The Teletype dance

(Mirror of a Hacker News comment I wrote, responding to someone who wondered what a serial connection actually looked like “on the wire.”)

I binged CuriousMarc’s YouTube series on teletype restoration a few months ago, so here’s a simplified (and possibly somewhat inaccurate) explanation of how one works.

The only electrical components in a Teletype are (a) a continuously-running electric motor, and (b) an electromagnet and a few switches. Everything else is completely mechanical.

The Teletypes in a circuit, and the electromagnet and switches within them, are all connected in series, forming a current loop. (Current, rather than voltage, is used because you can use a constant-current power supply to get the same power at each electromagnet regardless of how many are in the circuit and how many miles of wire are between them.)

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RAAF Syko machine manual

The Syko machine was a British strip cipher used during WWII for encoding radio communications between aircraft. There's not much information on the Web about these machines; a scan of the Syko Manual (PDF, 2.4MB) is available but it's rather hard to read as it's an old document and has had extra annotations added.

In this post, I have transcribed the entire document to make it easier to read and find information about how the Syko machines were operated.

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