Build it Yourself Modem for under $50 (Byte Magazine, 1980)
archive.org
archive.org
- They are using acoustic pickup for some reason (possibly for legal reasons?), there are plans on how to construct interfaces from home depot materials.
- The modem is fixed speed, 300bps and uses FSK. There is no dialing, pickup/release or ring detection, this is all manual. It is RS232 lines connected to modulator/demodulator and then to phone line.
- For modulator, they use NE567 which is designed as a tone decoder, but can be abused to act as tone generator instead. It can be muted by demodulator's "carrier detect", so the modem can auto-shut-off if the other side is gone.
- For demodulator they use XR2211, which is apparently special PLL designed specifically for the modems.
- Every other page is a full-page ad, plus there are ads inside the text, too. So 50%+ area is advertisement. If you thought that the web invented pervasive advertisements, you are wrong.
mx614 circuit - https://www.cmlmicro.com/wp-content/uploads/2017/06/FAQs-MX6...
attiny style - https://sites.google.com/site/wayneholder/attiny-4-5-9-10-as...
direwolf softmodem - https://github.com/wb2osz/direwolf
From this article, I (possibly re-)learned that originating and receiving modems use different frequencies, but I guess that should be obvious from being full duplex. Also I didn't know that most of the handshake is ignored. I'm assuming that means in 300 baud or less, since the handshake also negotiates data speeds.
Shocking is the $30/kbyte RAM chips advertised in between, yet the wire stripper/wrapper/unwrapper on page 33 is pretty much the price you would expect it if on sale today.