Question Dell U2412M, input switching problem after replacing a PC

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For the past 10 months, this configuration has worked beautifully:

Dell U2412M monitor (VGA, DVI-D, DisplayPort inputs)
Ancient Dell XPS 8100, Win10 Home, NVIDIA GeForce 220 (VGA, DVI-D, HDMI out)
Dell Latitude 7400, Win 10 Pro, Dell D6000 USB-C dock (DisplayPort, HDMI out)

XPS8100 - NVIDIA GeForce 220 DVI-D out --> DVI-D cable --> U2412M DVI-D in
Dell Latitude 7400 --> Dell D6000 docking station --> DisplayPort cable --> U2412M DisplayPort in

I use the source select on the monitor to go back and forth between machines.

I have replaced the XPS 8100 with a new XPS 8940 (NVIDIA GeForce GTX 1660 Super, Win10 Home), also connected via DVI-D. The GTX 1660 has DVD-D, DisplayPort, and HDMI out.

Now, with both machines on, when I change the monitor to the DisplayPort input, the screen goes black and the light on the power button flashes at a two second interval. I can't see the laptop and cannot change the source back to DVI-D. Here's where it gets weird. As soon as I unplug the DVI-D cable, the power light returns to a solid state, the Dell logo appears, and the laptop video appears on the monitor. I can then plug the DVI-D cable back into the XPS and switch the source back to DVI-D. But it's a one-way trip without unplugging a cable.

I suspect it has something to do with the GeForce GTX 1660 Super, but I have no idea what. Both the old and new XPS are running the same basic default settings (1920x1200, 60Hz, 8 Bit RGB, SDR). It's almost as if something from the GTX 1660 "poisons" the DVI-D connection in the monitor logic.

I'm about ready to buy a DisplayPort KVM, but hesitant, as they seem to introduce other problems.

Appreciate any thoughts. Thank you.

 

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No luck solving this problem, so I settled on the following KVM and it's working really well.

IOGEAR 2-Port HDMI Cable KVM Switch with Cables and Audio, GCS62HU ($70)
Monoprice 102661 3ft 28AWG High Speed HDMI to Adapter DVI Cable with Ferrite Cores ($7)

The key item, from a connection standpoint, is that HDMI and DVI appear to be electrically equivalent. A cheap, passive cable or adapter will connect computer HDMI to monitor DVI.

The second critical item is that the GCS62HU version of this KVM includes EDID support. This maintains the monitor state information when switching between sources. If a KVM doesn't support this, a laptop in Multiple Displays > Extend These Displays configuration will "lose" the external monitor when switching away. All open windows will collapse back to the laptop screen. With EDID support everything remains where you left it.

EDID support doesn't seem to be very common and appears to be missing completely from "budget" DisplayPort KVM's.

For some, the max resolution of 1920 x 1200 may not be enough. That's the optimal resolution for this monitor, so it's a perfect match. I am not using the Audio switching; some report problems with this feature.