ThinkPad T480 and high refresh monitors

Basically bought two 24” monitors, that go all the way to 180Hz and 200Hz. I wanted to connect them through a dock. And one couldn’t be recognized. This is a guide on understanding why and how to fix this issue.

The answer

TL;DR, the T480 is indeed old by now. So it has limitations to its hardware, my ThinkPad T480 uses an Intel 8th Gen graphics (UHD 620). Its USB-C/Thunderbolt ports have a limited “pipe” for video data (DisplayPort 1.2 bandwidth). If I try to push 180Hz and 200Hz signals through one single dock/cable is like trying to fit two firehoses into a garden pipe.

Bandwidth Math

The T480 uses DisplayPort 1.2 via USB-C/Thunderbolt ports

• A standard 1080p @ 60Hz monitor uses about 3.2 Gbps
• Because of drivers/hardware, we can’t really go towards the 200Hz so:
  • A 180Hz would want around 9.6 Gbps
  • A 165Hz would want around 8.8 Gbps
  • Total ~18.4Hz
  • The T480’s single-cable output (the dock) will be capped at 17.28 Gbps

So in short, it’s a bandwidth issue.

Workarounds

With this in mind, it’s easy to draw a comparison and the base logic to try and get the setup to recognize the monitors:

The higher the refresh rate, the more bandwidth it will require. So you can start
at the lowest, and start going up, testing to see at what frequency everything breaks.

So that’s it, you can start doing stuff like:

• Disconnecting one monitor
• Switch refresh rate of the plugged-in monitor to the lowest, plug-in the other monitor, see if signal is fed
• Start going up in refresh rates until one monitor dies

My use case and actual workaround

In my case, nothing was working. So I instead decided the Two-Cable Solution.

This is nothing out of the ordinary: if we are trying to put all that bandwidth through one cable, what if we had two cables to evenly distribute said bandwidth?. And also, make usage of the Thunderbolt port of the T480 which has more bandwidth than the normal USB/C port

And so:

• I connected the dock to the Thunderbolt port
• Disconnected the second monitor from the Dock, plugged-in directly the HDMI cable to the T480’s side HDMI port
• Both monitors started getting signal
  • The HDMI monitor is capped at 120Hz, and that is because the side port is limiting the refresh rate at that amount
  • The Display Port monitor is at its full 180Hz, it’s directly plugged into the dock and said dock goes straight into the T480’s Thunderbolt port.

So, it’s not that bad, compromised 80hz on a 200hz monitor for the fact that I can actually use the two monitors

Conclusions

Had no idea that this was a thing, I was about to attribute it to the usual it’s not the year of the Linux Desktop yet argument. But after some research, troubleshooting, and reflection, it turns out it was nothing that Linux was guilty for (yaaay).

So lesson learned, when dealing with old hardware, be extremely careful as to what peripherals you want to add onto it, it might limit them.