Question about DVI KVM

[jw]

I have a 4-port IOGEAR DVI KVM switch, It works fine with two
machines that have DVI connections. I have a third machine that does
not have that connection - just a RGB VGA connection.
In order to use that machine with this KVM, I bought a DVI to VGA
adapter. The result is no raster on my HD monitor.

What is wrong? Should not this have worked?

Thanks

Duke

 

[Paul]

I have a 4-port IOGEAR DVI KVM switch, It works fine with two
machines that have DVI connections. I have a third machine that does
not have that connection - just a RGB VGA connection.
In order to use that machine with this KVM, I bought a DVI to VGA
adapter. The result is no raster on my HD monitor.

What is wrong? Should not this have worked?

Thanks

Duke

A DVI to VGA adapter, picks out a subset of signals on the DVI-I
connector.

video card DVI-I ------ digital
------ analog --- dongle ---- VGA 15 pin

DVI-I has both digital and analog signals, on the video card output
of the computer. When you connect the DVI to VGA dongle to that, it
is selecting signals which are already there. (It picks off the analog
VGA signals and puts them in the familiar 15 pin VGA format. A purely
passive, wire routing function.)

Now, reverse the direction of signal flow. If you connect the DVI to
VGA dongle to a VGA source, all the dongle can do, is put the
analog signals on the DVI-I connector. There are still no digital
signals on the connector. The KVM may be looking for digital signals,
and it won't be finding them. And that gives the black screen.

(DVI-I)
digital --------------- KVM (no signal)
VGA 15 pin ---- dongle --- analog ------

When you buy any KVM, it is critical to understand what interworking
it supports between VGA and DVI. I've seen some devices, where it
looks like they have both DVI and VGA capability, but it doesn't state
whether one signal type is converted to the other kind of signal. The
product advertising may be careful to *not* state how the product works,
presumably to increase the odds of buying junk.

For example, this one looked promising, but as near as I can tell,
it is two sets of independent monitor connections. A signal from a
VGA computer won't be "leaking" to the DVI side.

*******

LINKSKEY LDV-DM714AUSK 4-Port Dual Monitor (DVI+VGA)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817402027

http://www.linkskey.com/detail.php?Productid=1007&ProductName=LDV-DM714AUSK

http://www.linkskey.com/drivers/linkskey/LDV-DM712&714AUSK_QIG_R1.6.pdf

*******

To convert VGA to DVI, there are undoubtedly active converters. This
one converts up to the limits of DVI single link (1900x1200).
And for the price, you could buy a pretty nice new video card, and
get DVI signals that way. So retrofitting a video card to the
computer in question, could be a lot cheaper.

http://www.gefen.com/kvm/dproduct.jsp?prod_id=4341

I don't know if there is a cheaper converter product out there. A quick check
is not showing any bargain units.

Paul

 

[Grinder]

I have a 4-port IOGEAR DVI KVM switch, It works fine with two
machines that have DVI connections. I have a third machine that does
not have that connection - just a RGB VGA connection.
In order to use that machine with this KVM, I bought a DVI to VGA
adapter. The result is no raster on my HD monitor.

What is wrong? Should not this have worked?

Paul and kony have given you good information. The good news is that
you can probably solve this scenario with a $30 video card. If you can
tell us what motherboard you have, we can probably make a specific
recommendation.

 

[jw]

Paul and kony have given you good information. The good news is that
you can probably solve this scenario with a $30 video card. If you can
tell us what motherboard you have, we can probably make a specific
recommendation.

Thanks to all three of you.

Well it would appear that I can't do what I wanted to do.
You see, I have two of my own machines, both having DVI video cards,
and both now connected to my new IOGEAR KVM, which is DVI.
They used to be connected to a VGA KVM, which had an unused port that
I used to service my neighbor's and friends' machines (VGA because
they were older) requiring my help. A very convenient set-up, because
all I needed was their tower.

Since my monitor is DVI, I want to use its DVI capability, so I bought
the IOGEAR. Now, to work on said machines, I have to use their VGA
monitor, PS/2 keyboard and PS/2 mouse, or keep mine handy.

I bought the DVI to VGA adapter in the hope that the 'service' port on
my KVM could then be made to select my DVI monitor via my IOGEAR for
my friends' machines. I guess not.

BTW, I can 'jump' my DVI monitor's VGA connection directly to my
friends' VGA connections, and select them with my IOGEAR, and work my
monitor that way, but it knocks out my machines' displays until I
power off my friends' machines. Odd, but true.

 

[Grinder]

Thanks to all three of you.

Well it would appear that I can't do what I wanted to do.
You see, I have two of my own machines, both having DVI video cards,
and both now connected to my new IOGEAR KVM, which is DVI.
They used to be connected to a VGA KVM, which had an unused port that
I used to service my neighbor's and friends' machines (VGA because
they were older) requiring my help. A very convenient set-up, because
all I needed was their tower.

Since my monitor is DVI, I want to use its DVI capability, so I bought
the IOGEAR. Now, to work on said machines, I have to use their VGA
monitor, PS/2 keyboard and PS/2 mouse, or keep mine handy.

I bought the DVI to VGA adapter in the hope that the 'service' port on
my KVM could then be made to select my DVI monitor via my IOGEAR for
my friends' machines. I guess not.

BTW, I can 'jump' my DVI monitor's VGA connection directly to my
friends' VGA connections, and select them with my IOGEAR, and work my
monitor that way, but it knocks out my machines' displays until I
power off my friends' machines. Odd, but true.

Your monitor is probably autoselecting which port to display. The first
one that gets a live signal gets shown. When that signal is turned off,
it scans for signals on the other port(s). I think that would match
your described behavior.

Most likely, you can also manually select the input. So, when you
viewing the VGA input, and want to flip over to the DVI input, being fed
by your KVM switch, just manually select that port and it will probably
work.

 

[jw]

Your monitor is probably autoselecting which port to display. The first
one that gets a live signal gets shown. When that signal is turned off,
it scans for signals on the other port(s). I think that would match
your described behavior.

I agree.

Most likely, you can also manually select the input. So, when you
viewing the VGA input, and want to flip over to the DVI input, being fed
by your KVM switch, just manually select that port and it will probably
work.

I don't know of a manual input type selector on my ACER HD monitor.

I am wondering.....

The cable that came with my IOGEAR KVM has a DVI cable to use to
connect the KVM to a computer. The video end is a male DVI-D
dual-link. The adapter has a female DVI-D dual-link also. Should not
the adapter be female DVI-I so as to accommodate the analog
requirement since VGA is analog? Please tell me if I am wrong - I
thought maybe I bought a wrong adapter.

Thanks

Duke

Duke

 

[Grinder]

I agree.


I don't know of a manual input type selector on my ACER HD monitor.

I am wondering.....

The cable that came with my IOGEAR KVM has a DVI cable to use to
connect the KVM to a computer. The video end is a male DVI-D
dual-link. The adapter has a female DVI-D dual-link also. Should not
the adapter be female DVI-I so as to accommodate the analog
requirement since VGA is analog? Please tell me if I am wrong - I
thought maybe I bought a wrong adapter.

If you're adapter is intended to translate the analog lines in a DVI-I
line to a legacy VGA line, then the analog pins present. They're the
ones around the big plus-shaped pin, as well as a few others.

See DVI-A images on right:
http://en.wikipedia.org/wiki/Digital_Visual_Interface#Connector

The two that I have here actually have *every* pin present on the (male)
DVI side, but obviously some of them do not connect to anything on the
(female) VGA side.

Just because you've made an electrical connection, of course, is no
guarantee that either of your devices (PC or KVM) will generate or
accept that analog signal.

 

[Grinder]

I don't know of a manual input type selector on my ACER HD monitor.

What model of Acer monitor do you have?

 

[jw]

What model of Acer monitor do you have?

ACER AL2223W

The DVI connector on it is a DVI-I single-link even though the cable
is a DVI-D dual-link.

Duke

 

[Paul]

ACER AL2223W

The DVI connector on it is a DVI-I single-link even though the cable
is a DVI-D dual-link.

Duke

If a KVM had circuits to route and select both digital and
analog signals, and the monitor had digital and analog signals,
and manual control over which input is used, you could do something
like this. In this example, the digital and analog of the corresponding
input, would be on the same connector. On the output side, you'd
need a cable, to split the DVI-I output to a separate DVI-D and VGA
signal cables.

analog DVI-I Splitter_cable
DVI-I #1 --+--> -----+-----+------+---> monitor
| | | | \ +--------
| -----+ | | \ |
| | | | \____| VGA
| -----+ | | |
| | | | ____| DVI-D
| -----+ | | / |
| | | / +--------
| digital | | /
+--> -----+-----+ +--->
|
-----+
|
-----+
|
-----+

The question is, is that what the KVM does internally ? And if so,
why does it do that ? They could have logic to pass both the analog
and digital signals, but to make use of it, would need messy cabling
if the monitor had separate inputs to do the selection. For example,
you might need to split DVI-I output to a DVI-D and VGA cable.
I don't see a good reason for a monitor to have an actual DVI-I
input and the ability to select between the analog and digital
signals, on the same connector. (I've heard of that being the
case, but it makes no sense. There are people who claim they've
compared analog to digital output, for signals traveling on
the same DVI-I cable. So there are monitors that do it.) And
if the monitor has only DVI-D input, then there'd be no way
to access the analog signal if present.

If a KVM has DVI connectors on it, it makes a bit more sense if it
is routing DVI-D only. The connectors on the faceplate of the KVM may
be DVI-I for simplicity of cabling, as you could use a DVI-I cable
without crushing the cross-shaped pins on the end.

monitor
+--------
|
|
|
____| DVI-D
/ |
/ +--------
digital /
-----+-----
|
-----+
|
-----+
|
-----+

I did see a "dual view" KVM, where separate VGA connectors are provided
for analog, and DVI-D for the digital portion. If the monitor had both
connectors, then you could flip between one interface network or the
other, and get a monitor signal. For example, if you had a dual view
KVM, and wanted to wire two digital and one analog VGA, it would look
like this. Since separate connectors are used, the cabling would be
simpler.

analog
computer #3 -----+----- monitor
| \ +--------
-----+ \ |
| \____| VGA
-----+ |
| ____| DVI-D
-----+ / |
/ +--------
digital /
computer #1 -----+-----
|
computer #2 -----+
|
-----+
|
-----+


http://www.startech.com/item/SV431DDUSB-4-Port-StarView-DVI-VGA-USB-KVM-Switch-with-Audio.aspx

http://www.startech.com/Share/Gallery/Large/SV431DDUSB.Dlarge.jpg

Another one from Linkskey.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817402027

What would be harder to do, is mixing analog and digital on the
same selector, like this. If you have to do analog to digital
conversion, and then digital to analog on the other side, so
that all possible signals are available, that is going to
make the KVM much more expensive. At least, if it employs
scaler technology like the Gefen adapter provides.

analog -----+----------> analog
| ------> digital
analog -----+
|
digital -----+
|
digital -----+

The problem is, with the introduction of DVI-I connectors,
you can't really tell what these products do, because the
KVM documentation lacks proper block diagrams.

Paul

 

[Grinder]

ACER AL2223W

Ok, just to reiterate the scenario as I understand it:

A) You have a couple of computers that are putting out digital video
signals.

B) You route those digital signals into a digital (DVI) KVM switch.

C) The DVI KVM switch connects to a monitors DVI-D port.

D) The DVI KVM switch does not seem to be able to route an analog
signal. Said another way, the KVM is DVI-D only, so DVI-A signals are lost.

E) The monitor also has a legacy analog (VGA) connector.

F) When you want to service a computer that only has analog output, you
can run a cable directly from that PC to the monitor's VGA
port--skipping the KVM. This allows you to see that PC's output.

G) After you have output to the analog (VGA) port on the monitor, you
cannot switch amongst your other PCs which are using the KVM and the
digial (DVI) port on the monitor.

I suggested that the monitor is probably just unaware that you want to
change its input from the analog port to the digial port, but that you
can manually make it switch.

You stated you did not know how to do that, and unfortunately there is
no INPUT button on your monitor. You can, however, change it in the
menu settings, and I've specifically seen how to do this in your
monitor's user manual.

I found a manual for an AL2223WD and an AL2423W, but not for the
AL2223W. The menus appear to be the same for both of those models, so
hopefully it will be the same for you.

1) Hit the MENU button. Of the 5 buttons on the front of the monitor,
the MENU button is second from the left. The onscreen display (OSD)
will appear.

2) Click the plus button until Input Signal icon is selected. It's the
second icon from the left in the bottom row. Click MENU to select the
input signal options.

3) Use the plus/minus button to select which port you want to display
the signal from. Click MENU. (If the OSD doesn't go away after making
this selection, use the AUTO button to back out.)

 

[jw]

Ok, just to reiterate the scenario as I understand it:

A) You have a couple of computers that are putting out digital video
signals.

Yes. Each has a DVI-I dual-link female port.

B) You route those digital signals into a digital (DVI) KVM switch.

Yes => IOGEAR 4-port. The cables have DVI-D single-link male
connectors on both ends. The cables came with the IOGEAR in the box.
The IOGEAR has a DVI-I dual-link female port. Compatibility, or lack
thereof, here?

C) The DVI KVM switch connects to a monitors DVI-D port.

The ACER AL2223W monitor has a DVI-I dual-link female port, as does
the IOGEAR.

D) The DVI KVM switch does not seem to be able to route an analog
signal. Said another way, the KVM is DVI-D only, so DVI-A signals are lost.

If a DVI-I connection can indeed pass BOTH digital and analog, but a
DVI-D can ONLY pass digital, then it would appear to me that the
switch ITSELF can pass both. Agree?

E) The monitor also has a legacy analog (VGA) connector.

Sure does.

F) When you want to service a computer that only has analog output, you
can run a cable directly from that PC to the monitor's VGA
port--skipping the KVM. This allows you to see that PC's output.

Sure can.

G) After you have output to the analog (VGA) port on the monitor, you
cannot switch amongst your other PCs which are using the KVM and the
digial (DVI) port on the monitor.

No I cannot - not until I power down the 'analog computer'.

I suggested that the monitor is probably just unaware that you want to
change its input from the analog port to the digial port, but that you
can manually make it switch.

Would appear so.

You stated you did not know how to do that, and unfortunately there is
no INPUT button on your monitor. You can, however, change it in the
menu settings, and I've specifically seen how to do this in your
monitor's user manual.

I'll look again. Have to hunt the manual though.

I found a manual for an AL2223WD and an AL2423W, but not for the
AL2223W. The menus appear to be the same for both of those models, so
hopefully it will be the same for you.

Hope so. Thanks for your effort here.

1) Hit the MENU button. Of the 5 buttons on the front of the monitor,
the MENU button is second from the left. The onscreen display (OSD)
will appear.

2) Click the plus button until Input Signal icon is selected. It's the
second icon from the left in the bottom row. Click MENU to select the
input signal options.

3) Use the plus/minus button to select which port you want to display
the signal from. Click MENU. (If the OSD doesn't go away after making
this selection, use the AUTO button to back out.)

Got it. Works for me. The monitor buttons and their usage is very
unfriendly.

Do you think if I bought cables with DVI-I dual-link male connectors
both ends that the switch would work for both analog and digital?

Thanks for your time and efforts. I really appreciate it.
I'll remember you in my will. :<)

Duke

 

[Paul]

Got it. Works for me. The monitor buttons and their usage is very
unfriendly.

Do you think if I bought cables with DVI-I dual-link male connectors
both ends that the switch would work for both analog and digital?

Thanks for your time and efforts. I really appreciate it.
I'll remember you in my will. :<)

Duke

But that to me, is the essence of the problem. I haven't looked
at that many KVMs, but the ones I have looked at, don't spell out
exactly what they do. And due to the connector issue (using
DVI-I connectors solely to prevent pins from being busted
off), we cannot rely on the appearance of the connectors,
to tell us what they do.

In a perfect world, the KVM would have DVI-D connectors if it
only supported digital. But then, if people had ready access
to DVI-I cables, the cross shaped bits would be busted off.

Maybe if the standards committee had come up with a simple
label to be put next to the connector (D+A, D, A), the
function would be more obvious visually.

To give an example, there are some extremely cheap video cards
($25-$30), where the card has a DVI-I connector on the faceplate,
but the analog signals are missing. It is only when a customer
connects a dongle, and attempts to access VGA on the connector,
that the truth becomes known. So the KVM is not the only
device, with documentation problems. (Another potential source
of surprises, is motherboards with integrated graphics, and
some of the limitations they've got with respect to the
connectors.)

Paul

 

[jw]

But that to me, is the essence of the problem. I haven't looked
at that many KVMs, but the ones I have looked at, don't spell out
exactly what they do. And due to the connector issue (using
DVI-I connectors solely to prevent pins from being busted
off), we cannot rely on the appearance of the connectors,
to tell us what they do.

Yup and yup.

In a perfect world, the KVM would have DVI-D connectors if it
only supported digital. But then, if people had ready access
to DVI-I cables, the cross shaped bits would be busted off.
Yup


Maybe if the standards committee had come up with a simple
label to be put next to the connector (D+A, D, A), the
function would be more obvious visually.

Sure is not now - Thank God for Wikipedia and the diagrams therein.

To give an example, there are some extremely cheap video cards
($25-$30), where the card has a DVI-I connector on the faceplate,
but the analog signals are missing. It is only when a customer
connects a dongle, and attempts to access VGA on the connector,
that the truth becomes known. So the KVM is not the only
device, with documentation problems. (Another potential source
of surprises, is motherboards with integrated graphics, and
some of the limitations they've got with respect to the
connectors.)

Yup