Date   : Sun, 13 Mar 2005 17:54:19 +0000
From   : Richard Gellman <splodge@...>
Subject: Re: BBC analogue joysticks for PC

Yup, its perfectly possible to do this, and in fact I did it with an old 
joystick I got from Watford Electronics way back. Essentially, the 
circuit design is identical: two "axes" consisting of a potentiometer 
between 0v and +5V, with the variable output connected to the input 
channel. The buttons then work as switches between +5V and the button 
inputs.

Here's the catch: Same circuit, different pinout. If you plug a BBC 
joystick into a PC, it wont work. It won't fry anything either, but you 
won't get any movement. For reasons which I assume are pure coincidence 
(the BBC ananlogue port was pinned just before the Game port was pinned 
for the PC) the two sockets use totally different pinouts, but require 
no "interpretation" circuitry.

So what you need, is one female 15-pin D, one male 15-pin D, some wire, 
a soldering iron, and pin diagrams of both connectors. Then you should 
be able to assemble an adapter that connects any BBC joystick to a PC.

Caveats:

The PC Game port has TWO joystick connections on it, thus making a total 
of FOUR axes, and FOUR buttons. On the BBC analogue port, there are 
similiar circumstances, 4 analgoue in lines, but only the two fire 
button inputs. How you wire these is entirely dependent on what you plan 
to connect, in terms of exotic hardware. All two-axis two-button 
joysticks will work fine if you split the PC game port into two and do a 
straight conversion. If however, you want to connect something with four 
axes, you'll need to sacrifice one PC game port for the two surplus axes.

The voltage levels between the PC and the BBC do vary somewhat. While 
Windows' game device calibration should take care of this, you find it 
well worth shunting a control potentiometer either in series or parallel 
with the axis potentiometer (depending on desired control type) to 
adjust how the axis is interpreted.

Have fun :)

-- Richard

Joe N wrote:

> Does anyone know if it is possible to make a hardware interface 
> between original BBC joysticks and a PC? I remember seeing a curcuit 
> diagram for a similar such thing for Amiga joysticks on a website 
> somewhere, but wondered if anyone had any ideas about the BBC?
>
> Thanks,
> Joe.
>
>