[geeks] KVM revisited

[nate at portents.com]

> Does that mean you cannot switch resolutions?  What if you boot Windows
> at 1600x1200, and then play a game at 1280x960?  Will the KVM not work
> then?  That seems a really serious limitation.

It has to do with EDID -

http://en.wikipedia.org/wiki/Extended_display_identification_data

"Extended display identification data (EDID) is a data structure provided
by a computer display to describe its capabilities to a graphics card. It
is what enables a modern personal computer to know what kind of monitor is
connected. EDID is defined by a standard published by the Video
Electronics Standards Association (VESA). The EDID includes manufacturer
name, product type, phosphor or filter type, timings supported by the
display, display size, luminance data and (for digital displays only)
pixel mapping data."

and specifically:

"Before DDC and EDID were defined, there was no standard way for a
graphics card to know what kind of display device it was connected to.
Some VGA connectors in personal computers provided a basic form of
identification by connecting one, two or three pins to ground, but this
coding was not standardized."

The EDID communication between the graphics card and the monitor happens
when a computer boots.  So let's say you boot computer 1 hooked up to a
KVM while that KVM is set to computer 2, which is already booted.  As I
understand it, the EDID communication coming from computer 1's video card
will go unanswered by the KVM, so computer 1's video card won't know the
exact capabilities of the monitor hooked up to the KVM.  That's my
understanding of the problem.  Now if the video driver on computer 1
allows you to switch resolutions (or something switches resolutions
anyway) despite the lack of EDID info, there's nothing in the KVM to stop
a video signal going from computer 1 through the KVM to the monitor.  KVMs
are, generally, not that intelligent of devices.  And when they have tried
to be intelligent (such as storing unique brand and model-specific USB
identifiers for USB devices connected to the KVM so they can be emulated
by the KVM even when they are not electronically wired to every connected
computer), the KVM makers have been sued, and it's my understanding that
KVMs offer at best generic USB device emulation.

- Nate