Posted By
gerliczer
on 2021-07-04
03:06:15
|  Re: Joypad for plus/4
Some sort of limited single line serial communication might be possible using the joystick select bits/pins.
I'm thinking about a very simple protocol like this:
- at power up, after reset and (****)time-outs behave like normal 4 direction, 1 fire button joystick
- if received (*)k select line toggles in (**)rapid succession enter identification mode where signature is transmitted in 4 consecutive (***)scans (e.g. LR/NF, UD/F, LRUD/NF, -/F)
- if received k-1 select line toggles in rapid succession enter extended data transmission mode where 20 bits of state data are transmitted in 4 consecutive scans
- if received k+1 select line toggles in rapid succession reset to normal joystick mode
* 3? 4? 5?
** double clock, border, no memory refresh interference, different NTSC and PAL clock rates must be considered
*** select joystick, read data, deselect joystick
**** longer than reasonable scan duration
Four scans and twenty bits status word length are arbitrary values used only as an example.
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