Cassette Interface Assistance

Alexandre Souza alexandre.tabajara at
Tue Feb 28 09:59:21 CST 2017

74hc(or ls)14

Enviado do meu Tele-Movel

On Feb 28, 2017 12:55 PM, "Jim Brain via cctalk" <cctalk at>

> Analog, which is my nemesis, curses me again.
> I have a cute idea for a cassette port project for the Tandy line of
> computers (the ones with the cassette port).  I have a Coco 3 on the bench,
> so I scoped the output line while doing 'csave "jim"'.  The signal looks to
> be just under 1V PtP (0-1V on the scope), and rests at about .3V when not
> sending data.
> I have tried 6 different ways to boost the signal to 5V digital, to no
> avail, and so I ask humbly if someone with analog knowledge might be able
> to assist.
> I first tried to boost the signal with a transistor (with variations using
> a N channel FET as well). Arguably, that was foolhardy, and it did not work.
> My second attempt was based on this link that was shared with me:
> The output from the Coco3 does not appear to be "loud" enough to work with
> this circuit.
> So, I finally decided a comparator solution would be required.
> First, I tried a design using a 741 op-amp, which failed miserably, but
> probably would have worked, but I tried to merge the design from the Coco1,
> and replace the LM339 in the Coco 1 design with the 741, and I feel I did
> not merge the designs well :-)
> I then tried using the comparator in an Atmel AVR, and had minimal
> success.  By biasing one input via a variable resistor to around .8V, I was
> able to get a digital stream, but it did not look like the data stream of
> the cassette format.
> I then pried an LM339 out of my Coco1 and replicated the circuit int the
> Coco 1, as noted in the tech manual:
> Color Computer Technical Reference Manual (Tandy).pdf <
> Hardware/Color%20Computer%20Technical%20Reference%
> 20Manual%20%28Tandy%29.pdf>
> I was shocked that I had no success with that design at all.  I assumed
> (wrongly, it appears) that the Coco cassette input circuit would read the
> output of it's output circuit.  Beyond the possibility that my components
> are defective or I wired it up wrongly, I can only theorize that Tandy
> assumed that all tape recorders would AGC the output and then feed a 2V PtP
> signal back to the Coco (the Coco 1 circuit looks to bias the comparator at
> 1.05V (not sure about the feedback resistor's impact))
> I can fiddle around with the AVR solution, which might work if I can
> smooth out the spikes and bias the comparator right, but it just bothers me
> that the Coco 1 circuit does not work, as I assumed I would at least have
> success by copying a working design.
> Jim
> --
> Jim Brain
> brain at

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