[GreenKeys] USB to TTY-loop possibility...

[epvgk at limpoc.com]

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. 
Looks at first glance like manufacturing cost would be about $15/ea, dropping 

to $10ish in quantity 60. (including parts) You'd still need the Teensy board 
from pjrc.com of course, and they're about $16 each. Still not bad. 

Some details and the circuit at http://heepy.net/mediawiki/index.php/Optocouplers_for_teletype_current_loop

I'm happy to send anyone the Eagle files or printable mask images if you want
to etch your own and try it. It's a single-sided board and should be very easy
to make. Or wait until i've tested it. :) 

eric




On Fri, Nov 02, 2012 at 12:34:19PM -0700, epvgk at limpoc.com wrote:
> It seems like lots of folks have been looking for an easy solution to 
> connecting a computer to a TTY loop... There are various options like 
> reprogramming the CP2102-based usb-serial adapters and using PC software
> to generate the baudot code for it. 
> 
> I just picked up a little breakout board for an Atmel cpu (Atmega32u4)
> which is tiny and has USB support builtin. The whole board is only like 
> 3/4" by 1" and costs $16.  I've been playing around a bit writing software
> to make it act as a complete USB to TTY adapter...
> 
> The idea is to make it look to the computer like a generic USB serial device,
> but internally handle all the character set conversion, shifts, and framing,
> and without a large internal buffer. So you talk to it from the computer
> in ASCII and it ignores whatever the computer thinks about baud rate, character
> size, etc, and produces correctly timed 7.42 code at 45.45 baud on the output
> end, and vice versa for receiving. It only picks up a new character from 
> the computer when it's done sending the current one to the loop, to avoid
> having to wait for a buffer full of data to transmit once you stop sending.
> 
> Anyway, it's nowhere near finished but i have the sending side more or less
> working as a proof of concept at least... if I can get it working well I'll 
> make the software available and anybody who wants to can get one of the boards
> and program it with my code... just thought I'd mention it in case anyone is
> interested, and to motivate myself to make time to finish it :) 
> 
> eric
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