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?
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
Best Answer
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