TTYs for the deaf

From: Tom Jennings <tomj_at_wps.com>
Date: Sat Dec 11 16:54:37 2004

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.

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 - 16:54:37 GMT