On Sat, 29 May 2010 20:23:19 +0100, "Michael Watterson" said:

> sounds like it is drivers for 90 pin parallel head printer, not voice 
> lines directly (but who knows?)
> So common ground, no opto isolators needed
> use resistor divider on each driver pin/pen to convert to 5V. Since it 
> might not be exactly 50V, a resistor divider to reduce volts to 6V out 
> from 48V in  and 4.7V zener .

Unless it's positive ground like lots of old phone company stuff seems
to be...

Hey, Andrew, have you reverse-engineered the part of the circuit you
need to monitor? Where is ground in reference to the 50 volts(positive
or negative)? Or, even better, is there some driver for these
pen-thingys that is driven with a logic-level signal? That would be LOTS
easier to monitor. I don't know what your level of electronics expertise
is, but the more you know about the problem, the simpler your solution
will be.

Cheerful regards,

Bob

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