Re: Game port

[Rubywand <rubywand@swbell.net>]

David Empson writes ...
> 
> Rubywand <rubywand@swbell.net> wrote:
> 
> > David Empson writes ...
> > >
 ....

> 
> >      Regarding pinout of 9-pin A2 mouses, is the following correct?
> 
> [Mouse diagram snipped]  Yes that looks correct.  It agrees with the IIc
> manual.
> 
> >      Does the IIc+ also omit a connection for PB2?
> 
> According to the IIc technical reference (2nd edition, which includes
> the IIc+), the IIc+ is identical to every other IIc as far as the game
> connector is concerned.

     Good to know; I'll modify the info to include the IIc+.

 
> > > You cannot reliably put a TTL level into a IIc.  The only officially
> > > supported input combinations are a switched +5V with pulldown (as on
> > > earlier machines), or a similar transistor configuration (fully
> > > saturated PNP, with Vce <= 0.3V, and current into the IIc >= 30 mA,
> > > according to the techincal reference).
> > >
> > > The IIc manual specifically excludes a schottky gate output, switched
> > > ground through a transistor (open collector), and emitter follower
> > > transistor configuration.
> >
> >      It seems like about the only way to exclude Open Collector is for
> > the //c PB inputs to include a resistor going to Ground.
> 
> The IIc inputs have both pullup and pulldown resistors (different
> values), so the input normally sits somewhere between +5V and ground if
> nothing is connected.  According to my calculations, this is about 3.2V
> for SW0/MSW and 2.9V for SW1/MID.
> 
> The signal then passes through either of two paths, effectively a 2.5V
> comparator on SW0/MSW and a 2.2V comparator on SW1/MID.  If the input
> signal is lower than the threshold, the mouse signal goes active.  If
> the input signal is higher than the threshold, the joystick signal goes
> active.  (This also goes through an optocoupler.)  There is probably a
> minimum current requirement to activate the signal reliably.
> 
> An open collector output would not provide any current into the pin, so
> it wouldn't activate either the joystick or mouse signal.  You would
> need an external pullup at the very least.
> 
> >      To interface such an input to TTL, you would be stuck with
> > something like ...
> >
> >             +5V
> >               \ E
> >                \ |        about 1k
> >                  |--B-----/\/\/\--------<--- Output from TTL gate
> >      PB input  / |
> > ---------o----/ C  PNP Transistor
> >          |
> >          Z
> >          Z
> >          |
> >         Gnd
> >
> > i.e. your saturated PNP transistor.
> >
> >      Would the above circuit work on a //c?
> 
> This type of circuit is given as an example of something that will work
> (assuming we are talking about the same type of transistor - some parts
> of my electronics knowledge are a little rusty; the diagram in the IIc
> manual shows an arrow pointing in towards the base, on the upper signal,
> which you have labelled as the emitter).

     Yes; it is hard to do a slanting arrow in ASCII. For PNP the
Emitter arrow points in; for NPN it points out.


> 
> There is no pulldown resistor on the pushbutton input in the IIc's
> diagram (though I have no idea why - there is one for the pushbutton
> example).  It also specifies a minimum input current of 30 mA.

     With the opto coupling, it does sound like the //c PB inputs are
mainly 'current mode. Thanks for all the information!



Rubywand