FW: [GreenKeys] ZCZC and NNNN

[Eric Scace K3NA]

-----Original Message-----
From: Eric Scace K3NA [mailto:eric@k3na.org]
Sent: 2003 December 2 Tuesday 16:14
To: John Poulton
Subject: RE: [GreenKeys] ZCZC and NNNN


   ZCZC (and other systems) were used as start of message indicators, and NNNN as an end of message indicator.  As far as I can
recall, these indicators came into use when semi-automated torn tape cross-office relay systems were developed to help sort incoming
messages for transmission on the proper outgoing circuit at message switching centers.

   The start of message sequence often contained two parts: a section containing sequence numbers, routing codes and message
priority codes, followed by the message itself.  These varied quite a bit depending on the organization.  For systems which used
ZCZC, the sequence simply started the process of recording a message to tape and figuring out where to send it.  Typically ZCZC was
preceding by 8 LTRS and followed by 5 spaces CR CR LF.

   ZCZC was chosen because:
a) it was unlikely to occur in plain text in either letters case or figures case (":":) in ITA2.
b) during idle time on a circuit (particularly radio circuits), it was unlikely to occur as a result of random atmospheric noise or
other hits.
c) as a 4-character group, it was unlikely to occur with cipher or code text employing 5-character groups.

   USA military systems with which I was familiar didn't use ZCZC.  These systems started with multiple LTRS to get the machines in
synch, then a string of 3-character channel identifiers and sequence numbers used in auditing traffic, followed by 5 spaces, CR CR
LF, message precedence and routing codes, another line identifying where the message was introduced into the network (and giving a
serial number), and then finally the military message header.  The incoming traffic was recorded on a tape reperf, which fed a
two-headed TD.  The first TD would read the tape looking for the line with the routing codes (after the SP SP SP SP CR CR LF) and
the second TD would pause, waiting for the routing codes to be read.  Once the routing codes had been read, a connection across the
switching center was formed to the machinery serving the proper outgoing circuits.  If the precedence code warranted it, any
outgoing traffic currently being sent would be interrupted (a complex task in its own).  Once the cross-office circuit(s) had been
seized, the second TD would then read the message all the way to the NNNNs.  At the outgoing circuit side, the cross-office circuit
recorded the message on tape again.  That tape queued up for transmission with another special TD that had 2 or sometimes 3 heads.
The first head contained a special tape used to pre-pend a channel number, and the second and third heads would then read the
traffic out onto the outgoing circuit.

   The end of message format was almost universally NNNN.  This shut off the cross-office routing connection and terminated the
recording process.  For easy visual identification of the end of a message, NNNN was often preceded by CR CR and some number of LF
(typically 8, but as low as 4 and as high as 12 were in common use).  On a printer this made a convenient point to tear the paper.

   Another nearly-universal requirement was the transmission of at least 12 LTRS after the NNNN.  This would be an easily recognized
place to tear paper tape, and the all-holes pattern made it easy to tear cleanly.

   It is likely there were submarine cable telegraphy precedents for some of these designs and code choices.  You'll have to ask
some of the Cable & Wireless historians for details about how their semi-automated systems worked.  These were quite advanced by the
time teletypewriter semi-automated systems were being developed, so it would not surprise me to see some carry forward of concepts
and lessons.  (A lot of what was done by C&W in the first half of the 20th century is still done today in a different media: optical
fiber.  But that's another story...)

-- Eric K3NA

> It's been a while since I've been active on RTTY.
>
> Someone please remind me of the meanings of ZCZC and NNNN
> and, if known, how they developed.
>
> TIA  Geoff, K6TFZ