Re: Cassette Interface Assistance

[allison via cctalk]

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@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
br...@jbrain.com
www.jbrain.com