22 Sep
2013
22 Sep
'13
4:10 p.m.
I'm thinking the only practical way to deal with it, other
that removing the -30v is to break the wire to the
Vcc of the opto, put a resistor and a zener to limit the
voltage.
It shouldn't need more than a 10 or 20 uA to the opto.
I recall trying to figure out the drawing for the teletype's
paper tape reader power supply. I think the draft
person that makes the drawing had no technical
background. They just needed to make it electrically correct.
Dwight
From: hilpert at cs.ubc.ca
Subject: Re: PDP-8/L current loop -> RS232 help
Date: Sun, 22 Sep 2013 12:22:36 -0700
To: cctalk at classiccmp.org
On 2013 Sep 22, at 8:10 AM, dwight elvey wrote:
Thanks Brent
I'd not noticed the diode path to -15v.
I know, it's horrible trying to figure things out when they're drawn
in a convoluted manner as it is. That's why I redrew it, and do so
with lots of things, so (as much as reasonably possible) the E
potential is consistently increasing in the vertical direction.
That pretty much clamps 3 to about -14V
on the positive direction.
Putting the zener wouldn't help there.
I get about 40mA as well.
I'll have to give it a little more thought.