence of a multibutton controller by assuming that a button does not exist until it has been seen to be released. The adapter supports this autodetection by leaving its button outputs open unless a Mega Drive controller is detected or the corresponding button input is pulled low.
To activate the second button when a Mega Drive controller is not connected, pin 9 on the controller input must be pulled low before the adapter starts driving the second button output. This means that the second button must be pressed once before it can be detected by the computer.
The third button output is open unless a Mega Drive controller is detected since there is no other source for a third button input.
* The astute reader will note that games are required to ignore button inputs until the button is released, otherwise a nonexistent button will appear to be continually held down.
The adapter has mostly been tested on 8-bit Ataris using Joy2B+ enhanced games.
Button two registers as the second mouse button / port 1 joystick button if the adapter is plugged into port 0. The ST does not support additional joystick buttons on port 1.
The C64, C128, and VIC-20 can also read the second and third buttons. The extra buttons on these machines are active high, which is opposite to Atari and Sega.
Pulling the /C64 pin (number 4) on the PIC low inverts the outputs for the extra buttons, allowing them to be read correctly by the 8-bit Commodores.
Because the joystick inputs of the 8-bit Commodores are somewhat sensitive, all of the pins are normally open (floating). The extra button pins are pulled high when active, all other signals are pulled low when active.
Unlike the 8-bit Commodores the extra buttons on the Amiga are active low, and unlike all the 8-bit computers they are digital inputs instead of repurposed paddle circuits. The Atari mode supports both Atari and Amiga by driving the pins for buttons two and three both high and low.
The adapter is theoretically compatible with a six-button Mega Drive controller, but this has not been tested. The extra X, Y, Z, and Mode buttons are not mapped to anything even if a six-button controller is connected.
Sega Master System controllers are also theoretically supported, including the second button.
The adapter is extensively tested with the 8BitDo Retro Receiver for SEGA. It's a nifty way to use wireless controllers on old hardware.
A controller or joystick with a second button that is not normally recognised by 8-bit Atari and Commodore computers may work through the adapter.
CAUTION: the second button must be on pin 9 and not pin 5. Pin 5 is the power pin, and pulling it low may damage your computer or the adapter.
Single-button joysticks will work but derive no benefit from using the adapter.
These devices will not work and should not be plugged in to the adapter.
xc8 --chip=16f1847 sega-adapter.c
JOY-2-PIC can program a PIC using an Atari 8-bit computer's joystick port. Boot from the .atr image attached to each release to start programming.
The .hex file can be used with a USB PIC programmer.
PIC 16F1847
+---------_---------+
| |
Atari 5 -| 1 RA2 RA1 18 |- Atari 1
| |
Atari 6 -| 2 RA3 RA0 17 |- Atari 2
| |
Atari 9 -| 3 RA4 RA7 16 |- Atari 3
___ | |
C64 -| 4 RA5 RA6 15 |- Atari 4
| |
Atari 8 SEGA 8 -| 5 GND +5v 14 |- Atari 7 SEGA 5
| |
SEGA 1 -| 6 RB0 RB7 13 |- SEGA 6
| |
SEGA 2 -| 7 RB1 RB6 12 |- SEGA 7
| |
SEGA 3 -| 8 RB2 RB5 11 |- SEGA 9
| |
SEGA 4 -| 9 RB3 RB4 10 |- X
| |
+-------------------+
Information on reading Sega controllers came from https://github.com/jonthysell/SegaController/wiki/How-To-Read-Sega-Controllers