Some cell phones are TTY/TDD (telecommunications device for the deaf) compatible, meaning a terminal can be plugged into the headset jack for text communications. Is it just a hidden RS-232 interface?


TTY/TDD uses tones in the audio band (similar but not compatible with the old Hayes modems) to communicate over telephone lines. The original protocol uses Baudot code at 45.5 or 50 baud. In 1994 the ITU approved a newer V.18 standard.

So I would presume the headset jack would just pass these tones back and forth to a TTY/TDD modem. It would not be a hidden RS-232 interface -- i.e. the cellphone is not acting as the modem.

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    \$\begingroup\$ Oh the horror. It appears the "newer" technology is called CTM (cellular text modem) which is a 4-tone modulation that will survive GSM compression. In other news I really hope nobody bothers to hook up a typewriter to a cell phone anymore, instead of a computer or smartphone. \$\endgroup\$ – joeforker Sep 24 '10 at 17:59
  • \$\begingroup\$ They've updated the standard several times since 1994-that was edition 1, we're now on edition 4 (Published 2002). See the full list of Series V standards on "Data communication over the telephone network" here: itu.int/itu-t/recommendations/index.aspx?ser=V The most recent spec is here: itu.int/rec/… [pdf] \$\endgroup\$ – Kevin Vermeer Sep 25 '10 at 15:40

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