The result of tonight's frivolities: a fully-functional USB adapter for the original Gravis GamePad Pro in GrIP mode, which enables all ten buttons and D-pad input to work over a standard 15-pin gameport.
This on its own is not special - at least two other projects out there replicate this exact functionality - but the key here is that *I understand how this works now*, and I can start work on supporting all the other #GravisGrIP controllers. Stay tuned!
The subject of tonight's frivolous nonsense is the second of five #GravisGrIP devices I'd like to support with my adapter. The BlackHawk Digital was the first Gravis joystick to support the GrIP protocol, and - without the fall-back analog gameport mode of the GamePad Pro or forwards-compatibility with USB - is basically lost technology at this point. This week - tonight, with any luck - I intend to change that.
With three analog axes, this will be tougher to discombobulate than the GPP.
The GamePad Pro's #GravisGrIP protocol is well-understood, and totally documented in the last paragraph in this archived project: https://github.com/prosper00/GRiP-duino
However, the same thing doesn't seem to exist for the BHD or other devices.
All but the GrIP MultiPort are supported by Linux, but the code here is totally opaque to me: https://github.com/ilbers/linux/blob/master/drivers/input/joystick/grip.c
I've cheekily reached out to Vojtech Pavlik, that driver's author from 24 years ago, to see if he's kept any notes - we'll see how lucky I am there.
I did find a Linux driver for the MultiPort, something Mr. Pavlik didn't end up writing. I've found it in the input drivers for Android Wear, of all things, but it seems to be what I think it is: https://android.googlesource.com/kernel/msm/+/android-wear-5.1.1_r0.2/drivers/input/joystick/grip_mp.c
That, terrifyingly, suggests the GrIP MultiPort has two-way comms (rather than just it reporting stuff to the PC like the gamepads or joysticks would), so implementing support for it in my adapter could be a difficult task.
Progress today on reverse-engineering the #GravisGrIP BlackHawk Digital joystick - there are indeed two different packets this thing sends! Bit 9 is a zero if it's updating digital inputs, and a one if it's changing joystick inputs.
This was confusing because the GamePad Pro's start sequence was always a 0, a number of 1s and another 0; I'd assumed that trailing zero was part of the start bits, but the GPP has no analog controls, so even if that's in the GrIP spec it'd never show a one there.
For reference, this is the Linux driver for Gravis GrIP devices: https://github.com/ilbers/linux/blob/master/drivers/input/joystick/grip.c
My programming skills are pretty simple, so this is an impenetrable wall of optimisation and bit-shifting to me, and I have no hope of reverse-engineering anything substantial out of here.
Just discovered someone has actually done the hard work and ported the XT portion of the Linux GrIP driver to Arduino: https://gist.github.com/Hyratel/4e689925743fb789b5cf3cca3812511a
Took me ages to stumble on it since it doesn't actually mention any of the devices, and it's still a complex program, but there's more clues for me to work from here.
There's a big difference between my code and anything approaching "official" - the Linux driver has CRC checking and all sorts of super-efficiency, whereas I'm just banging two packets' worth of bits out of the gameport, picking one complete packet out of that, and joysticking as needed.
I don't want to be a protocol-completionist here - I just want to make a thing you plug a joystick into and it works well enough to play some games with it, even if I'm not catching every single packet.
@hyratel right, thanks - this is starting to make sense. The Xterminator joystick adds an analog thumb-stick and the gamepad has two analog rear buttons, I guess that's what P2 is for.
CRC would explain why multiple bits change where I expected to see only one...
Where's the true start of the packet? How does your code find/sync to it? With the answer to that I should have all the information I need to understand these controllers.
Tim 🎮
hyratel