TTYs for the deaf

From: John C. Ellingboe <john_at_guntersville.net>
Date: Sat Dec 11 20:42:43 2004

Tom Jennings wrote:
>
> On Sat, 11 Dec 2004, Fred Cisin wrote:
>
> ASR-33's and their ilk are ASCII and 110 baud (43's do 300
> I think).
>
> The TTY-for-the-deaf tech is based upon older ITA2 (aka "baudot"
> but it's not -- no teletype-like device EVER used Baudot's code,
> it was for manual (finger) input) 5-bit code with two code tables
> (aka cases) selected with FIGS and LTRS. They mostly run at 60
> wpm, aka 45.45 baud.
>

Teletype Corp. Models 14, 15, 19 and 26 for a few plus some by other
companies used Baudot code. Some of those were interfaced to early
micros for lack of a better/cheap console at the time.
 
> Besides the speed issue, which requires good buffering or
> per-character flow-control handshaking (not commonly done well,
> and not possible on msoft OSs past DOS), you need a little state
> machine for code table/case translation each direction, and then
> about 1/2 the ASCII code set simply doesn't exist in ITA2, and
> there are no control codes in common except CR, LF, bell and NUL.
> Plus unless you re-wire them they are all half-duplex; pressing
> a key prints locally. ASR33's are 20mA current loop, but the
> older devices want 60mA, and 100 volts on the loop with a big
> series resistor; the current risetime requirements make it so.
Received on Sat Dec 11 2004 - 20:42:43 GMT