Cassette Interface Assistance

Jim Brain brain at
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 
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 

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 Brain
brain at

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