Looking for PC analogue/Gameport to BBC analogue port joystick diagram.

[BBCNoob]

As the title states, I'm looking for a PC analogue/Gameport to BBC analogue port for PC joysticks.

I've found one for digital Atari style joysticks to Beebs, but for the life of me I can't find one for PC to Beeb/Electron.

I see from here viewtopic.php?t=2649 that there was a star on talking about them here, but it stopped for some reason?

I also see that's there's some chap on ebay from Germany selling already converted Joysticks, but sadly, it would cost just as much to have one sent, as it would for the joystick.

I'm in talks with a friend who knows electronics miles more than I do.

My idea is to have a circuit that has an Atari 9 pin socket, and a 15pin PC gameport, but with some kind of magic (that's way over my head), that would then plug into the Beeb. Giving me the choice of hooking up a digital Atari joystick (maybe two), or a PC analogue joystick. In such a way, neither joystick needs to be modded if possible.

Is there diagrams to this in some old Beeb magazine? Or someone has already figured this out, and there's no need to reinvent the wheel?

Thanks...

[vanekp]

in this viewtopic.php?t=5612#p82597 article there is a diagram of one.

[BBCNoob]

in this viewtopic.php?t=5612#p82597 article there is a diagram of one.

Is that not for Digital joysticks? I'm looking for PC analogue to Beeb.

[vanekp]

The diagram is a analog joystick as it used potentiometers for the X & Y axes.

[sydney]

I don't think such a diagram exists. You could start by looking at a PC analogue joystick pinout and comparing it to the Beeb's. I think a PC uses 5v and the Beeb's 1.8v but I could be wrong. I don't think its going to something simple to create.

[vanekp]

Surely its just a question of finding an analog joystick and wiring the two X/Y pot's as per this diagram and the fire button.

[1024MAK]

The method used by PC Joystick interfaces (often part of the soundcard in older machines) to detect the position of the Joystick is completely different to the way the Acorn analogue joystick system works.

They are both analogue, but the PC uses analogue time to determine the position of the joystick while the Acorn system uses a DC analogue voltage. Hence they are wired differently and they use different value potentiometers.

It may be possible to convert some PC joysticks to work on a BBC, it depends on the internal construction of the joystick.

In theory it may be possible to build a conversion circuit, but I don't know of an existing design.

Mark

[crj]

Arguably, the value of the potentiometers in an Acorn joystick is largely irrelevant; it's only the relative resistance each side of the divider that matters.

I'd not realised PC joysticks worked differently. Actually, truth be told, I'd not realised PC joysticks were analogue. Googling finds a diagram of how they're wired, though:


That's annoyingly fiddly to convert; I guess you have to do something grim with an op-amp and half a dozen resistors?

[MartinB]

Most PC analogue sticks, like the one pictured, don't seem to use the pot as a three-connection potential divider as is used in the Beeb. I'm wondering therefore if the Beeb layout is intended for very high impedance inputs whereas the PC ones assume a little current will be drawn in order to produce a voltage. I think perhaps you'd need to be careful when porting a PC one that you don't just repeat that two-connection wiring of the pots because it would, at one end of the travel, present too little resistance to the input. Maybe...?

[BBCNoob]

As I don't understand electronics, does this mean anything to anyone?

I've found this - https://www.andreas-pernau.de/Computer/Acorn_BBC-B.htm

There seems to be a mod there?

[vanekp]

Basically it is showing a rewiring of the potentiometers to that they are wired from vref and the other side to ground and the center connector the joystick output. ans normally the one one is floating and not used for pc use.

[1024MAK]

PC analogue joysticks are simply two variable resistors. Hence why only one end of the potentiometer track and the wiper are used.
In the PC joystick interface, these variable resistors are each used in a timing circuit to charge a capacitor. The system normally is based around a quad 555 timer chip so that two joysticks can be supported. The software running on the PC times how long it takes for each of the capacitors to charge. If the stick is pushed one way, it takes less time for the capacitor for that channel to charge (timing quicker), if pushed the other way, it takes longer.
As each joystick is different (has a different resistance), this is the reason that PC joysticks have to be calibrated before they are used.

Mark

[tricky]

I mentioned this before, but here is the EEVBLOG episode about PC joysticks and the beeb even gets a shout out