[Collins] ZCZC ZCZC ZCZC ZCZC ZCZC ZCZC ZCZC

[Gerald]

On Sat, 2005-07-23 at 23:13 -0400, Jim Miller wrote:
> http://www.nadcomm.com/ascii_code.htm
> NNNN (4 N's) was also an International signal that disconnected the call and 
> turned off the radios used to convey the message in International radio 
> message traffic. This is (or was) necessary with RCA, ITT, WUI, MacKay Radio 
> to ships at sea, etc..
> 
> NNNN: the international standard EOM indicator.
> LLLL: an EOM signal used on FAA circuits.
> 
> ZCZC: the international standard SOM indicator. An SOM may not have been 
> sent or was unrecognizable when received.
> 
Maybe after 1000 repeats, I'll remember ZCZC or to look at some
references first.

I believe ZCZC and NNNN were used on teletype circuits on wire long
before they were used on radio circuits. For sure the procedure of ZCZC
with several CR/LF pairs allowed time for a teletype motor to come up to
speed before the message arrived but the ZCZC may not have printed in
the old days unless there was a paper tape memory.

These four character sequences were surely chosen to be unlikely in text 
(which precludes sending tutorial messages about them) but not all that hard
to program in the hardware teletype "stunt" boxes. Especially for shut down
that could easily be done in the model 28 stunt box with just for coded bars 
and a bit of linkage. There had to be something like that for motor starting 
when the printer wasn't running, but I've forgotten that detail and I have 
no model 28 system manuals anymore. Hardware that I'm glad to have forgotten.

One FAA circuit that I built a computer to monitor (long about 1978, ran
until 1996 continuously) the 604 circuit, ASCII characters STX and ETX
were used around messages. No extra ZCZC or NNNN. And as such data has
migrated to internet servers one can't depend on any of these message
separators and often has to recognize the format of the WMO headers to
know a new message has arrived in the data stream. And if air craft
momentarily blocked the satellite down link things can be really mixed
up.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer