by Darius Kazemi, May 15 2019

In 2019 I'm reading one RFC a day in chronological order starting from the very first one. More on this project here. There is a table of contents for all my RFC posts.

Correspondence models

RFC-135 is titled “Response to NWG/RFC #110” and is authored by Wayne Hathaway of NASA Ames on April 29th, 1971.

The technical content

This is a response to RFC-110, which discussed ways to handle the IBM 2741 teletype terminal. The main issue of that RFC was that there are some characters on the 2741 that aren't supported in ASCII, and there are some ASCII characters that have no key on the keyboard for the 2741.

This RFC exists because RFC-110 was apparently written with one model of the 2741 in mind. As explained in the Wikipedia article for the 2741, there were two models manufactured: a “correspondence coding” model and a “PTT/BCD coding” model. The solutions proposed by RFC-110 only apply to the PTT/BCD coded 2741.

Most of the document generalizes the information in RFC-110 to apply to both kinds of 2741 terminal.

The author recommends using the cent sign (¢) on the 2741 as an escape character (recall that the cent sign was not supported in the first revision of ASCII, but it exists on the 2741 keyboard). He also recommends an interesting method of sending ASCII control characters: you would type ¢, then @, and then you would type the English abbreviated mnemonic for an ASCII control character. So for example, if you wanted to send “escape” (ESC), you'd type ¢, then @, then the letters E, S, C.


The last part about sending control characters via mnemonics is really interesting to me! Some of you might have had to do things like enter special characters by looking up or remembering their actual hexadecimal ASCII values, which are a pain in the ass to remember. Supporting mnemonics seems like it would be pretty helpful.

Further reading

Modern operating systems have many ways to support entering keys that are not present on the keyboard but are present in the Unicode character set. It's pretty haphazardly implemented across operating systems and operating system versions so it seems like not much has changed since 1971. (Now that I think of it, “not much has changed since 1971” might be the central thesis of this blog.)

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About me

I'm Darius Kazemi. I'm an independent technologist and artist. I do a lot of work on the decentralized web with both ActivityPub and the Dat Project. You can support my work via my Patreon.