[GreenKeys] ZCZC and NNNN

[jhaynes@alumni.uark.edu]

Well I know this much.  They were used in Western Union's Plan 55 system
for the USAF, late 1950s.  Which suggests they were probably standards for
the U.S. military as a whole in that time period.  ZCZC has the property
that the bit patterns are exactly the opposite for the two characters, so
it is unlikely to be generated falsely.  In some of the switching 
equipment it is linefeed-NNNN that is detected as the end of message 
signal, so NNNN occurring just anywhere isn't taken as the end of message.

These signals are still used in military systems.  See ACP127(G) (which
I found somewhere on the web, but I don't remember where.)



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jhaynes at alumni dot uark dot edu