[GreenKeys] Two very newbie questions

[Jim Cooper]

> Now, is Mark when current flows and
> Space when there is no current? 

yes, that is exactly right.
 
> Part B to the second question... I
> never thought of the loop being a
> voltage device. Yes, I know that
> there is a rather stiff voltage to
> push the current through the loop
> wires and electro-magnets, and that
> the loop must be energized. 

You will find that the ratio of the Voltage
and the resistance/inductance (of the sel. 
magnet) affects the squarishness of the 
desired square wave -- that is why the 
voltage was chosen to be so high.

> Now, I am studying the manuals for a
> FSK unit (military), and there are
> two options to key the unit: a)
> shorting two terminals together,
> which would require a relay in series
> with the loop; and b) by applying
> voltage from the loop on another set
> of terminals. 

I would not mess with the "apply a voltage" 
unless you know exactly what voltage is 
required ... and better, see a schematic to 
see what it is doing. 

> Even though the relay set up is more
> complicated, I think that it would be
> actually easier to implement as I
> would not have to figure out what
> wire is "positive" and what is
> "ground". 

indeed ... BUT it has to be a very sensitive 
'relay' with contacts that do not bounce, etc. 

"in the old days" this was accomplished in 
most teleprinter systems with a POLAR RELAY ... 
a very delicate and complex item that includes 
windings, magnets, sensitive contacts, etc. 

in the not so old days, sometimes sealed 
mercury-wetted contact relays were used
instead; the advantage being they are sealed 
and need no special adjustment. 

w2jc