[GreenKeys] HRS312 TTY Interface

[John Nagle]

    I've been looking at the USB problem.  I'm considering
using some Atmel AVR board to provide a 45.45 baud serial
port.  This one looks promising:

     http://www.olimex.com/dev/avr-usb-stk.html

It's out of stock, but wait two weeks, and Microcontroller
Shop should have some.

Annoyingly, it won't quite work with my 120mA interface
board, because both sides are a DCE, and I need one side
to be a DTE and supply a positive DTR signal to power my
board.  A special cable between the two, with a connection
to +5VDC, will be required.

I don't know if it's worth the trouble, but the ideal
solution to driving old Teletypes would be a USB board
that provides a serial port, can switch a 120V 60mA
current loop, and generates the 120V supply from the 5V
supply with a boost converter. Might even be able to power
the whole thing from the USB port; I need to crunch the
numbers on that.  This is quite feasible,
but the market is rather tiny.

We've been discussing means of powering a selector magnet
from a low voltage supply over on sci.electronics.design.
It looks quite possible, and there are some SPICE models
now.  The key observation is that you only need 120VDC for
about 1-2ms of each bit time, at each SPACE to MARK transition,
to overcome that huge 4H inductance. The other 90-95% of the time,
you only need about 4V.

					John Nagle