Refresh rate through VGA switch box

  • Thread starter Thread starter yaugin
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yaugin

I got a couple of switch boxes but they don't seem to operate at
anything other than 60 Hz when hooked up to a CRT at home. I tried the
switch on my computer at work and it seems to work okay with a flat
panel monitor.

Home PC:
Radeon 4670 outputting 1024x768 @ 75 Hz
CRT 4:3

Work PC:
GeForce 7300 SE/7200 GS outputting 1440x900 @ 75 Hz
Flat panel 16:10

So, without a switch involved, the monitor at home reports 75 Hz on
the OSD. But when a switch is between the video card and the monitor,
the OSD reports 60 Hz and there is some flicker.

On the work monitor, the OSD reports 75 Hz regardless of whether a
switch is used or not.

The cable between the video card and switch is literally the same for
both, as I brought the one from home to test on the work PC. The cable
going from the switch to the monitor is different, since the monitor
at home has built-in cabling I cannot exchange it. But it does support
75 Hz when directly attached to the video card so I wouldn't think
quality should be an issue there. However, it seems to be the only
difference so I am looking for suggestions on what else would stop my
setup from working.

The two switches I've tried: One is an unpowered 2-port VGA switch
with a mechanical toggle. The other is a 4-port USB-powered KVM switch
with an electronic toggle.
 
I got a couple of switch boxes but they don't seem to operate at
anything other than 60 Hz when hooked up to a CRT at home. I tried the
switch on my computer at work and it seems to work okay with a flat
panel monitor.

Home PC:
Radeon 4670 outputting 1024x768 @ 75 Hz CRT 4:3

Work PC:
GeForce 7300 SE/7200 GS outputting 1440x900 @ 75 Hz Flat panel 16:10

So, without a switch involved, the monitor at home reports 75 Hz on the
OSD. But when a switch is between the video card and the monitor, the
OSD reports 60 Hz and there is some flicker.

On the work monitor, the OSD reports 75 Hz regardless of whether a
switch is used or not.

The cable between the video card and switch is literally the same for
both, as I brought the one from home to test on the work PC. The cable
going from the switch to the monitor is different, since the monitor at
home has built-in cabling I cannot exchange it. But it does support 75
Hz when directly attached to the video card so I wouldn't think quality
should be an issue there. However, it seems to be the only difference so
I am looking for suggestions on what else would stop my setup from
working.

The two switches I've tried: One is an unpowered 2-port VGA switch with
a mechanical toggle. The other is a 4-port USB-powered KVM switch with
an electronic toggle.
Click to expand...

The video card normally communicates to the monitor and exchanges
information via an I2C bus known as the EDID. If those pins are not
switched/carried through the switch, the video card will not know what
type of monitor is connected to it.
 
The video card normally communicates to the monitor and exchanges
information via an I2C bus known as the EDID.  If those pins are not
switched/carried through the switch, the video card will not know what
type of monitor is connected to it.
Click to expand...

Thanks, you guys are right about this. I did some digging and it seems
that ATI's driver is very defensive when it cannot read the DDC. It
will lock the refresh to 60 at the card even if your Windows display
setting is set higher. I had to disable the "use EDID" setting in
Catalyst before it would let me use another refresh rate. The GeForce
driver on the other hand just lets you choose whatever you want.

However I am now having a different problem trying to run games. Many
of them seem to query the monitor directly, and since they too are
unable to read the DDC/EDID they refuse to display any kind of signal.
I just get a black screen after a frequency over range popup. Is there
any way to work around this or will I not be able to play games while
using a KVM?
 
The video card normally communicates to the monitor and exchanges
information via an I2C bus known as the EDID. If those pins are not
switched/carried through the switch, the video card will not know what
type of monitor is connected to it.
Click to expand...

Thanks, you guys are right about this. I did some digging and it seems
that ATI's driver is very defensive when it cannot read the DDC. It
will lock the refresh to 60 at the card even if your Windows display
setting is set higher. I had to disable the "use EDID" setting in
Catalyst before it would let me use another refresh rate. The GeForce
driver on the other hand just lets you choose whatever you want.

However I am now having a different problem trying to run games. Many
of them seem to query the monitor directly, and since they too are
unable to read the DDC/EDID they refuse to display any kind of signal.
I just get a black screen after a frequency over range popup. Is there
any way to work around this or will I not be able to play games while
using a KVM?

I had the same problem - I have a monitor connected to and xbox(which doesnt
care what its connected to) and a computer - throiugh a mechanical toggle
switch - only solution I could come up with was o permanently add 4/5 more
wires (not sure if all are needed - depends on comms protocol) IIRC it was
pins 4, 11,12,15 oh and pin 9 - the hardest thing was to ensure that these
pins were not grounded(mounted on pcb)

HTH
 
I had the same problem - I have a monitor connected to and xbox(which doesnt
care what its connected to) and a computer - throiugh a mechanical toggle
switch - only solution I could come up with was o permanently add 4/5 more
wires (not sure if all are needed - depends on comms protocol) IIRC it was
pins 4, 11,12,15 oh and pin 9 - the hardest thing was to ensure that these
pins were not grounded(mounted on pcb)

HTH
Click to expand...

So if I understand correctly, the reason this isn't working is because
the pins aren't wired inside the switch? And you were able to get it
working just by adding the wiring yourself? If so I might have to look
into it.
 
Are there any switches that have these already? I am not a handy man
since I have disabilities (can't even use a screwdriver or open computer
cases).
Click to expand...

Yes, there are. The lesson I learned from this is to check the
features on the box. It will say DDC is supported, if that is the case.
 
Yes, there are. The lesson I learned from this is to check the
features on the box. It will say DDC is supported, if that is the case.
Click to expand...

Thanks. I guess these DDC features didn't exist in early 2000s. :
( What I usually have to do is disconnect my monitor's VGA cable and
connect to the video card directly and then reconnect to the KVM
switch. Good thing it is rare.
 
Ant said:
Thanks. I guess these DDC features didn't exist in early 2000s. :
( What I usually have to do is disconnect my monitor's VGA cable and
connect to the video card directly and then reconnect to the KVM
switch. Good thing it is rare.
Click to expand...

They make boxes, to send a "fake" EDID to a computer.
You put a box like this, in the middle of the cable
feeding the computer. Of course, the connectors on these,
are never the ones you're looking for (and the chain
of cables and adapters, to make the right connectors,
would cost a small fortune). I would have thought the
main market for these, would be VGA.

http://www.gefen.com/pdf/EXT-DVI-EDIDN.pdf

http://www.gefen.com/pdf/EXT-HDMI-EDIDP.pdf

The boxes typically have the ability to be connected
directly to some display, and take a copy of the EDID.
Then, you flip the box to write protect mode, and
the contents of the chip inside is read-only. Then,
whenever that box is put inline, between the computer
and something else, the EDID data is played back.
And that is used to fool the computer into allowing
the desired resolution. When in read-only mode,
the box provides continuity for the video signals
(RGBHV or whatever), and the box becomes the source
of the EDID signals (DDC_Clock and DDC_DATA).

DDC
<------ EDID
Computer ------> Box ------> Display
Video Video

So there are solutions, but they're not really intended
to fix a KVM. Because for the price, you could likely
buy another KVM, before equipping each computer with
its own EDID box. Those EDID boxes, are for solving
specific "one computer to one custom display device"
problems. Like a $2000+ VGA equipped projection device,
with no EDID.

Paul
 
Ant said:
Interesting, crazy, and complex. :) I will keep that in mind if I ever
need to get a new KVM switch box. I currently use the Belkin cables that
came with the OmniCube KVM switches (2-ports and 4-ports).

Just curious, how does the boxes memorize the EDID? Is there a button
for that?
Click to expand...

The DVI Detective has a push button.

This one, on the other hand, does the "EDID copy" whenever the 5V supply
is connected. If you later connect the box to a computer, without the
provided 5V supply connected to the EDID box, then it's read only.
So this one is automatic or implicit copying, based on whether the power supply
is connected or not.

http://ca.startech.com/media/products/DVIEDIDDET/Manuals/DVIEDIDDET.pdf

On a DVI connector, there is +5V on pin 14. That would be the power
source, used by the EDID box, when the computer is attempting to
read the EDID. A similar situation would exist on a VGA computer
with external VGA EDID box. But in that case you have to be
careful, as at least some VGA, don't have +5V on pin 9. I have
at least one VGA thing here, where pin 9 is missing.

http://www.interfacebus.com/Design_Connector_Digital_Visual_Interface_DVI_Bus.html

Paul
 
Mark said:
Seems very complex. Why can't the switch boxes just pass the signal
through?
Click to expand...

That's a question you'd have to ask their designer :-)

If you have two computers and one display device, the KVM should be
able to switch the DDC signals, such that one computer at a time, is
connected to the display. It's also possible to do more
sophisticated things within the KVM, to keep both computers
fooled into thinking a monitor is always connected. But
switching the wires is the simplest thing they could do.
Or just leave that interface disconnected entirely (which
has side effects).

I don't think DDC is multi-master, so you can't have
two computer connected to the same DDC bus at the same
time. An ultra-cheap solution to that, is to not wire it
up.

Paul
 
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