[GreenKeys] USB to TTY-loop possibility

[John Nagle]

On 1/5/2013 7:35 PM, greenkeys-request at mailman.qth.net wrote:
> On Jan 5, 2013, at 7:10 PM, epvgk at limpoc.com wrote:
>> > Just a follow-up. I've been using an AVR microcontroller breakout board
>> > called "Teensy 2.0" from http://pjrc.com/teensy as a USB-to-TTY adapter with
>> > ascii/baudot translation. The board is nothing more than a breakout for the
>> > Atmel ATMEGA32U4 cpu.  To actually connect it to a loop, you need to convert
>> > voltages, etc.  I believe George has tried the software, or is planning to 
>> > soon. I'm using a Teensy 2.0 with a breadboarded adapter pretty successfully. 
>> > 
>> > I just did a quick layout of a PC board to interface it to a TTY loop. With 
>> > this plus the "Teensy 2.0" and the software, you would have a device that looks 
>> > to a computer like a USB serial port and looks to a TTY loop like a teletype 
>> > machine, including ASCII/Baudot conversion, so the computer does not have
>> > to know or care about weird baud rates or 5 bit characters or Baudot.

   You can certainly do that.  One problem is how to get configuration
parameters from the upstream computer to the board.  When you're doing
ASCII to Baudot conversion, it's useful to be able to set

	- baud rate
	- does machine have automatic LF on CR?
	- does machine have unshift on space?
	- USTTY, ITA2, or FRACTIONS font?
	and perhaps
	- Half duplex or full duplex connection?
	- Max line length
	- Number of extra LTRS to send after a CR

I ended up having to support all those in "baudotrss".  I have
four Teletype machines, one Model 15 and three Model 14 machines.
One is USTTY, two are ITA2, and one is FRACTIONS.

I've found it useful in ASCII->Baudot conversion to have the
software represent the shift state as LTRS, FIGS, or UNKNOWN.
If a character which requires a specific shift is sent in
state UNKNOWN, either LTRS or FIGS has to be sent first, which
changes the state in the software and on the real machine.
The state starts at UNKNOWN after a power up or motor start,
so there will be an initial LTRS or FIGS sent.  After a SPACE
in FIGS mode, the state becomes UNKNOWN, so machines with or
without unshift on space are handled.  If you do that, the
conversion software doesn't have to know if the machine has
that feature.

			John Nagle