[GreenKeys] TTY protocol

[Jim Haynes]

I'll comment on several recent postings in one posting.

With regard to M28 motor control, the one we had at W5YM was pretty
ordinary and came with the stuntbox coded for FIGS-BLANK-H to turn
off the motor.  There was a latching relay on the LESU, and the
stuntbox contact operated that relay into the motor-off position.
Any break in the signal line would operate the relay to the other
position and turn on the motor.

Back in the days of Irv Hoff there was an amateur radio autostart net
out West.  Some of them had built selective calling circuits using ICs,
essentially an electronic stunt box.  And they used ZCZC to turn the
motor on and NNNN to turn it off.

As I recall some of the Western Union stuff in Plan 55 actually required
LF-NNNN as the end of message signal, so you could have NNNN anywhere in
the message text and it would not cause a shutoff; it had to be at the
start of a line; and an unusual start of line at that since the usual
protocol is to follow LF with a LTRS.   One of the problems here is that
they were sometimes sending offline encrypted text, where the message
headers and all were in the clear and the message body was in five-letter
groups.  So you have to guard against the chance that a crypto group
will include NNNN.  Maybe it's the line feed that takes care of that.

On the FAA systems such as ADIS used for weather distribution there were
unusual sequences of stunts used for control purposes.  I don't begin to
remember much about them, but one was CR-CR-LTRS and another was
FIGS-CR-LTRS.  So if you encounter an ex-FAA Model 28 you'll find things
like that coded into the stunt box.  The weather sequence messages are
ordinarily very short, one liners.  A station on the circuit polls all
the stations to collect their messages in sequence.  The polling calls
are the three(usually)-letter location identifiers, and as I recall the
messages from the stations repeated the location identifier at the 
beginning.




jhhaynes at earthlink dot net