On Sun, 26 Jul 1998, Ake Hedman wrote:

> I'm in real need for a monitor switch. One of those that are capable of
> switching between several PC machines using one mouse, keyboard and VGA
> monitor. The obvious solution would of course be to go out and buy one but I
> kind of like to build things.....
>
> Have anyone one the list done a design of this kind or have any thoughts on
> the subject? What special considerations should one have regarding switching
> video streams?

I have designed/built several prototypes. It looks easy but it is NOT
easy. The monitor switch is relatively easy. The safest way is to switch
all 15 poles, with the added problem of not wanting crosstalk induced by
stray capacitance between R,G,B and anything else. Thus, get a large
switch and use the (stacked) contacts such that between each R, G and B
switchover stack segment, there is a GND-siwtching segment. The case
grounds of all the units connected must be wired together (beware !).

Some OSes/SVGA cards get confused when the feature bits are interrupted
during operation. This may mean lock-ups and lost work or even crashes.
The simple solution is, not to switch the feature bit signals through the
changeover switch, and to provide a small DIP switch for each computer's
SVGA feature bits. Set the switches to suit your setup BEFORE starting
either computer. You will lose any VESA monitor control functionality like
this. Also, if you happen to be using certain video gaming software aka
mainstream OS (I will quote no names here), you will likely have to use
'safe mode' boot and set up the video/monitor mode after starting to use
this setup (i.e. turn off the VESA SVGA monitor support). Also, spend some
*money* on cables. I mean it, unless you like ghosts. For monitors with no
internal 'smart' control, frequent switchover between sources that require
multisync (H freq.) switchover spells early and spectacular destruction.

Now, to more complicated things:

Keyboard switchovers are straightforward, but one must make arrangements
for both machines to be able to boot without a keyboard, and to make sure
that the keyboard receives power when either computer is powered, and
stays powered all the time (diodes on Vcc). The signal lines in all
directions must have defined states while a switch changes sides (pull
ups). Some software constantly updates LEDs and the like in the keyboard,
which implies continuous transmission to the keyboard. Such software
cannot be used with a simple (mechanical)  switch, it will lock up the
keyboard at least, and may crash the machine(s) connected. Of course you
don't want to switch while holding down any key. This is a PIC job really.
Did I mention that the lines to/from each computer and the keyboard need
to have defined states during switchover ?

Mouse switchovers are straightforward for serial mice, if an only if, both
machines use the mouse in the same mode. This is not very easy to do, as
some 'extended' and 'improved' mice are used with proprietary drivers.
Also, you cannot practically use a mechanical switch for this. If the
mouse moves even one pixel when you switch, it being on the same table and
attached by a wire, you are likely going to break a mouse protocol packet,
and lock up or crash both machines immediately. Even Linux X servers die
with this treatment. This is also a PIC job, a protocol buffering PIC job,
to be more exact, so you have to know the protocol. Steer clear of
multi-mode mice and 'extended functionality' for best results. Plain Mouse
Systems protocol serial 3-button works ok, and covers UNIX [tm] (3
buttons).

So, now that I've said so much, I'd like to recommend the system switches
from Rose, which I know to work well, and seem to be durable, or that you
embark in a medum size PIC project, that will likely be more expensive in
the end, than commercial solutions, due to the amount of mechanical manual
work required to make such a box look good ;)

My conclusion on the 'homebuilt' is, that it is not worth your while. A
mechanical monitor switch is about the limit for easy homebuilt, and 2
good ethernet cards plus a reasonable networking OS that lets you work
remotely (not made in Redmond) are cheaper than the time to do all this in
one-off mode (and cheaper than the commercial switches too).

hope this helps,

        Peter