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No signal.

This 80s calculator, a TI 74 has a function that lets you save BASIC programs to a cassette tape. But, I don't have the cable to connect to a tape deck (or MP3 recorder?) So, I put my oscilloscope on the contacts (The ground pin and 6th pin): this shows that the digital signal is easy to detect. Can I run this through an amplifier and record it? Or do I need additional hardware?

You can see the signal becoming live.

You can see the digital signal.

UPDATE I soldiered together an audio cable and this video shows what the TI-74 sounds like when saving a program to tape. I was not able to load the program back on to the TI-74 using this sound file, I think I need to adjust the pinouts.

https://www.youtube.com/watch?v=q_TumkmldJk&feature=youtu.be

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    \$\begingroup\$ Possibly related: retrocomputing.stackexchange.com/questions/845/… \$\endgroup\$ – MarkU May 4 '17 at 4:39
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    \$\begingroup\$ There are two ICs in the CI-7 cassette interface module that plugs into the dockport. There is special software involved in exchanging data and audible tones for recording use. I'd be worried about just connecting it to something. Google up what you can about what I mentioned. You might get lucky. \$\endgroup\$ – jonk May 4 '17 at 4:44
  • \$\begingroup\$ I think those ICs may be for the computer connection? I found a schematic for connecting to a DOS PC via the serial port. I don't have any old PCs that could do that so I hoped the audio route would be simpler. \$\endgroup\$ – futurebird May 4 '17 at 4:46
  • \$\begingroup\$ if you play it back, it should work; tapes are not complex, and neither is the signaling. \$\endgroup\$ – dandavis May 4 '17 at 5:15
  • \$\begingroup\$ Here's a cassette interface to examine: petlibrary.tripod.com/cassette.html and here is the technical manual: progettosnaps.net/manuals/pdf/ti74.pdf where, starting on page 17 you can read quite some detail about the connector and protocols (schematics of main board [not CI-7] near bottom.) \$\endgroup\$ – jonk May 4 '17 at 5:26
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I realized such an Interface severals years ago. The "Data Output" pin is pin #6 --> D3. The "Data Input" pin is pin#3 --> D0.

Recording:

you are on the right way: just use a voltage divider (e.g. 2 resisitors) to match the voltage requirements your recorder. A 100nF capacitor between voltage divider and mic-input could help.

Playback:

You have to clean and amplify the recorded signal, it must be a square wave, with low state = "GND" and high state = "VCC".

I used a common LM358N as non inverting Schmitt-Trigger to obtain a clean wave. It'easy, you need only 4 resistors. I played wit several resistor values, I obtained the best results with a 0,3V treshold.

You can supply the LM358N from Pin#1. Ok, it drains power, but not noticeably ;-)

I used the same interface with a Sharp PC-1260: VCC: pin#2, GND; pin#3, data in: pin#6, data out: pin#7

Have fun! :-)

Paolo

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Most probably, you will need simple amplifier based on, let's say, LM4558, but you may start even without it at all seeing if your capture device will save it properly.

You have two tasks - ensure (1) you save signal with level enough to (2) be reproduced with required level.

I also played with such implementations, and, if you choose MP3 format, use maximal settings. MP3 is lossy format, and it may reproduce modified square-like waveform which TI-84 will not capture. The best format is, of course, WAV, but the file size will be large. That's why there're programs out there to convert such files to binary files, which then can be converted back to output square waveforms using special program and target device's protocol.

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  • \$\begingroup\$ mp3 far exceeds the bandwidth of 80-era cassette tapes; especially on plain signals without competing noise that invokes the ATH model, then it's almost CD-quality. arguably. \$\endgroup\$ – dandavis May 4 '17 at 5:14
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    \$\begingroup\$ I can tell you that 44kHz/16bit MP3 worked with my 8-bit computer, 22kHz/16bit or 44 kHz/8bit did not. Arguing per theory is one thing, but experience proves it is not that simple. Also, to argue about applicability of MP3 you should know its internals very well. I know it relatively well (up to trying writing it in Verilog). \$\endgroup\$ – Anonymous May 4 '17 at 5:18
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    \$\begingroup\$ OP's signal is only ~1Khz; a lot less, but i agree about real vs math. make sure to use mono and cbr and apply pass filters to maximize the mp3's concentration... \$\endgroup\$ – dandavis May 4 '17 at 5:20
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    \$\begingroup\$ But MP3 is designed for compressing music so a human (allegedly) can't tell the difference, not compressing data so a computer can't tell the difference. Doesn't matter how much bandwidth MP3 can manage compared to the data, if the compression algorithm still mangles the data beyond what the computer is able to recover. (Of course, @Anonymous's experience tells us that it does work in some cases). \$\endgroup\$ – Muzer May 4 '17 at 8:52
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    \$\begingroup\$ Ok, you know that file size will be relatively big. If you are really into this subject, I recommend you joining some retro TI community, it is a good chance that someone has already designed application I mentioned (playing small binaries rather than large WAV files), and may even have a set of circuits to connect external recording and playback devices. \$\endgroup\$ – Anonymous May 4 '17 at 21:06
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The TI-74 was very advanced for its day, and some of the peripherals were intelligent.

You can look here at the manual that shows how to interface to it. From that you will see that the interface is a nibble I/F and you need to externally serialize the data to record it. Their tape interface was called the Wafertape. I have never seen an Audio tape interface.

The Computer Archive has a complete technical manual with schematic and a great document on the HeXbus (the nibble wide port).

They also have a schematic of the nibble wide PC-port, that should give you ideas for a simple MCU interface.

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The TI-74 cassette function expects a CI-7 cassette interface plugged into the "Dockbus" port on the back of the calculator. The "TI-74 Programming Reference Guide" is available here;

http://ftp.whtech.com/hexbus_cc40_ti74/ti-74/

& details connecting & operating a cassette recorder attached to the CI-7. More info on the Dockbus port is contained in the "TI-74 Technical Reference" pdf in that same directory. HTH.

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