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.
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
