ASR-33 problem

From: vrs <vrs_at_msn.com>
Date: Sat Jan 24 18:16:48 2004

> > 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.

I did find a couple of paragraphs about the theory of operation, and yes,
the mark current is supposed to cancel the default negative bias of the
little transistor, which acts rather like an inverter stage before the power
transistor. I am still lost about the wierd emitter bias potentimeter and
the "regenerative feedback" it is supposed to provide.

> 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
Received on Sat Jan 24 2004 - 18:16:48 GMT