windows 7 - Dell Latitude E5540 doesn't recognize external display

I stumbled over this problem when I connected a projector to my E5540's VGA port and the laptop simply failed to detect it. Even restarting the laptop did not make it recognize the external display. The projector didn't receive any signal from the laptop and the laptop didn't show any signs of recognizing an external display.

Instead of showing the projector, the Intel HD Grafics utility insisted on a Digital Display VMM2300 ROM being connected to the laptop, regardless of whether the projector was plugged in or not.

I did some more tests and came to the result, that monitors attached to the laptop's VGA or the docking station's VGA and DVI ports were not recognized. A monitor plugged into the laptop's HDMI port showed up in Intel HD Grafics utility and was usable.

None of the above made that misterious VMM2300 ROM monitor disappear. Doing a driver update did not change the behaviour.

Mostly by accident I did then discover that the external monitors do work when the are already connected when the laptop powers up. This requires a true shutdown and fresh power up, so only using Windows Restart option or putting it to sleep and waking it up doesn't do the trick.

Anyway, there still appears to be a VMM2300 ROM attached to the laptop in addition to any devices that are really present.

Now it is possible to use external display(s), at least as long as I'm prepared to shut the laptop down, let it switch off, connect the displays, switch it back on and wait for Windows to boot.

Answer

Dell Pro support quickly came up with this being a hardware problem, offering to replace the Motherboard of the laptop. But as the external displays worke flawlessly when connected before powering the laptop, I couldn't agree with this explanation.

A web search for VMM2300 returns one hit from superuser, but this one is for Latitude E7440.

The solution however is very similar: Downgrade the Synaptics VMM2320 MST HUB Firmware to a version lower than the (pre-installed) A05 / 2.22.

In the beginning, I installed A01 / 2.15, which already solved the problem for me.
I did then upgrade step by step to A02 / 2.17 and A03 / 2.21, which do also work.
Installing A05 / 2.22 brings the error back.

So my conclusion for now is: Use the A03 / 2.21 version of the firmware. With this one, I can connect and disconnect external monitors, dock and undock the laptop, restart it and it does reliably recognize each change to the display devices.

Note: The Dell Support pages have quite detailed installation instructions. For each of the firmware installations, it is required to shut down the laptop, either dock it or connect an external monitor to the laptop itself (it depends on which firmware you are going to install whether dock or external monitor is required) and then start the installation.