Cassette Interface Assistance

[Tony Duell]

On Tue, Feb 28, 2017 at 6:10 PM, Jim Brain <brain at jbrain.com> wrote:

> A digital one.  The current goal is to feed the incoming data stream into an
> Atmel AVR running at 5V for parsing.  The final goal is to do the same, but
> run the AVR at 3V on two coin cells.

OK. I assume you don't want to be running special software on the CoCo, since
there are easier places to get a digital signal from if you can run
special software
(such as the RS232 port).

>
>

[Comparators]

> I saw the notes in the datasheet about the oscillation, and the recommended
> way to overcome this.  I also noticed the Coco 1 schematic includes this

Yes, they do oscillate with the slightest provocation....

> exact recommendation.  I then wired up the exact circuit, using the LM339
> from my Coco1, and the exact resistors and other passives as specified in
> the schematic.
>
> Where do you get the 0.5V reference from? Well, if your 5V supply is stable
> you
> can use that, divided down with a couple of resistors. Say 9.1k and 1k in
> series
> across the supply, comparator input to the junction.
>
> The Coco1 creates a 1.05V reference via the 15K and 56K resistors.  After I
> noticed that the analog signal was centered around .5V, I quickly added
> another 15K resistor in parallel with the first one, to create a 56K/7.5K
> divider, which yields .591V as a reference.  I am using a lab PSU, running
> at 5.00V, but the resulting circuit did not respond to changes in the Coco
> cassette data signal.  I rechecked my circuit last night, but will recheck
> it tonight in case I made some stupid wiring mistake.

OK... Time to check the obvous. Firstly you have power to the comparator
chip (Easily forgotten, I speak from experience). You have a pull-up resistor
(say 1k, or 4k7) from the comparator output to +5V.

Now measure the output of that resistor divider. Make sure it _is_ 0.6V or
so. Use a 'scope in DC coupled mode (you do not want to lose the DC
level of the signal by using AC coupling) to look at the signal from the
CoCo on the other comparator input.

-tony