Cassette Interface Assistance

[allison]

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 classiccmp.org>
>> 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:
>>>
>>> http://labs.rakettitiede.com/12kbps-simple-audio-data-transfer-for-avr/
>>>
>>> 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 <
>>> http://www.colorcomputerarchive.com/coco/Documents/Manuals/
>>> 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 jbrain.com
>>> www.jbrain.com
>>>
>>>
>
>