[GreenKeys] jackfields

[Jim Haynes]

On Tue, 17 Feb 2009, WA5CAB at cs.com wrote:

> 
> Maybe someone can refresh my memory, which just went indeterminate. Isn't a
> Normalling jack one wired such that plugging a patch cord into it puts the
> equipment wired to to the jack onto the patch cord and shorts the loop
> across the jack? Or am I mixing audio with TTY terminology?
> 
I take "Normalling" to mean that with no patch cords plugged in something
is connected so it works.  You only have to patch to change things.

Now addressing the matter of loop voltage being present on the jack 
sleeves, there are some circuits where a normally-open contact in the
jack lets the sleeve float until a plug is inserted into the jack, so
that the exposed jack sleeves are not connected to anything.

Repeating myself, note that what you want in a jack field in a telegraph
office is not the same as what you want in a radio station or a museum
or repair shop.  In the old telegraph office you are most concerned with
wires failing, or later, carrier channels.  Also you are making up long
circuits by connecting circuit segments in tandem, and you are making up
multipoint circuits by connecting point-to-point circuit segments together
through repeaters.  In the radio/museum/repair environment you are more
concerned with patching out a broken machine or radio set, or with getting
a particular combination of machines working together locally.

A case can thus be made for equipping each machine with something like
a loop-to-RS232 converter and doing all the patching and switching at
low voltages, and with no loops that have to be kept closed and no 
machines that have to be always in loops to keep them from running open.
Modify the RS232 scheme slightly by having diodes in all the sending
sources so that you can connect them in parallel and any one of them
can drive the resulting hub to spacing.  And the hub can drive practically
any number of receivers.

The downside to the above is that you have to equip all your machines
and T.U.s and whatever with loop-to-RS232 converters; you can't just
bring a Model 15 in the door and plug it right in, unless you have
provided some extra level converters for the purpose.  So you'd like to
have a little box with its own power supply that sits under the TTY
machine or behind the RTTY T.U. and does the level conversion.  It
gets tedious having to make all those boxes.  But the technology is
simple, especially now that we have opto-isolators.

Jim W6JVE