Joystick Port Pin 8 (COM) to GND?

[MickGyver]

Hi! I'm new here at this community and not very familiar with the Amstrad range of computers. I have developed a gamepad for the Amiga CD32 that has some nice features. I'm currently working on a MSX and C64 versions (two button support and fully configurable, including mapping of the dpad to other buttons etc.)


I was looking into the Amstrad CPC, I read that the joystick port is compatible with the MSX so I started looking into the possibility of using my gamepad on the CPC. Problem is that the joystick port does not have a +5V signal. The voltage of the power supply is +5V so I thought I could make an adapter that takes +5V directly from a splitter cable.


This leads me to asking, is it safe to have Pin 8 on the joystick port (COM) connected to GND? My gamepad requires +5V so it would need a ground connection.


Grateful for any insight!

[Bryce]

No, it's not particularly unsafe, but it will stop the keyboard from working and the screen will fill with characters. The COM pin is a pulsed 0V signal that's part of the keyboard scanning circuitry.

Bryce.

[MickGyver]

Thanks, damn, the circuitry of the gamepad would need to be reworked then...

[GUNHED]

To connect COM to GND happens more often and works, for example here:

http://cpcwiki.eu/index.php/Atari-ST_mouse_adapter

I used this solution routinely and it works fine (with both, Trackball and ST mouse)


Maybe I mixed something up, then Bryce please correct me :-)

[Bryce]

Yes, you are mistaking the floating negative of a battery and CPC GND, which are not the same thing. MickGyver was talking about connecting the CPC GND to COM, which essentially would be the same as permanently setting I1 of the keyboard matrix to 0V and cause clashes in the keyboard scanning.
In the example you show, the battery minus is only equal to GND during the negative pulse of the COM signal, the rest of the time the ST Mouse GND is floating.

Bryce.

[GUNHED]

Uups got it, the direct shortcut I wouldn't suggest either