Latitude 7480 USB -C to DVi issue connecting to external monitors

@AC94  a "Billboard device" is a device class created for USB-C peripherals.  Its intent is to give those devices a way to display some kind of error message (the "billboard") on the system when there's a compatibility issue that prevents the device from performing its normal function, so that the user knows why it's not working as expected.  For example, USB-C ports can support video output, system charging, and USB 3.1 Gen 1 or 2, but all of those capabilities are optional, so if you try to use a peripheral that requires a capability like that with a system that doesn't support it, the billboard device (which communicates over USB 2.0, which is mandatory in the USB-C spec) could be used to inform you of that type of problem.  That said, it sounds like you're not seeing an error message, and considering that the Latitude 7480 definitely supports video output over USB-C as standard, I'm surprised you're having an issue here.

It could be a problem with the cable.  Do you have another way to test video output from that system's USB-C port to rule that out?  If not, you mention that you're also connecting another display using HDMI to VGA.  In that case, what display inputs do you have across each of your displays?  You could potentially use an HDMI to DVI cable/adapter to connect the currently non-functional display via the system's HDMI output, and then a USB-C to VGA adapter to connect the display that's currently running off your HDMI output.  Or better yet, perhaps there's a way to stop using VGA entirely?  VGA will always look noticeably worse than anything else because it's the only analog video signal still in use; everything else is digital.

I also encountered similar problem. Connecting usb-c to displayport through a converter can work, but it does not work to connect to a monitor using the dvi port. Very strange. I tested three different 7480 laptops, none worked.

@sglaoli  Are you saying you used a USB-C to DisplayPort adapter and then tried to use a DisplayPort to DVI cable to connect that dongle to the display?  If so, that typically will not work.  Typical passive DisplayPort to DVI cables rely on the source port supporting Dual Mode DisplayPort, aka DP++, which is an optional feature that allows a DisplayPort output to support sending a native HDMI/DVI signal.  USB-C ports do not support this.  That's why USB-C to HDMI adapters -- and USB-C to DVI adapters for that matter -- have to incorporate active signal converter chips to switch that DisplayPort signal over to HDMI/DVI.  There are however also active DisplayPort to DVI cables that incorporate that type of signal converter chip.  They're designed to be used with DisplayPort sources that don't support Dual Mode DisplayPort or where using that capability would create a problem, such as AMD EyeFinity setups.  That type of cable would have paired with a USB-C to DP adapter, as long as the adapter supplied the amount of power that is supposed to be available from a full-size DP output according to the standard.  But the much easier solution would have been a USB-C to DVI cable.  I've used a USB-C to DVI cable with my own Latitude 7480 with no issues at all.

Hi @jphughan I have tried several cases:

1. USB-c to DVI adapter =>DVI cable =Monitor: didn't work

2. USB-c to DisplayPort adapter =>DisplayPort cable =>moniter: it works;

3. USB-C to DisplayPort adapter => DisplayPort to DVI adapter => DVI cable =>monitor: didn't work

@sglaoli  I realize this is a basic question, but I'll ask it because this has been the answer in cases like this in the past: Are you switching the display over to its DVI input when switching over to using a DVI cable?  Otherwise, if you've only tested a single USB-C to DVI adapter thus far, there might be an interoperability issue between that specific adapter and the Latitude 7480.  I've seen this on USB-C to HDMI adapters where a user will report that some adapter works on certain systems and not others even though it should work on both, and in all cases, the fix was to just try another brand of adapter.  Evidently those active signal converter chips can be finicky.

Hi @jphughan , yes, did the monitor port selection.

About the usb-c to dvi adapter: tested 2 dell 7480, didn't work.

Later my colleague tested the same usb-c to dvi adapter using a newer DELL laptop model and it worked!

That's strange, made me assuming that something might be wrong for the 7480.

Not sure if anybody has the experience of using usb-c to dvi adapter on a DELL Latitude 7480 and it worked?

@sglaoli  Sounds just like the interoperability issues that can occur with USB-C to HDMI adapters. I’d suggest trying another cable. For what it’s worth, I’ve consistently had good results with products by Cable Matters.

I'm having a similar issue: I have two Samsung SyncMaster S24B300 monitors. When connected with a BENFEI USB-C to DVI cable, the display is recognised by the OS, and can be enabled, but the monitor doesn't display anything - have tried both monitors. The monitor  menu shows that it is configured to use DVI, and you can select between AV and PC mode, but neither option produces an image. This happens regardless of whether the system is set to duplicate the display or extend it onto the USB-C connected monitor.

The other monitor is connected via an HDMI to DVI adapter cable and works perfectly.