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Old October 18th 18, 10:31 AM posted to alt.comp.hardware
Paul[_28_]
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Default Connection Monitor to PC ???

wrote:
On Wed, 17 Oct 2018 22:46:52 -0400, Paul
wrote:

wrote:
I discovered something that surprised me - maybe someone can tell me
why?

I connected my monitor to two PCs, one DVI, the other VGA. I planned
to used my two PCs together, but never at same time. Guess what? My
PC connected with VGA worked alone. buy stopped working once I
connected the DVI cable. The DVI PC worked fine. I even unplugged
power to the DVI PC!! Now I am plugged to only the VGA PC and all is
fine. What don't I understand?

This is all because my MINIVIEW SYMPATH 4-VGA-PC
monitor-mouse-keyboard-connector quit working after several years use.
Just quit. Further, my cables no longer want to want on the Sympathy.
Seems strange.

I am discouraged to say the least.

Johnny

They share some resource they were not supposed to have shared.

If this was my kit to debug, I'd have the ohmmeter out and have
all three devices disconnected from power. See whether the VGA pins
on the DVI-I connector on the monitor, are in common with the
VGA pins on the VGA connector on the monitor. The two monitor connectors
could also share a common SDA/SCK (serial bus for EDID monitor resolution
info).

Using the make and model number of LCD monitor, I would be looking
for Newegg customer reviews for this particular monitor. Someone else
will have seen this happen.

Paul


Then maybe what I canf irst try is a second monitor I have?
J


My theory is, that there is some "sharing" between the two
connectors on the monitor, on the inside. Maybe SDA/SCK are
shared, or maybe the DVI-I and its VGA pins are shorted
to the VGA pins on the LCD monitor VGA connector. Neither
scheme is guaranteed to cause a black screen, but that's
about the only thing I could think of, where they'd be
joined together.
(Inside)
Computer #1 -------- VGA --------- ----+--- LCD Monitor
|
Computer #2 -------- DVI-I ------- ----+

I don't know how a second mystery monitor will help
this situation. Sure, you can run two computers
and two monitors, but there would not be much mystery
if you do this. Just more desk space.

Computer #1 -------- VGA --------- LCD Monitor #1

Computer #2 -------- DVI-I ------- LCD Monitor #2

What did you have in mind as a test case with the
second monitor ? Does the second monitor have its
own VGA and DVI-D input.

The computer end would typically be DVI-I. That has
both DVI-D and VGA pins on it, and supports either.
Usually, the DVI cable has DVI-D on the end for
the monitor, and in such a case, there's no continuity
on the VGA blades on the DVI-I connector. The input
on the monitor end should be DVI-D (digital only).

Wikipedia has an article on both DVI and VGA, if you
need pinouts. There might be as many as five connector
diagrams for DVI :-) The nice thing about standards,
is you can have so many of them.

Paul