I need to use my single monitor (HDMI, Samsung SyncMaster B2430) and keyboard / mouse (USB, Microsoft Sculpt Ergonomic) with 2 laptops, to be able to switch between the two easily with one click of a button.

My 1st laptop is my old Samsung Series 9 NP900X4C, running Windows 10 Home (with a micro HDMI port).

My 2nd laptop is a Dell Precision 5520, running Windows 10 Enterprise (with full HDMI port)

Up till a few months ago I was using a cheap HDMI Switch, (3-port Duronic).

It was working fine (I was switching the keyboard separately by swapping the dongle), but at some point it stopped recognising my Samsung Laptop. I thought it was something wrong with the HDMI switch, given its such a cheap device, and didn't bother doing any more troubleshooting.

Instead I went ahead and bought a Bowu 2 USB HDMI KVM-Switch. This one also takes USB so with one button I also switch the keyboard and mouse, not just the monitor. The USB switching works perfectly between the two laptops on the new KVM switch.

The problem is that when I connect my Samsung laptop through the new switch the monitor does not recognize the signal. But when I connect them directly it works, so I know that the HDMI port on my laptop is not busted. It is also strange that both HDMI switches are behaving in this way.

Samsung Laptop -> Monitor = OK

Samsung Laptop -> KVM Port 1 -> Monitor = NOK
Dell Laptop -> KVM Port 2 -> Monitor = OK

Dell Laptop -> KVM Port 1 -> Monitor = OK
Samsung Laptop -> KVM Port 2 -> Monitor = NOK

I also tried swapping cables to eliminate the possibility of a faulty cable.

Given that this used to work before with my old HDMI switch, and suddenly it stopped, I am starting to suspect it must be some driver or windows update issue that screwed this up, or some display setting which somehow is not compatible, although I don't recall changing any resolution or display settings.

Is there any way or any tool I could use to debug what is going on when the HDMI handshake takes place between my laptop and monitor through the KVM?

  • Try to power cycle the KVM On/Off several times. – harrymc Jan 28 at 15:08
  • @harrymc No luck. Tried also the old HDMI switch, same symptoms. The Dell laptop works fine on both. The Samsung laptop works fine direct, but not through the switcher. Powered off and on multiple times. – jbx Jan 28 at 16:37
  • I wonder if the Samsung's video adapter is weakening, so cannot punch the signal through. – harrymc Jan 28 at 19:46
  • It's an Intel HD graphics 4000. I reinstalled the driver today, still no luck. Any way to test the signal hypotheses, or to measure it in some way? Would a shorter hdmi cable help? – jbx Jan 28 at 21:44
  • HDMI is digital signal, so would need several meters for degrading it. An HDMI Cable Signal Tester is not cheap. Trying a better cable wouldn't hurt, anyway, as one test. You might also have a look if the video driver for the Samsung was updated at about the time the problem started, and study its available settings. – harrymc Jan 29 at 7:45

After a long exchange with the poster, and quite a few tests, the relevant points that are left are listed below:

  • The Samsung Series 9 no longer works through two different KVMs
  • Two other computers have no problem getting through both KVMs
  • The Samsung Series 9 can directly connect to the monitor, just not through any KVM.

My conclusion is a hardware degradation on the Samsung's video adapter. Perhaps a weaker video signal has some problem punching through a KVM.

As the video adapter is integrated into the motherboard, it's likely not worthwhile to further invest in this computer.

  • Thanks for your patience. In the end I solved the problem with a workaround, connected the good old VGA port (the laptop has a mini VGA port), directly to the VGA port of the monitor. So the KVM is unfortunately only serving for the keyboard, but with an extra push on the monitor I switch the source. Choosing your answer so that you get the bounty. – jbx Feb 4 at 22:01

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