TTYs for the deaf

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.