Cassette Interface Assistance

allison ajp166 at
Tue Feb 28 12:08:21 CST 2017

On 2/28/17 12:09 PM, Paul Berger via cctalk wrote:
> 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.
Simply bias it in the middle of the range it works well then.

All digital circuits start as analog designs.

> Paul.
> 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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