3rd button for DB9 double-triggers or so

[dfilskov]

Concerning the 3 button specialized version for DB9: When using the third button in combination with one of the other buttons (like a modifier - or vice versa) something wrong happens. It's supposed to produce sth like:
button 2 down
button 3 down
button 2 up
button 3 up
... but it doesn't seem to. I've tested this on MiSTer trying to use it for a 2-button combination for opening the OSD (menu) but it doesn't work - other USB joysticks do but not the 3 button version of the DB9 firmware.