Cassette Interface Assistance

Paul Berger phb.hfx at
Tue Feb 28 11:09:21 CST 2017

74LS of 74HC gates are not going to work, the signal level is only 1V 
the threshold for 74LS is 2V and for 74HC it is 3.7V.  I would probably 
use something like a compatator or an opamp but I don't have a circuit 
handy to use, but tehy should be easy to find.


On 2017-02-28 11:59 AM, Alexandre Souza via cctalk wrote:
> 74hc(or ls)14
> Enviado do meu Tele-Movel
> On Feb 28, 2017 12:55 PM, "Jim Brain via cctalk" <cctalk at>
> wrote:
>> 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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