@Jayokc fyi when asking technical questions like this, it helps to provide technical information. In this case, it would help to specify the resolution(s) of the display(s) you're trying to run. But as general info, as noted in the WD15's product page, FAQ page, and manual available on support.dell.com, it's only meant to run dual displays up to 1920x1200 each or a single display up to 2560x1600 (or a 4K display but only at 30 Hz rather than the typical 30). This is related to bandwidth limitations on USB-C ports especially when they're also carrying USB 3.x data, as they would be set up to do when connected to the WD15. If you've got 3 displays working, then you're likely using those displays at a low resolution and/or refresh rate.
In addition, all Intel GPUs as of this writing are limited to running no more than 3 simultaneous independent displays total, regardless of resolution, how they're connected, or any other limitations that may exist. And even among laptops that have discrete GPUs, most or all of the display outputs are still controlled by the Intel GPU, so the discrete GPU doesn't help you avoid that limitation. This is true of the Precision 5520, where its USB-C and HDMI outputs are controlled by the Intel GPU.
The only way to run more displays would be to use an adapter that relied on "indirect display" technology such as DisplayLink -- not to be confused with DisplayPort. DisplayLink displays don't count toward the GPU maximum because they're not directly driven by the GPU, but DisplayLink's technology comes with some drawbacks that can be significant in certain use cases. I wrote about those in detail in the post marked as the answer in this thread. But if those drawbacks are acceptable to you, then yes you could plug a DisplayLink-based USB to DisplayPort/HDMI/DVI/VGA adapter (there are all kinds available) into a USB port on the WD15 and that display would work, after you installed the DisplayLink software. That display wouldn't count against the WD15's normal display bandwidth limitations because its data would be carried as USB data, not native video data.
If DisplayLink's drawbacks would not be acceptable to you, then as I said, laptops in general are limited to 3 simultaneous independent displays. There are only a few models that allow the discrete GPU to have direct control of the display outputs that would be exceptions to that rule, but the Precision 5520 isn't one of them. And again, even with the 3 displays you're running, if you're trying to run all of them through a WD15, chances are they're not going to perform well unless they're actually very low resolution to begin with.
@Jayokc just edited my above post since I see that you did in fact specify your system model, so I've updated my reply to take that into account. Also note that since you're using a Precision 5520:
- Make sure your WD15 is using a 180W power supply, not a 130W power supply it could also be ordered with, otherwise the WD15 won't be able to pass through the full 130W that the Precision 5520 is designed to run on, in which case the Precision 5520 would reduce its battery charging speed and possibly throttle performance to try to operate within a reduced power budget.
- The Precision 5520 supports Thunderbolt 3, which means you could use a Thunderbolt dock like the WD19TB, which would be able to tap into 4x more display bandwidth than the WD15 can, since it only uses regular USB-C. So the result is that while the WD15 can only do dual displays up to 1920x1200 each, the WD19TB can do dual displays up to 4K 60 Hz each, as well as triple displays up to 2560x1600 each.
By the way, that "square hole" connector you referred to is called Mini-DisplayPort, and it isn't quite square.
jphughan
9 Legend
•
14K Posts
0
April 5th, 2020 13:00
@Jayokc fyi when asking technical questions like this, it helps to provide technical information. In this case, it would help to specify the resolution(s) of the display(s) you're trying to run. But as general info, as noted in the WD15's product page, FAQ page, and manual available on support.dell.com, it's only meant to run dual displays up to 1920x1200 each or a single display up to 2560x1600 (or a 4K display but only at 30 Hz rather than the typical 30). This is related to bandwidth limitations on USB-C ports especially when they're also carrying USB 3.x data, as they would be set up to do when connected to the WD15. If you've got 3 displays working, then you're likely using those displays at a low resolution and/or refresh rate.
In addition, all Intel GPUs as of this writing are limited to running no more than 3 simultaneous independent displays total, regardless of resolution, how they're connected, or any other limitations that may exist. And even among laptops that have discrete GPUs, most or all of the display outputs are still controlled by the Intel GPU, so the discrete GPU doesn't help you avoid that limitation. This is true of the Precision 5520, where its USB-C and HDMI outputs are controlled by the Intel GPU.
The only way to run more displays would be to use an adapter that relied on "indirect display" technology such as DisplayLink -- not to be confused with DisplayPort. DisplayLink displays don't count toward the GPU maximum because they're not directly driven by the GPU, but DisplayLink's technology comes with some drawbacks that can be significant in certain use cases. I wrote about those in detail in the post marked as the answer in this thread. But if those drawbacks are acceptable to you, then yes you could plug a DisplayLink-based USB to DisplayPort/HDMI/DVI/VGA adapter (there are all kinds available) into a USB port on the WD15 and that display would work, after you installed the DisplayLink software. That display wouldn't count against the WD15's normal display bandwidth limitations because its data would be carried as USB data, not native video data.
If DisplayLink's drawbacks would not be acceptable to you, then as I said, laptops in general are limited to 3 simultaneous independent displays. There are only a few models that allow the discrete GPU to have direct control of the display outputs that would be exceptions to that rule, but the Precision 5520 isn't one of them. And again, even with the 3 displays you're running, if you're trying to run all of them through a WD15, chances are they're not going to perform well unless they're actually very low resolution to begin with.
jphughan
9 Legend
•
14K Posts
0
April 5th, 2020 13:00
@Jayokc just edited my above post since I see that you did in fact specify your system model, so I've updated my reply to take that into account. Also note that since you're using a Precision 5520:
- Make sure your WD15 is using a 180W power supply, not a 130W power supply it could also be ordered with, otherwise the WD15 won't be able to pass through the full 130W that the Precision 5520 is designed to run on, in which case the Precision 5520 would reduce its battery charging speed and possibly throttle performance to try to operate within a reduced power budget.
- The Precision 5520 supports Thunderbolt 3, which means you could use a Thunderbolt dock like the WD19TB, which would be able to tap into 4x more display bandwidth than the WD15 can, since it only uses regular USB-C. So the result is that while the WD15 can only do dual displays up to 1920x1200 each, the WD19TB can do dual displays up to 4K 60 Hz each, as well as triple displays up to 2560x1600 each.
By the way, that "square hole" connector you referred to is called Mini-DisplayPort, and it isn't quite square.