Microsoft-Paul Allen

[Tony Duell]

On Sat, Oct 20, 2018 at 8:28 PM Fred Cisin via cctalk
<cctalk at classiccmp.org> wrote:
>
> >> Similarly, although they sold a joystick board, they didn't sell
> >> joysticks.  DA15 connector for two joysticks.
> >> In some of the documentation, the sketch of a joystick was clearly the
> >> Radio Shack Coco joystick (which needed a different connector)
>
> On Sat, 20 Oct 2018, Tony Duell wrote:
> > And is electrically different.
> > The CoCo Joystick is a potential divider across the 5V rail. Moving the
> > joystick varies the output voltage (2 voltages per joystick, X and Y). This
> > is fed into a 6 bit ADC (actually a 6 bit DAC, a comparator and some
> > firmware).
> > The IBM PC Joystick (like the Apple ][ ones) is a pair of variable
> > resistors. This are the timing resistors in monostable circuits, acutally
> > an NE558 chip (Think of it as being 4 555 timers always connected as
> > monostables in one package). The software triggers the monostables
> > then sees how long they take to flip back again.
>
> Could you provide some remedial tutoring on what I am misunderstanding?
>
> All of that circuitry is in the "controller".
> The joystick itself consists of pushbuttons and two potentiometers.  NO
> other active circuitry.
> Moving the joystick does not itself vary the output voltage, when it is
> not so connected.  Moving the joystick varies the wiper position along a
> resistive element.   (admittedly, if the ends of the resistive element are
> connected to voltage and ground, then the wiper connection provides
> a variable output voltage)

That last is basically what I am getting at.

The variable resistor consists of a resistive track with a wiper on it. It
has a total of 3 connections -- the 2 ends of the track and the wiper.

Now you can use it in essentally two ways :

1) As a 'potentiometer' [1] . All 3 connections are used. The ends of
the track are connected across a power supply (in the case of the
CoCo Joystick to +5V and logic ground). The wiper thus has a voltage
that depends on the postion of the wiper.

2) As a variable resistor. Only one end of the track and the wiper are
used [2]. This is the case of the PC Joystick. Here one side goes to
+5V, the other to the timing resistor input on the monostable.

Now because the CoCo Joystick's 2 axes use the same power supply,
corresponding ends of the 2 resistive tracks are linked inside. Which is
a minor problem if you want to use it with the PC as the 'unused' ends of
the 2 tracks should not be connected. You have to change the wiring
slightly.

[1] So called because originally it was used to measure 'potential'
(voltage). The ends of the track were connected to a stable voltage
supply, the wiper to one side of a sensitve current detector (galvanometer).
The unknown input voltage was connected between the other side of
the galvanometer and the 'bottom' end of the track. When no current
flowed through the galvanometer, the 2 connections to it were at the
same voltage, meaning the unknown voltage could be determeined
as a fraction (the fractional position of the wiper on the track) of
the stable voltage supply across the whole track.

[2] Although just to be confusing it is conventional to connect the
unused end of the resistive track to the wiper. Then if there is a
bad contact at the wiper you get the whole track resistance in the
circuit rather than infinite resisitance

>
> If the resistance is compatible, then what modifications need to be made
> to convert a "voltage divider" potentiometer to a "rheostat"/"variable
> resistor"?
> (wire to wiper and one end of the resistive element (already present),
> disconnect the connection at the other end of the resistive element)

That's it. You have to disconnect one end of the resistive element.

>
> YES, it was "intended" to be used differently.
>
> If one KNOWS that it is impossible to make it work, then it makes it more
> difficult.
>
>
> Could one safely ignore the "No user serviceable components inside"
> sticker on the bottom of the joystick?  :-)

Well, technically it's true. If I am servicing something I can't be using it
at the same time so I am not a user....

> And, of course, there is the difficulty of locating a DA-15 connector!
> There are some on eBay, but the seller says that they are DB-15  :-)

I would think most of the large component suppliers (RS components
(who are not Radio Shack), Farnell/Newark, Mouser, Digikey, etc)
would have them but no idea what they are called...

-tony