Cassette Interface Assistance
brain at jbrain.com
Tue Feb 28 09:55:30 CST 2017
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
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
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.
brain at jbrain.com
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