Multiple external displays for 7420

Question

I'm looking at getting one of the new 7420 laptops but I'm trying to figure out how many (and what resolution) monitors it can drive? Ideally I'd to have a couple of 4K monitors in the mix. If a dock like the WD19TB is necessary for this, is it important that the monitors support DisplayPort 1.4? (I was looking at the S2721Q but noticed that only has DP1.2)

jphughan · Accepted Answer

@flcdrg1 I'll assume that you're talking about a Latitude 7420 since you posted in the Latitude section, but fyi it's good to specify the product line because Dell reuses model numbers across multiple lines and sometimes people post in the wrong forum section.  But yes that system can run dual external 4K 60 Hz displays and the built-in display all simultaneously.  If you're using the WD19TB, it's not important that the displays themselves support DisplayPort 1.4.  The DisplayPort revision typically comes into play in two scenarios: The DisplayPort revision your system supports on its USB-C/TB3 port determines how much total video bandwidth it can push out to a dock and therefore what display setups can be run from the dock.  But when Thunderbolt 3 is being used, even DP 1.2 is enough for dual 4K 60 Hz. The DisplayPort revision the display supports can matter if you're trying to run a daisy chain, since that affects the amount of total bandwidth available along the chain and therefore what displays you can put onto the chain. But since each display will be directly connected to the dock, you won't have to worry about daisy chaining.  DP 1.2 is enough for a single 4K 60 Hz display, and your system will push enough total bandwidth to the dock to support dual 4K 60 Hz.

jphughan · Answer

@flcdrg1  Just adding to my earlier reply, in terms of how many displays and maximum resolution, according to Intel's specs for that CPU (and therefore its integrated GPU), it can run up to 4 total displays, and up to 8K 60 Hz on a single display.  But the maximum total display quantity and maximum per-display resolution are just part of the equation.  You also have to look at the bandwidth requirements of your envisioned setup and how you plan to connect them to the system.  You wouldn't be able to run 4x 8K 60 Hz displays through a WD19TB, for example.  In fact you can't even run a single 8K 60 Hz display through the dock because the bandwidth requirements of one of those would exceed what's available over Thunderbolt 4, even before considering the need to run non-video traffic for things like the USB ports, Ethernet, etc.  (If DisplayPort DSC were in play here, that might change the equation, but there's no indication that the WD19TB supports that.)I'm pretty sure the most you can get from that system when paired with a WD19TB is dual 4K 60 Hz + single QHD 60 Hz, and even achieving that requires using specific combinations of outputs on the dock because the dock isn't infinitely flexible in terms of how it allocates video bandwidth from the attached system.  You may want to check the WD19TB User Guide available on support.dell.com.  There's a Display Resolution Table that summarizes your options pretty well.  Make sure you look at the table for a Thunderbolt-capable DisplayPort 1.4/HBR3 system, since that's what the Latitude 7420 is.

jphughan · Answer

@flcdrg1  Happy to help!  In terms of the follow-ups, yes you'd be able to use the laptop's built-in display along with the external 4K displays.In terms of the third external display, you should be able to connect it to the HDMI output and use everything simultaneously.  I've seen some Latitude laptops that have 3 types of video outputs (USB-C, HDMI, and VGA) that had internal wiring such that only 2 could be used at any given time, so in theory it's possible that running dual displays via TB3 might render the built-in HDMI output unusable, but I'd be quite surprised if that was the case.  But another option you'd have would be to get a USB-C to HDMI adapter/cable to run that HDMI display from the downstream TB3 port on the back of the dock, thus preserving single cable connectivity to your system.  In a dual 4K 60 Hz scenario, the third display on the WD19TB would have to be connected to the USB-C downstream TB3 port, but that doesn't mean it has to be a native USB-C/TB3 display.Finally, fair warning here: I have not tested this exact setup, and it is the nature of the tech world that not everything that SHOULD work actually DOES work.  As just one example, the XPS 13 9300 when it first launched was very glitchy when trying to run dual 4K 60 Hz displays through a WD19TB, even though that setup is supposed to work.  So especially when using a brand new system model that hasn't received a batch of firmware updates yet AND pushing it near the edges of its capabilities, you might find that things don't actually perform as the specs suggest they can and should.  Just something to keep in mind here.

flcdrg1 · Answer

Thanks for the confirmation, and yes I meant the Latitude 7420 A followup question.. If I had WD19TB dock configured with two 4K monitors, I presume I'd still be able to use the display on laptop as well? If I added a 3rd (non-4K) external monitor to the mix, can I plug that in to the laptop's HDMI port? I had taken a look at the WD19TB display resolution table and if I'm reading it correctly for HBR3 then if you have 2x 4K monitors then an additional 3rd display needs to use 'USB Type-C TB' (a HDMI port is not given as an option). With a 3rd external monitor would the laptop display work then?

flcdrg1 · Answer

Awesome. Appreciate that! I like the idea of the USB-C to HDMI adapter. Looks like they're pretty cheap so I could possibly reuse an existing HDMI monitor and confirm it works alongside everything else without as you say having to buy a special USB-C display.

RD2013 · Answer

Hi, Great advice on this issue!!  I have another related question. Using the WD19TB dock with the Latitude 7420, is it possible to run two 24' monitors with higher refresh rate. For example, with the Nvidia Iris card and the dock support 1 75hz or ideally 2 125hz displays? I have been told that this boost in refresh rate can make a noticeable difference when it comes to reducing eye fatigue.

RD2013 · Answer

That was a typo!  I want to run a pair of 24' monitors with higher refresh rate than 60hz. I am hoping to go to as high as 125hz.

flcdrg1 · Answer

@RD2013probably best posting a separate question. FYI I ended up running two S2721Q monitors through the WD19TB dock, and a third S2721D monitor directly attached to the 7420 laptop HDMI (it can connect via the dock but the quality was not as good).

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