Everything you wanted to know about Joystick ports
The de facto standard for joysticks on 8bit (and 16bit) machines is a DE-9
connector, first introduced on the Atari VCS console. Unfortunatly, some
machines implemented it with some variations, resulting in compatibility issues
and confusion. This page attempts to summarize the main problems that you may
The original port on the Atari VCS
The original port has 5 digital inputs and two analogue ones. The digital inputs
are up, down, left, right and trigger (button) on pins 1, 2, 3, 4, and 6.
Pins 5 and 9 are analogue inputs, wired to potentiometers.
Pin 7 is a +5V power supply, and pin 8 is ground.
All Commodore machines also fully implement this standard (complete with
Amstrad CPC and Sam Coupé
These machines manage to handle two joysticks with only one connector. Support
for analogue input is missing.
The joysticks inputs are wired as part of the keyboard matrix. It's expected
that joystick 1 will connect the input pins (up, down, left, right, or button)
to the COM1 line (pin 8), and joystick 2 will connect them to the COM2 line
(pin 9). These lines are forced to 0 by the computer when scanning the respective
joystick, and left floating (with pull-up resistors) otherwise.
On the Sam Coupé, pin 7 is still a +5V supply, and a real ground is found on
pin 5. You can use this to power up some electronics in the Joystick.
The Amstrad CPC, however, wanted more buttons, so pins 5 and 7 are used for
two extra buttons. As a result, there is no ground and no +5V signal. This
makes it difficult to have "intelligent" electronics in the joystick, as there
is no simple way to power it. All the input and output lines are connected to
the PPI and AY chips IO pins in the CPC.
The idea for this port is that the first joystick would have a connector on
its base to connect the second one. This connector would swap pins 8 and 9,
so the second joystick would work without any specific changes.
To avoid any problems with keyboard scanning, the input pins should be forced
to 0 only when the respective COM pin for the joystick is 0, and left floating
otherwise (or directly connected to the COM pin).
The situation on Thomson machines is yet again a bit different. This time there
are two ports on the machine. However, their up/down/left/right pins are also
wired to other devices. On the MO6, they are used as outputs for the printer
port. On the TO8, they are used for the audio output. To avoid interferences,
similarly to the Amstrad and Sam Coupé ports, pin 8 is not an actual ground,
but is forced to 0 only when the joystick is being scanned (and pulled up with
a resistor otherwise). Take care of the fact that the pins are used for output
by the computer, and make sure this will not damage your controller.
Note that Thomson's own mouse does not do this properly, and it will interfere
with sound output, if you move it, or even if you don't move it but left it in
a position where it's forcing one of the lines to 0. Nice work, Thomson!
There is a +5V supply on pin 5, and a ground this time on pin 9.
Pins 6 and 7 are used for two button joysticks (in a way compatible with the
Amstrad CPC ones).