Driving Eve Spectrum with MST DP output of Dell P2415Q
The Dell P2415Q 4K monitor I use since 2015 supports so called daisy-chaining: for this purpose, it’s equipped with not just a DisplayPort (DP) input for connecting a video source, but also with a DP output for connecting another monitor.
This was an interesting feature to investigate in combination with Eve Spectrum.
The tests were done with the latest Eve Spectrum’s firmware 102 rev. 875 (2021-07-12).
- Only Dell P2415Q was visible to the system. Both monitors displayed the same image.
- Computer boot was visible and stretched like it is always on Dell P2415Q alone.
- 4K@60Hz was possible on Eve Spectrum with MST disabled in Dell P2415Q settings.
- 640x480@75Hz worked on Eve Spectrum with pixel-perfect scaling, though EDID lacks the mode.
What is daisy-chaining?
Daisy-chaining is the feature the DisplayPort’s MST (Multi-Stream Transport) mode is intended for. It’s meant for connecting two monitors in a serial way, using just one DP output of the video source:
- the video source (computer) is connected to a DP input of a monitor called primary;
- another (secondary) monitor is connected to the DP output of the primary monitor.
Both monitors should be visible to the system and usable as separate displays just like if they were both connected directly to the computer.
Trying with MST disabled in Dell P2415Q settings (default)
So I connected my computer to Dell P2415Q’s DP input, and Eve Spectrum to Dell P2415Q’s DP output, using the DP 1.4 cable by Hama and DP 1.2 cable by VCOM correspondingly.
Only Dell P2415Q visible to system, same image on both
By default, the daisy-chaining/MST mode is disabled in Dell P2415Q settings: the “Display” → “MST” option is set to “Off”.
In this mode, the same image was output to both monitors. The only monitor visible to the system was Dell P2415Q, and available video modes (resolution + refresh rate) were limited to those of Dell P2415Q, so e.g. no HFR video modes like FHD@144Hz supported by Eve Spectrum were available.
4K@60Hz was possible on Eve Spectrum
Probably as a result of that Eve Spectrum and therefore its EDID (with its apparently not-quite-correctly reported data about supported video modes) were invisible to the system, Eve Spectrum finally displayed 4K at 60 Hz sent by Dell P2415Q, instead of just 30 Hz available previously when connecting my computer directly to Eve Spectrum.
In an attempt to use Dell P2415Q as a sort of DisplayPort “adapter” between my computer and Eve Spectrum, I tried to turn Dell P2415Q off with its “Power” button. But as a result, Eve Spectrum said “No signal” and switched to standby mode. Turning Dell P2415Q on again restored the image on it, but Eve Spectrum was still in standby mode. Turning Eve Spectrum off with its “Power” button, then on, did help — both monitors displayed the same image again.
Of course this is not an option for long-term use because:
both monitors have to be turned on at the same time and show the same image;
Spectrum-specific HFR modes like FHD@144Hz are not available;
and controlling Eve Spectrum’s brightness and other settings programmatically via DDC/CI is impossible (see below).
Computer boot is identical to Dell P2415Q alone
In terms of computer-boot visibility and scaling during boot, everything was identical to when Dell P2415Q is the only monitor connected to my computer:
Computer boot was visible on both monitors.
The video mode displayed as the current one in menus (OSD) of both monitors was 3840x2160@60Hz.
The image was stretched to entire screen and blurry on both monitors.
Pixel-perfect scaling feature of Eve Spectrum had no effect.
Given that during computer boot, Dell P2415Q, when connected to computer as the only monitor, always displays 3840x2160@60Hz as the current video mode in its menu/OSD, I suspect it performs some implicit internal conversion of unsupported video modes such as 720×400@70Hz (apparently used during computer boot on computers with non-UEFI BIOS) to 4K@60Hz. So Eve Spectrum apparently received the same video signal already prescaled to 4K by Dell P2415Q, therefore further scaling was impossible.
Attempt to turn daisy-chaining/MST on
Enabling daisy-chaining/MST in the Dell P2415Q menu with it set to be primary (“Display” → “MST” → “Primary”), resulted in that both monitors switched to standby mode. I was forced to blindly turn my computer off via a sequence of
Win+D (to minimize all windows),
Alt+F4 (to show the Windows’ shutdown window), and
Enter (to shut the computer down). After turning my computer on again, the computer self-restarted multiple times in several seconds after boot start, cyclically.
After several iterations, including turning off the power of the computer and both monitors, Dell P2415Q said there is no signal and suggested to try switching to “Secondary” as the value of its “MST” option. (Later I read in the Dell P2415Q manual that there is a way to manually force displaying this option. That’s called RTFM. ) I accepted and computer was finally able to boot. As a result of switching Dell P2415Q to the “Secondary” value, both monitors displayed the same image again just like in case of the default “Off”, but now with 30 Hz as the refresh rate of both monitors.
In case of DP 1.2 and Dell P2415Q, 30 Hz is the expected refresh rate due to the limited DP 1.2 bandwidth. But if daisy-chaining worked, both monitors should have been visible and usable as separate displays by the computer, while in fact the only display available to the system was Dell P2415Q.
So looks like Eve Spectrum 4K monitor does not support operating as a secondary monitor in multi-display configurations based on daisy-chaining/MST. According to the Dell P2415Q manual (PDF, 6.7 MB), daisy-chaining is not tied to the specific monitor model and should work with any DP-1.2-capable monitor.
No way to control brightness etc. via DDC/CI
Because Eve Spectrum was not visible to the system as a separate display, and the only visible one was Dell P2415Q, it was not possible to control brightness and other settings of Eve Spectrum programmatically via DDC/CI e.g. with the ClickMonitorDDC utility.
So the only way to change brightness of Eve Spectrum according to the usecase was using the monitor menu/OSD with its physical joystick: e.g. during regular desktop work (web surfing, file management, programming, etc.), enough brightness is 0 (zero) (at constrast of 25 due to the current impossibility to set it below 25 programmatically), while watching movies needs a brightness of 30 or higher.
Eve Spectrum worked fine in unsupported 640x480@75Hz
Dell P2415Q supports 640x480@75Hz video mode which Eve Spectrum’s EDID lacks. But with the daisy-chaining/MST serial connection, the mode was displayed fine on Eve Spectrum too:
- The mode was indicated correctly in the Eve Spectrum menu.
- Pixel-perfect (integer) scaling did work fine.
This is probably another confirmation that Eve Spectrum is capable of displaying modes missing in its EDID, along with 960×540 under Linux via HDMI port #2, and 1920×1078 (1080i under Linux) via HDMI port #1.