RE: Commodore joystick ports

From: Scott McDonnell (
Date: 2007-04-09 08:22:16

> -----Original Message-----
> From: 
> [] On Behalf Of Jim Brain
> Sent: Monday, April 09, 2007 12:37 AM
> To:
> Subject: Re: Commodore joystick ports
> You must have missed my theory in the last post:

Actually I thought I was just agreeing with that theory. 

> > contents of the counter available while it is running, or is it 
> > latched into a register after the 512 cycles are up? If it is 
> > available to read while free running, this would definitely 
> increase 
> > the usefulness.
> >   
> The counter is not visible, just the registers.  That's why I'm 
> theorizing about the 9 bit counter, but it makes perfect 
> sense.  Use the 
> high bit to clamp the inputs, and use a comparator to latch 
> the counter 
> value into the SID register.

That's unfortunately what I figured. Makes sense, though. 
I was hoping I was wrong. :)

> It's a shame all of this good info isn't of current help to 
> the original 
> poster. 
> Jim

But interesting, nonetheless! The SID is one of my favorite parts
of the C64.

Anyway, as I get some time this week, I will try to get something
nailed down as far as what is needed. Right now, I am picturing
a NPN resistor with the control port 5V on the collector, control
port ground on the emitter, and a very large value resistor across
the collector and emitter (the transistor in parallel.) The emitter
is connected to the POT line. Since the transistor and the resistor
are in parallel, I can control the total resistance with the transistor.

I am really interested in the op-amp solution, but think it is beyond
my skills at the moment.

As a related story:

During the day, I work on a lot of older products (military) from
the same era as the Commodore 64. It is cool when I find that some
circuit is using a HW trick that is shared by the C64. They just don't
teach that stuff in the textbooks anymore. So, my knowledge of the C64
hardware has come in pretty handy in my career.

I also found out that my company was huge into the C64 and actually 
bought one for all the techs and engineers. They were also used for 
automated testing. I still see some Commodore equipment around and 
keep my eye out for them to let it go.

Scott McDonnell

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