[GreenKeys] Special sequences

[jhaynes@alumni.uark.edu]

Thanks for the posting - I'm aware of that book but never read it.  Guess
it's time to add it to the reading list.

Different systems use different sequences for control purposes.  I won't
try to make a complete list here, but just give some examples.

Upper-case H, or the sequence FIGS-H, or FIGS-H-LTRS, or FIGS-BLANK-H
were popular in Bell System applications as end of message or motor stop
signals.  Automatic switching systems typically used two-character
addresses, separated by spaces, and carriage return as the end of address
signal.

Western Union's public message system used two-charcters followed by a
space as the address code.  There was only one address per message.
A double carriage return was the end of message code.  On a tape printer
this prints as a large period.

Federal Aviation Agency systems used various nonprinting combinations such
as CR-CR-LTRS, FIGS-CR-LTRS, FIGS-BLANK-LTRS for control purposes.
Somewhere I have a manual with a bunch of these in it.

ZCZC as a start of message sequence began in some military systems, I
believe.  Note that ZCZC is like RYRY in that it alternates opposite
bit patterns.

NNNN as end of message comes from these military systems also.  In some
cases the requirement is actually LF-NNNN.