[GreenKeys] Of current loops and voltages

[John Nagle]

> Date: Tue, 15 Sep 2015 23:15:44 -0500
> From: "drlegendre ."<drlegendre at gmail.com>
> Subject: Re: [GreenKeys] Of current loops and voltages.. (M15 20mA /
> 	60mA)
 >
> Yeah, it's almost seeming like I got "lucky" with my machine, in that it
> appears to be almost totally original, in terms of the wiring layout. And
> it's all in very nice shape, to boot - I wouldn't dream of ripping it out
> with the aim of 'simplifying" matters.
>
> My guess is that a well-placed jumper will put the KB / TU in series, and
> give a local echo of the transmission. But that dang terminal block - why
> so many terminals when we 'only' need two each for KB and TU? And while
> some of the terms are un-wired, many are - so Teletype Co obviously had
> something in mind for all that crap! ;-)
>
> So my machine has KB on #32 & #34 - and TU on #45 & #46. Of the top of
> anyone's head, where should the 'local echo' jumper go?

    Some machines had a polar relay in a big round can, so that the
incoming signal powered the relay and a local power supply powered
the selector magnet. There's also a remote motor turn on/off option.
Most of the extra connections are to support those.

    There is no standard "local echo" jumper on Model 15 machines.
That's a function of what the machine plugs into.  In Bell
installations, there's a box under the machine table to handle
the connections.  In Western Union installations, there were
standard "desks" for various jobs, and  they had the wiring
and switching.  Generally, a stock machine has two 1/4" two
conductor phone plugs, one for the typing unit and one for the
keyboard.  All five of my machines are set up that way, and
I have my own boxes into which they plug.

>> >Other question - how is the Send / Rec / Break device used? Break seems
>> >obvious - you press down on the 'small' switch paddle, which open-circuits
>> >the KB line. If the KB were in series with a selector coil, it would cause
>> >the coil to let go and the machine to start printing continuous NULLs. It's
>> >the same situation as with old-school telegraph - you open aka space
>> >("BREAK") the line to interrupt the other party to get their attention.
>> >This is clear..
>> >
>> >But how is the 'large' switch paddle used? How does it control Send &
>> >Rec?  I can't seem to make it do anything..

    It's all in the manual.  All this stuff is documented quite well.
See "Keyboard locking mechanism", page 24, Model 15 Description.
http://www.aetherltd.com/public/model15manuals/144_Model15_Descr_Feb31.pdf

    Think of several Model 15 machines, all on one current loop.
Anything typed on any keyboard appears on all machines.  If two
people type at the same time, you get garbage.  That lever and
the mechanism behind it is a simple system to coordinate who's
sending.  When you're not typing, all levers are in RECV.  To
type, you switch your lever to SEND and start typing.  In RECV,
the keyboard is out of circuit.

    If you send a long BREAK, or two successive BLANK characters, the
levers on all machines will move to RECV.  This stops everyone
else from sending.  Then you move your lever to SEND and send
your urgent message. This follows old wire-line telegraphy
practice.

			John Nagle