24 Jan
2004
24 Jan
'04
6:16 p.m.
> So, is there a theory of operation somewhere for
the selector magnet
driver?
I can't
follow what the schematic is doing at all.
IIRC, it's strange. The selector magnet driver applies a current to the
Rx Loop that is _cancelled_ by the normal loop current. Now, I can't really remember the schematic, but is
there some kind of
clamping diode across the input, I wonder. If that was open-circuit, the
selector magnet driver could provide the 20V open-circuit voltage at the
loop terminals.
I'll go check that it really does clamp the negative bias to 5V like it is
supposed to.
I am still unsure where the +20V is coming from, since the +20V side is
apparently connected to the "-" side of the computer interface. Seems like
everything else should be more negative than that.
Hmm. Maybe I just partially answered my own question. When the transistor
in the interface is on, the "+" and "-" ends are
"connected". That would
surface the 20V+ on the "+" side of the TTY interface.
But that would mean something tied the "-" end of the TTY's 20V supply to
ground. Aren't those supplies supposed to float?
Vince