Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 3/1/2017 11:56 AM, dwight via cctalk wrote:

I'm not sure how something as simple as a cassette interface could be causing 
so much issues.

Can you post a schematic of what you have now and what the signal amplitude is?
Hmm, It's currently a jumble of wires and clips and such on the bench.  
I'll see if I can make some sense of it.


Jim

Re: Cassette Interface Assistance

[dwight via cctalk]

I'm not sure how something as simple as a cassette interface could be causing 
so much issues.

Can you post a schematic of what you have now and what the signal amplitude is?

Dwight



From: cctalk  on behalf of allison via cctalk 

Sent: Wednesday, March 1, 2017 3:10:50 AM
To: cctalk@classiccmp.org
Subject: Re: Cassette Interface Assistance

On 03/01/2017 01:40 AM, Jim Brain via cctalk wrote:
> On 2/28/2017 2:34 PM, Brent Hilpert wrote:
snippage
> Obviously, given the LM339 issues, this is unworkable in the end, but
> it did get me to realizing some outputs on the LM339, so thanks to
> both of you.
>
> I can plainly see the oscillation on the comparator transitions, so it
> is obvious I need a better comparator.  I'll have to source a few
> different comparator options from Digikey to fill my parts box.
> Suggestions are welcome.
OR  a bit of positive feedback.

>> Using the internal AVR comparator sounds like a better final solution
>> (fewer components), but in devising an
>> input circuit for the AVR you may be running into the same issue of
>> loading a high-impedance source.
> Though it plays into Tonly's notes about my analog understanding (or
> lack thereof), I was so disillusioned by the analog uncertainties that
> I initial focused on the AVR comparator.  By correctly biasing the
> comparator, I was able to get much better results tonight, but the
> lack of any ability to add hysteresis into the design hindered my
> success.  At transition, the signal simply bounces badly.  I tried to
> construct a low pass filter (22pF and 5K6), and some variations, but I
> was not able to overcome.  I then tried to deal with it in SW, but I
> have not yet been successful.  I'll try again, but lack of hysteresis
> is a big issue.
>
Single transistor audio amp before it.

Allison


> Jim
>
>

Re: Cassette Interface Assistance

[allison via cctalk]

On 03/01/2017 01:40 AM, Jim Brain via cctalk wrote:
> On 2/28/2017 2:34 PM, Brent Hilpert wrote:
snippage
> Obviously, given the LM339 issues, this is unworkable in the end, but
> it did get me to realizing some outputs on the LM339, so thanks to
> both of you.
>
> I can plainly see the oscillation on the comparator transitions, so it
> is obvious I need a better comparator.  I'll have to source a few
> different comparator options from Digikey to fill my parts box.
> Suggestions are welcome.
OR  a bit of positive feedback.

>> Using the internal AVR comparator sounds like a better final solution
>> (fewer components), but in devising an
>> input circuit for the AVR you may be running into the same issue of
>> loading a high-impedance source.
> Though it plays into Tonly's notes about my analog understanding (or
> lack thereof), I was so disillusioned by the analog uncertainties that
> I initial focused on the AVR comparator.  By correctly biasing the
> comparator, I was able to get much better results tonight, but the
> lack of any ability to add hysteresis into the design hindered my
> success.  At transition, the signal simply bounces badly.  I tried to
> construct a low pass filter (22pF and 5K6), and some variations, but I
> was not able to overcome.  I then tried to deal with it in SW, but I
> have not yet been successful.  I'll try again, but lack of hysteresis
> is a big issue.
>
Single transistor audio amp before it.

Allison


> Jim
>
>

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 2:34 PM, Brent Hilpert wrote:

In answer to the final point, and the interests of some analog education, 
looking at your reffed pdf, page 75, the reason (or one reason)
your duplicated Coco input circuit is not working is the gross impedance 
mismatch between the coco tape output and input circuits.
To get to the point, this advice was golden.  After stripping away the 
220, 8K2,6K8 and the 56K to Vcc, the unit started functioning. A nasty 
60Hz blip was removed by slightly biasing the negative input with a 1M 
resistor to GND.  The 1M drive to ground pulled the input over so much 
lower, making the center around .7V, so I used Tony's idea to create a 
.7V reference on the + end with a forward biased diode and a week pullup 
to Vcc, which made things ever so much better.


Obviously, given the LM339 issues, this is unworkable in the end, but it 
did get me to realizing some outputs on the LM339, so thanks to both of 
you.


I can plainly see the oscillation on the comparator transitions, so it 
is obvious I need a better comparator.  I'll have to source a few 
different comparator options from Digikey to fill my parts box. 
Suggestions are welcome.

Using the internal AVR comparator sounds like a better final solution (fewer 
components), but in devising an
input circuit for the AVR you may be running into the same issue of loading a 
high-impedance source.
Though it plays into Tonly's notes about my analog understanding (or 
lack thereof), I was so disillusioned by the analog uncertainties that I 
initial focused on the AVR comparator.  By correctly biasing the 
comparator, I was able to get much better results tonight, but the lack 
of any ability to add hysteresis into the design hindered my success.  
At transition, the signal simply bounces badly.  I tried to construct a 
low pass filter (22pF and 5K6), and some variations, but I was not able 
to overcome.  I then tried to deal with it in SW, but I have not yet 
been successful.  I'll try again, but lack of hysteresis is a big issue.


However, considering you have a known and fixed source circuit (the coco tape 
output) you might try something even simpler for the
AVR comparator input, such as just a 1K series R followed by a 220K to 470K R 
to GND. The comparator should then be seeing essentially the coco output
wave shape and levels, without you having to adjust the software detection for 
biasing introduced by the input circuit.

I will try this tomorrow.

Jim


--
Jim Brain
br...@jbrain.com
www.jbrain.com

Analog design: was: Cassette Interface Assistance

[Chuck Guzis via cctalk]

On 02/28/2017 03:05 PM, Noel Chiappa via cctalk wrote:

> I seem to have some bizarre brain dysfunction where I have a very
> hard time understanding even the simplest analog circuits. You, or
> someone like Brent Hilpert (whose explanation of core memory drivers
> I still remember :-) can explain something with sufficient clarity
> for me to 'get' it, but I can almost never work it out on my own.

I always enjoyed reading the columns of the late Bob Peases in EDN.
When the rest of the industry seemed to be nothing but digital, Bob was
a great way to get your head cleared of all of the concentration on
tricks with ones and zeroes.

More than anything, when he wrote about analog circuits, it was more
than just a twist on some binary algorithm; he made you think a bit
about circuit design.

I miss him and his floobydust.

--Chuck

Re: Cassette Interface Assistance

[Noel Chiappa via cctalk]

> From: Tony Duell

> I fail to see how anyone can be a good digital designer and not
> understand analogue electronics.

It's easy! As long as your devices are being run in a domain where their
behaviour is purely, well, digital, one can get away with it! :-)

I'm a perfect case in point!

I seem to have some bizarre brain dysfunction where I have a very hard time
understanding even the simplest analog circuits. You, or someone like Brent
Hilpert (whose explanation of core memory drivers I still remember :-) can
explain something with sufficient clarity for me to 'get' it, but I can
almost never work it out on my own.

However, I've designed a number of digital devices, at least one of which
became a product!

Noel

Re: Cassette Interface Assistance

[Ed Sharpe via cctalk]

if anyone has a cable for the trs 80 model 100 small compact cassette drive to  
the 100 please lest us know. need this for demoing and display of the model 100 
in the museum...
thanks ed# www.smecc.org 

Sent from AOL Mobile Mail

On Tuesday, February 28, 2017 allison via cctalk  wrote:
On 2/28/17 1:43 PM, Jim Brain via cctalk wrote:
> On 2/28/2017 12:17 PM, Ethan Dicks via cctalk wrote:
>> It sounds like he is contemplating a battery-powered tape drive
>> emulator that plugs on the outside of the machine for users to use
>> without opening their boxes. It would then be portable to other
>> machines. Sounds like a great idea.
> This year's CocoFEST is open to all TRS-80 models, not just the Coco, 
> and I noticed that many of the models have a similar cassette port 
> connector (I think they have variations on the format), and so I 
> thought of a neat project that might be of interest to all of the 
> participants of the show.
>
The TRS-80 used two formats! THe base 4K with 4K basic was a slower 
format than the 12K basic machine.
The MM100 was a modem (bell 103 compatable).
The pocket computer was again different I forget the format.
COCOs I think were not all the same.

Lots of ground to cover.

Allison
> As you note, picking off 5V from inside, while entirely possible, is 
> far from optimal, and a uC is the intended recipient. Besides, as 
> Tony implies, I should know this stuff, but I have indeed managed to 
> move along in my efforts for so many years without needing to use a 
> comparator :-)
>
>

Re: Cassette Interface Assistance

[allison via cctalk]

On 2/28/17 1:43 PM, Jim Brain via cctalk wrote:

On 2/28/2017 12:17 PM, Ethan Dicks via cctalk wrote:

It sounds like he is contemplating a battery-powered tape drive
emulator that plugs on the outside of the machine for users to use
without opening their boxes.  It would then be portable to other
machines.  Sounds like a great idea.
This year's CocoFEST is open to all TRS-80 models, not just the Coco, 
and I noticed that many of the models have a similar cassette port 
connector (I think they have variations on the format), and so I 
thought of a neat project that might be of interest to all of the 
participants of the show.


The TRS-80 used two formats!  THe base 4K with 4K basic was a slower 
format than the 12K basic machine.

The MM100 was a modem (bell 103 compatable).
The pocket computer was again different I forget the format.
COCOs I think were not all the same.

Lots of ground to cover.

Allison
As you note, picking off 5V from inside, while entirely possible, is 
far from optimal, and a uC is the intended recipient.  Besides, as 
Tony implies, I should know this stuff, but I have indeed managed to 
move along in my efforts for so many years without needing to use a 
comparator :-)

Re: Cassette Interface Assistance

[Brent Hilpert via cctalk]

On 2017-Feb-28, at 7:55 AM, Jim Brain via cctalk 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 
> 
>  
> 
> 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.

In answer to the final point, and the interests of some analog education, 
looking at your reffed pdf, page 75, the reason (or one reason)
your duplicated Coco input circuit is not working is the gross impedance 
mismatch between the coco tape output and input circuits.

The output is high impedance: a 75K/24K voltage divider.
The input is quite low impedance: a 220-ohm resistor to GND right at the front.
When you connect them together they form another voltage divider which drops 
the coco tape output down to a few millivolts across
the 220-ohm R, which then is the input to the 339 comparator (pin 10).
The trip point for the comparator is set at 1.05V by the 56K/15K voltage 
divider at pin 11.

If you want to play with the 339 comparator circuit, you could try adjusting 
the comparator trip point voltage divider
(replace one of the 56K/15K appropriately) to get, say, 0.25V at pin 11;
strip away all the input stuff at pin 10 and feed it directly from the tape 
output.
(Could be a good exercise, watching the 399 output along with the tape output 
on a 2-chan scope).

Using the internal AVR comparator sounds like a better final solution (fewer 
components), but in devising an
input circuit for the AVR you may be running into the same issue of loading a 
high-impedance source.
The 8.2K/6.8K R divider in the rakettitiede AVR circuit you tried is still 
quite disparate to the coco tape output and will load it,
reducing the level seen by the AVR comparator. The blocking cap and biasing 
also changes the levels you would be looking for at the comparator.
You could try multiplying the Rs of that divider up into the 10-100Ks range 
(probably best to reduce the 0.47 shunting C by a similar factor or more).

However, considering you have a known and fixed source circuit (the coco tape 
output) you might try something even simpler for the
AVR comparator input, such as just a 1K series R followed by a 220K to 470K R 
to GND. The comparator should then be seeing essentially the coco output
wave shape and levels, without you having to adjust the software detection for 
biasing introduced by the input circuit.


(This topic reminds me of my first assembly program as a teenager in the 70s, 
to make tape storage for a MEK 6800 SBC.
I wanted to keep the hardware as minimal as poss

Re: Cassette Interface Assistance

[Sean Conner via cctalk]

It was thus said that the Great Tony Duell via cctalk once stated:
> On Tue, Feb 28, 2017 at 4:21 PM, allison via cctalk
>  wrote:
> 
> 
> >
> > The tape recorders used had no agc.  They were simple portables and used the
> > mic or line input and headphone output.
> 
> I have to disagree with you there. I have just looked at the documentation 
> for a
> couple of the Radio Shack tape recorders commonly used with TRS80s, and
> both have automatic gain control on recording.

  I recall using a normal casette tape recorder with my Coco back in the
day.  I don't recall a gain control, but I do recall setting the volumn
control way up.  Never had a problem reading back tapes.

  -spc

Re: Cassette Interface Assistance

[Fred Cisin via cctalk]

On Tue, 28 Feb 2017, Jim Brain via cctalk wrote:
This year's CocoFEST is open to all TRS-80 models, not just the Coco, and I 
noticed that many of the models have a similar cassette port connector


yes.  even the IBM PC 5150 uses the same connector and pinout, and cable 
(Radio Shack #26-1207)



(I think they have variations on the format),


for some meaning of "variation"

Re: Cassette Interface Assistance

[Tony Duell via cctalk]

On Tue, Feb 28, 2017 at 4:21 PM, allison via cctalk
 wrote:


>
> The tape recorders used had no agc.  They were simple portables and used the
> mic or line input and headphone output.

I have to disagree with you there. I have just looked at the documentation for a
couple of the Radio Shack tape recorders commonly used with TRS80s, and
both have automatic gain control on recording.

It is actually quite rare to see a simple portable cassette recorder which
doesn't have automatic gain control (automatic level control). The early
Philips machines like the EL3300 are the only ones I've come across.


-tony

Re: Cassette Interface Assistance

[Tony Duell via cctalk]

On Tue, Feb 28, 2017 at 6:10 PM, Jim Brain  wrote:

> A digital one.  The current goal is to feed the incoming data stream into an
> Atmel AVR running at 5V for parsing.  The final goal is to do the same, but
> run the AVR at 3V on two coin cells.

OK. I assume you don't want to be running special software on the CoCo, since
there are easier places to get a digital signal from if you can run
special software
(such as the RS232 port).

>
>

[Comparators]

> I saw the notes in the datasheet about the oscillation, and the recommended
> way to overcome this.  I also noticed the Coco 1 schematic includes this

Yes, they do oscillate with the slightest provocation

> exact recommendation.  I then wired up the exact circuit, using the LM339
> from my Coco1, and the exact resistors and other passives as specified in
> the schematic.
>
> Where do you get the 0.5V reference from? Well, if your 5V supply is stable
> you
> can use that, divided down with a couple of resistors. Say 9.1k and 1k in
> series
> across the supply, comparator input to the junction.
>
> The Coco1 creates a 1.05V reference via the 15K and 56K resistors.  After I
> noticed that the analog signal was centered around .5V, I quickly added
> another 15K resistor in parallel with the first one, to create a 56K/7.5K
> divider, which yields .591V as a reference.  I am using a lab PSU, running
> at 5.00V, but the resulting circuit did not respond to changes in the Coco
> cassette data signal.  I rechecked my circuit last night, but will recheck
> it tonight in case I made some stupid wiring mistake.

OK... Time to check the obvous. Firstly you have power to the comparator
chip (Easily forgotten, I speak from experience). You have a pull-up resistor
(say 1k, or 4k7) from the comparator output to +5V.

Now measure the output of that resistor divider. Make sure it _is_ 0.6V or
so. Use a 'scope in DC coupled mode (you do not want to lose the DC
level of the signal by using AC coupling) to look at the signal from the
CoCo on the other comparator input.

-tony

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 12:36 PM, allison via cctalk wrote:

A sound recorder, how novel.  My I phone and androids already do that.
Before that I use tape recorders.
Using a classic machine, how novel.  My iPhone and androids already 
compute everything I need.  Before that I used other machines still more 
capable than classic systems.
AVR can measure small signals using the A/D and then do a bit of 
looking for zeo crossing to

detect bit times.
Even that is too much work, since the AVR has an internal comparator 
(that will fire an interrupt on crossing), no need to measure voltage 
levels.  But, as I noted in the original email, I already tried that, 
and did not have success.


Jim


--
Jim Brain
br...@jbrain.com
www.jbrain.com

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 11:42 AM, Seth Morabito via cctalk wrote:


Hi Jim,

When I was building a cassette circuit for my homebrew 6502 computer,
I essentially stole the Synertek SYM-1 circuit wholesale. See:

http://www.loomcom.com/blog/2013/01/04/retrochallenge-update-comparator-vs-zero-crossing-detector/

http://www.loomcom.com/blog/2013/01/05/comparator-success/

Synertek used an LM311 (an LM358 would work just fine) to build a
comparator circuit. I _think_ you could use this exact circuit to
take the analog output from the CoCo and turn it into a 5V sqauare
wave.
I will give it a try.  Obviously, I'll need to reset the bias down to 
.5V from 2.5V in your design, but I can do that.  I notice the design 
has no feedback, but has R93 feeding the reference into the inverting 
input.  Any idea what the function of that resistor is?


Jim

Re: Cassette Interface Assistance

[allison via cctalk]

On 2/28/17 12:56 PM, Chuck Guzis via cctalk wrote:

On 02/28/2017 09:42 AM, Seth Morabito via cctalk wrote:


Synertek used an LM311 (an LM358 would work just fine) to build a
comparator circuit. I _think_ you could use this exact circuit to
take the analog output from the CoCo and turn it into a 5V sqauare
wave.


I don't know a thing about the CoCo cassette format.  I know that back
in the day when things like the KC standard were the rage, a NE567 PLL
was considered the cat's whiskers for data recovery.  But again, I don't
know what the CoCo cassette standard looks like

Only works for FM and frequency shift based formats.

Everyone had their own favorite.  I used to flip a bit doing biphase in 
software

at 2400 baud.  Minimal parts to none.

Allison

Also, you may want to look at the IBM  5150 (PC) cassette circuit.  My
recollection was that it was pretty robust, even if few made use of the
facility.

--Chuck

Re: Cassette Interface Assistance

[allison via cctalk]

On 2/28/17 1:17 PM, Ethan Dicks via cctalk wrote:

On Tue, Feb 28, 2017 at 11:21 AM, allison via cctalk
 wrote:

On 2/28/17 10:55 AM, Jim Brain via cctalk wrote:

I have a cute idea for a cassette port project for the Tandy line of
computers (the ones with the cassette port)


Why not just go inside and grab the signal at it source before any of the
analog?  it starts life at TTL waveforms (not 5V).

It sounds like he is contemplating a battery-powered tape drive
emulator that plugs on the outside of the machine for users to use
without opening their boxes.  It would then be portable to other
machines.  Sounds like a great idea.

A sound recorder, how novel.  My I phone and androids already do that.
Before that I use tape recorders.

Why are you trying to get to 5V in the first place?

Digital input to the AVR.

Sounds more complicated than need be.

AVR can measure small signals using the A/D and then do a bit of looking 
for zeo crossing to

detect bit times.

Allison

-ethan

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 12:17 PM, Ethan Dicks via cctalk wrote:

It sounds like he is contemplating a battery-powered tape drive
emulator that plugs on the outside of the machine for users to use
without opening their boxes.  It would then be portable to other
machines.  Sounds like a great idea.
This year's CocoFEST is open to all TRS-80 models, not just the Coco, 
and I noticed that many of the models have a similar cassette port 
connector (I think they have variations on the format), and so I thought 
of a neat project that might be of interest to all of the participants 
of the show.


As you note, picking off 5V from inside, while entirely possible, is far 
from optimal, and a uC is the intended recipient.  Besides, as Tony 
implies, I should know this stuff, but I have indeed managed to move 
along in my efforts for so many years without needing to use a 
comparator :-)



--
Jim Brain
br...@jbrain.com
www.jbrain.com

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 11:56 AM, Chuck Guzis via cctalk wrote:

On 02/28/2017 09:42 AM, Seth Morabito via cctalk wrote:


Synertek used an LM311 (an LM358 would work just fine) to build a
comparator circuit. I _think_ you could use this exact circuit to
take the analog output from the CoCo and turn it into a 5V sqauare
wave.


I don't know a thing about the CoCo cassette format.  I know that back
in the day when things like the KC standard were the rage, a NE567 PLL
was considered the cat's whiskers for data recovery.  But again, I don't
know what the CoCo cassette standard looks like.
I admit my previous cassette format knowledge is Commodore based, where 
they basically shoved a 5V digital signal to the cassette deck, and 
expected the deck (which they produced) to handle it. Thus, interfacing 
with it is a digital exercise.  But, I am not picky on circuitry, so I 
will take a look.  Any pointers to a schematic?


Also, you may want to look at the IBM  5150 (PC) cassette circuit.  My
recollection was that it was pretty robust, even if few made use of the
facility.
I will do so.  Right now, a success, even if it's not a final state 
option, would be helpful.  I had this great idea, but held off on it for 
a few weeks because I suspected I'd end up in this state...


--Chuck



--
Jim Brain
br...@jbrain.com
www.jbrain.com

Re: Cassette Interface Assistance

[Ethan Dicks via cctalk]

On Tue, Feb 28, 2017 at 11:21 AM, allison via cctalk
 wrote:
> On 2/28/17 10:55 AM, Jim Brain via cctalk wrote:
>> I have a cute idea for a cassette port project for the Tandy line of
>> computers (the ones with the cassette port)
>>
> Why not just go inside and grab the signal at it source before any of the
> analog?  it starts life at TTL waveforms (not 5V).

It sounds like he is contemplating a battery-powered tape drive
emulator that plugs on the outside of the machine for users to use
without opening their boxes.  It would then be portable to other
machines.  Sounds like a great idea.

> Why are you trying to get to 5V in the first place?

Digital input to the AVR.

-ethan

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" 
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

Re: Cassette Interface Assistance

[allison via cctalk]

On 2/28/17 10:55 AM, Jim Brain via cctalk 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.



Why?

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 
 



The 339 is a very strange beast and can behave very badly.  The 741 is 
slow but should work as an amplifier and as a comparator
with minimal grief.  Trying to use 339 or 714 circuits for each other 
will usually fail as they behave differently.


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))


The tape recorders used had no agc.  They were simple portables and used 
the mic or line input and headphone output.


The big thing is most of those circuits took advantage of the audio out 
of the recorder (low level and fairly low impedance)

easily but larger they would overload (sometimes badly).

The output was for a low level input and loading it usually lowers the 
output more (high impedance source).


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.


Why not just go inside and grab the signal at it source before any of 
the analog?  it starts life at TTL waveforms (not 5V).


Why are you trying to get to 5V in the first place?


Allison


Jim

Re: Cassette Interface Assistance

[Jim Brain via cctalk]

On 2/28/2017 11:43 AM, Tony Duell via cctalk wrote:

I fail to see how anyone can be a good digital designer and not
understand analogue
electronics.

A couple points:

 * It is entirely possible I am not a good digital designer
 * I believe I have a basic understanding of analog, but I may not be
   competent.



To get back to the problem. What do you _actually_ want the output to become? An
analogue signal or a digital one?
A digital one.  The current goal is to feed the incoming data stream 
into an Atmel AVR running at 5V for parsing.  The final goal is to do 
the same, but run the AVR at 3V on two coin cells.



If you want a digital signal then look at a comparator chip like the LM339. This
compares the voltages on the 2 inputs, the output changes state as they pass
each other (if you see what I mean).
I understand the comparator function, though I will admit I have rarely 
used one.  As I had some success on the AVR internal comparator circuit, 
I was able to play around with the bias and see how it affected the output.

So you cassout signal to one
input, a stable
voltage of, say 0.5V on the other. There are couple of gotchas with
this chip. The
first is that it has open collector outputs, so you need a pullup
resistor. The second
is that it tends to oscillate at switchover. You can add hysteresis
with a couple
of resistors to prevent this.
I saw the notes in the datasheet about the oscillation, and the 
recommended way to overcome this.  I also noticed the Coco 1 schematic 
includes this exact recommendation.  I then wired up the exact circuit, 
using the LM339 from my Coco1, and the exact resistors and other 
passives as specified in the schematic.


Where do you get the 0.5V reference from? Well, if your 5V supply is stable you
can use that, divided down with a couple of resistors. Say 9.1k and 1k in series
across the supply, comparator input to the junction.
The Coco1 creates a 1.05V reference via the 15K and 56K resistors. After 
I noticed that the analog signal was centered around .5V, I quickly 
added another 15K resistor in parallel with the first one, to create a 
56K/7.5K divider, which yields .591V as a reference.  I am using a lab 
PSU, running at 5.00V, but the resulting circuit did not respond to 
changes in the Coco cassette data signal.  I rechecked my circuit last 
night, but will recheck it tonight in case I made some stupid wiring 
mistake.


Jim

--
Jim Brain
br...@jbrain.com
www.jbrain.com

Re: Cassette Interface Assistance

[Paul Berger via cctalk]

Now that I read your whole email I would like to ask what you plan to do 
with this boosted signal?   I have never had a CoCo, but most cassette 
interfaces on these old computers pretty much all worked like a modem 
but you where recording the signal instead of sending it down a phone line.


From what I read the CoCo uses   1200Hz for zero and 2400 Hz for one, 
what I was reading also went on to claim that it is a single cycle of 
the tone to represent a bit.  If you intend to feed this into a computer 
converting the signal to a square wave would be ok as long as the 
software monitoring the port had a high enough sampling rate to be able 
to differentiate the width of the pulses, which should not be hard as 
one is twice as long as the other.  A comparator would be the best way 
to produce a square wave representation, as they normally have a sharp 
threshold which you would want to adjust so that it switches over at 
about the 50% level of the input signal.  If you have a dual channel 
scope you can compare the input signal to the output to see if that is 
what is happening.  That might be hard to see on an analog scope, this 
is where a storage scope or a digital scope comes in handy because you 
can  freeze the signal.


Paul.





On 2017-02-28 11:55 AM, Jim Brain via cctalk 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 
 



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

Re: Cassette Interface Assistance

[Chuck Guzis via cctalk]

On 02/28/2017 09:42 AM, Seth Morabito via cctalk wrote:

> Synertek used an LM311 (an LM358 would work just fine) to build a 
> comparator circuit. I _think_ you could use this exact circuit to 
> take the analog output from the CoCo and turn it into a 5V sqauare 
> wave.


I don't know a thing about the CoCo cassette format.  I know that back
in the day when things like the KC standard were the rage, a NE567 PLL
was considered the cat's whiskers for data recovery.  But again, I don't
know what the CoCo cassette standard looks like.

Also, you may want to look at the IBM  5150 (PC) cassette circuit.  My
recollection was that it was pretty robust, even if few made use of the
facility.

--Chuck

Re: Cassette Interface Assistance

[geneb via cctalk]

On Tue, 28 Feb 2017, Tony Duell via cctalk wrote:


On Tue, Feb 28, 2017 at 5:12 PM, Tapley, Mark via cctalk
 wrote:

On Feb 28, 2017, at 9:55 AM, Jim Brain via cctalk  wrote:


Analog, which is my nemesis, curses me again.


In my opinion, I fail to see how anyone can be a good digital designer
and not understand analog electronics.



Fixed that for ya. :D

g.

--
Proud owner of F-15C 80-0007
http://www.f15sim.com - The only one of its kind.
http://www.diy-cockpits.org/coll - Go Collimated or Go Home.
Some people collect things for a hobby.  Geeks collect hobbies.

ScarletDME - The red hot Data Management Environment
A Multi-Value database for the masses, not the classes.
http://scarlet.deltasoft.com - Get it _today_!

Re: Cassette Interface Assistance

[Tony Duell via cctalk]

On Tue, Feb 28, 2017 at 5:12 PM, Tapley, Mark via cctalk
 wrote:
> On Feb 28, 2017, at 9:55 AM, Jim Brain via cctalk  
> wrote:
>
>> Analog, which is my nemesis, curses me again.

I fail to see how anyone can be a good digital designer and not
understand analogue
electronics.

>>
>> 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.

That sounds about right. IIRC the CoCo 3 uses a custom chip that contains a
6 bit DAC for this. Older machines just use a couple of TTL output
port lines and
a few resistors. The CoCo3 service manual suggests the output is 1V peak-to-peak



>>
>> 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.
>
> Warning, I am not an electrical engineer at all, only opinionated.
>
> What about:
>
> (analog out) -> A/D conversion -> tight software loop -> D/A conversion -> 
> (analog in) ?

Oh for $deity's sake. Why does everyone now insist on using a million components
when a dozen will do? I stick to classic computers to get away from this. In any
case it won't help that much. You should have some analgue 'signal conditioning'
circuitry before the ADC (to get the input signal into the right
voltage range for
the ADC), ditto after the DAC. Otherwise you are wasting the performance of said
converters.

To get back to the problem. What do you _actually_ want the output to become? An
analogue signal or a digital one?

An op-amp will boost the signal. Look at the LM324 (a very cheap and common
part, I have chosen it because the input voltage range goes down to 0 so you
can run it off a single 5V supply). You could use it as a *5 amplifier by adding
2 or 3 resistors.

If you want a digital signal then look at a comparator chip like the LM339. This
compares the voltages on the 2 inputs, the output changes state as they pass
each other (if you see what I mean). So you cassout signal to one
input, a stable
voltage of, say 0.5V on the other. There are couple of gotchas with
this chip. The
first is that it has open collector outputs, so you need a pullup
resistor. The second
is that it tends to oscillate at switchover. You can add hysteresis
with a couple
of resistors to prevent this.

Where do you get the 0.5V reference from? Well, if your 5V supply is stable you
can use that, divided down with a couple of resistors. Say 9.1k and 1k in series
across the supply, comparator input to the junction. Or perhaps 0.7V from a
forward biased silicon diode (diode cathode to ground, anode to +5V via a 4k7
or so resistor, take the 0.7V from the diode anode).

Do you have 'The Art of Electronics'? If not buy it. Yes, it's expensive But it
explains all this and more.

-tony

Re: Cassette Interface Assistance

[Seth Morabito via cctalk]

* On Tue, Feb 28, 2017 at 09:55:30AM -0600, Jim Brain via cctalk 
 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.

Hi Jim,

When I was building a cassette circuit for my homebrew 6502 computer,
I essentially stole the Synertek SYM-1 circuit wholesale. See:

http://www.loomcom.com/blog/2013/01/04/retrochallenge-update-comparator-vs-zero-crossing-detector/

http://www.loomcom.com/blog/2013/01/05/comparator-success/

Synertek used an LM311 (an LM358 would work just fine) to build a
comparator circuit. I _think_ you could use this exact circuit to
take the analog output from the CoCo and turn it into a 5V sqauare
wave.

-Seth

Re: Cassette Interface Assistance

[Tapley, Mark via cctalk]

On Feb 28, 2017, at 9:55 AM, Jim Brain via cctalk  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.

Warning, I am not an electrical engineer at all, only opinionated.

What about:

(analog out) -> A/D conversion -> tight software loop -> D/A conversion -> 
(analog in) ?

Use a dedicated (Beaglebone? Raspberry Pi?) system in the middle, and o-scope 
the analogs to make sure it’s clean enough.
The dedicated system should be able to 5V (or whatever you want) on the output, 
and I would think be fast enough to generate a reasonable sine-wave coming out 
if it only has to be audio frequencies. 

Once you have it working that way (and the software loop should allow you to 
tune it at will), you could build a simple dedicated circuit to multiply the  
same way the software did.

This is killing a mosquito with a sledgehammer, I know. With any luck, one of 
the regulars who *is* an EE will chime in before you read this with the correct 
answer, in which case disregard.

> 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.

Can a plain transistor do both + - ways on voltage?

> 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.

Even if it did, this looks like it implies all square waves. I think CoCo is 
designed to work with modulated sine-waves, and there could be some frequency 
components of the square waves (3x frequency, etc) that are causing problems 
somewhere. 

I don’t know that an AVR can run fast enough to do the sine-waves. I would 
guess you need to sample and digitize at least 10 times faster than the carrier 
(so maybe 100kHz?) and same for the output. 

> So, I finally decided a comparator solution would be required.

Still square-wave? I could be on the wrong track here completely, but I’m 
thinking you want sinewaves. 

> 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
> 
> 
> 
> -- 
> Jim Brain
> br...@jbrain.com
> www.jbrain.com
- Mark

> 

Re: Cassette Interface Assistance

[Paul Berger via cctalk]

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" 
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

Re: Cassette Interface Assistance

[Alexandre Souza via cctalk]

74hc(or ls)14

Enviado do meu Tele-Movel

On Feb 28, 2017 12:55 PM, "Jim Brain via cctalk" 
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
>
>

Cassette Interface Assistance

[Jim Brain via cctalk]

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 
 



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