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'[~OT] Switching for 1 monitor to many machines'
1998\07\26@144510 by Ake Hedman

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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?

/Ake

======================================================================
Ake Hedman - Soft.Dev.Eng.
Eurosource, Gruvbyn 415A, S-820 50 LOOS, SWEDEN
Phone: +46 657 413422      Fax: +46 657 10612
WEB: http://www.eurosource.se

Carpe diem quam minimum credula postero
======================================================================

1998\07\26@160720 by Peter L. Peres

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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

1998\07\26@173107 by Ake Hedman

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Yes, I understand that this can be troublesome. But to know which keyboard
to use among a table full of them is troublesome to....

Anyway I have tested a simple mechanical switch for 8 computers
controlling five NT servers and two linux machines ) and it worked quite
fine. The only problem we had was that we needed to switch to the server we
booted otherwise we lost the mouse on the NT machines. A "feature" that was
a little annoying after a power failure.....

Mark Willis suggests using 4051 doing the job and I think that sounds like a
good idea. The question is if it can handle the video signals or if one must
use some other means for that.

Most mouse devices today is compatible with a well known mouse from a
Redmond based company so a PIC that handles that protocol should solve the
"mouse problem".



/Ake

> {Original Message removed}

1998\07\26@174807 by Peter L. Peres

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On Sun, 26 Jul 1998, Ake Hedman wrote:

- SNIP -
> Anyway I have tested a simple mechanical switch for 8 computers
>  controlling five NT servers and two linux machines ) and it worked quite
> fine. The only problem we had was that we needed to switch to the server we
> booted otherwise we lost the mouse on the NT machines. A "feature" that was
> a little annoying after a power failure.....

Hmm, another feature from the same house. I'll remember this. Have you
tried to set the BIOS such that there is NO keyboard ?

Mechanical switches were invented mainly for Novell and Lantastic servers,
and for use by personel in-the-know. They work well for that, too.
Anything else, is troubles...

> Mark Willis suggests using 4051 doing the job and I think that sounds like a
> good idea. The question is if it can handle the video signals or if one must
> use some other means for that.

4051 can't switch any signal above about 40 MHz, can't separate them worth
a damn, and can't drive 50 ohm or 75 ohm lines. Mitsubishi, Philips and
others (including JRC or whatever they are called now) make excellent
video switches, including multiple ones, usually in 8 and 16-18 pin SOIC
cases. Some are available in DIL. Any video switch that carries 100 MHz
bandwidth at 50/75 ohms eats a lot of power and runs hot. The other
option, is a binary relay tree. This is known to work well, but remember
that you are routing digital signals at 100 MHz !

hope this helps,

       Peter

1998\07\26@180714 by Peter L. Peres

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On Sun, 26 Jul 1998, Ake Hedman wrote:
> Most mouse devices today is compatible with a well known mouse from a
> Redmond based company so a PIC that handles that protocol should solve the
> "mouse problem".

Beg pardon, I will say nothing of the consequence of that company in
breaking its own standards. I will only say that my mention of Mouse
Systems mouse protocols was not accidental. The Mouse Systems protocol is
supported by all M$ products, offers 3 buttons to the benefit of Unices
and is not likely to change in the future, so no-one is going to pull the
carpet from under your design in say, like 6 months or so ? with a
'bugfix' that will put you out of business as far as this product will be
concerned. Of course, supporting both protocols is better. An automatic
detection is trivial, based on the packet length which is different, and
which you can determine by examining a few 'blocks' of consecutive packets
for the number of bytes in them (to find one that is divisible by 3 and
not by 5 and vice versa ;).

sorry for being blunt <g>,

       Peter

1998\07\26@182620 by Morgan Olsson

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At 20:25 1998-07-26 +0200, you 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?
>
>/Ake

Maxim have very nice Muxes and video amps with more than sufficient bandwidth.

They even come Mux + buffer amn in same package.

I have designed VGA switches with theese and it worked great.
You got to know something about HF to get tracks on PCB correct etc.
I also used a ground plane.

For the synk etc standard logic can be used LSTTL or HCT, both with
protection of course.

If interested, I think my design is still in production by another company.
It is a special design for very high quality use in bank central office.


We thank Peter L. Peres for his wide covering writing on PC switching. :)

/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  spam_OUTmrtTakeThisOuTspaminame.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /

1998\07\27@133621 by Peter L. Peres

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On Sun, 26 Jul 1998, Morgan Olsson wrote:
- snip -
> Maxim have very nice Muxes and video amps with more than sufficient bandwidth.

Yes ! But somehow I don't get to use them often here ;(

- snip -
> For the synk etc standard logic can be used LSTTL or HCT, both with
> protection of course.

Yes, but getting multiple mux/switches (many of the same kind in a package
- 3 or 4) will get you there easier and keep delays impedances etc
uniform.

- snip -
> We thank Peter L. Peres for his wide covering writing on PC switching. :)

I'm glad you read it <g>, but I'd like to apologize for the long post,
again, and mention that I think that you do not use the term 'monitor
switch' in as broad a sense as we do... anyway it was fresh in my pointy
little head so...

Peter

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