Silent 4K X-Terminal (Experience report)

Skylake-based

• Zotac ZBOX nano CI523 (a fanless mini-computer) plus 2x8GB DDR3L RAM (less probably would have been sufficient, but would have saved little money). Alternative hardware, cheaper with less CPU power: Shuttle XPC slim DS67U (there are also versions of both boxes with more CPU power).
• Samsung U32E850R 31.5in monitor.

Experience

• The combination produces the full resolution at 30Hz with HDMI, and at 60Hz with the provided DP-miniDP cable.
• After power loss, if only the DP-miniDP cable is connected, the ZBOX beeps for four times and does not display anything on the screen (it obviously wants to display on a different output; there is no firmware option to change that), but reacts to keyboard input and boots an OS if available. However, all OSs we tried (Windows 10 (which we tried), Ubuntu 16.04, and a Debian-based LTSP recognized that the monitor was connected via DP and all give the full resolution at 60Hz. Then, after rebooting without power loss, the firmware displays on DP, too. So, the sequence to change firmware settings is: Power on, boot an OS, then reboot into the firmware (no idea what happens if you cannot boot the OS).
• Network (PXE) bootin of the Zbox did work with the (UEFI) settings we tried initially. We finally got it to boot by switching to "Legacy only", then "save and exit" from the firmware, then go into firmware again to tell it to boot with PXE (from the network); you have to set up the ability to boot with PXE in one of the earlier submenus of the firmware, or you don't get this option in the Boot submenu.
• Once we could boot from the network, we tried the LTSP setup based on Debian 8 (Jessie) with the 3.16 kernel that we also used for some older Zbox-based X-Terminals, and much to my surprise, it works fine in our tests up to now (I did not expect this old kernel and X server to drive the Skylake's graphics at 4K correctly).
• The power consumption of the Zbox when idling is 10.7W (and 24VA). The CPU heats up to about 50 degrees C (30 above the surroundings) at this power consumption if the Zbox is standing flat on the table (as apparently envisioned by the designers; there are only legs provided for this position). I played a bit around with firmware options for C-states and such things, resulting in saving 0.6W, but also in some malfunction that made me revert to the original conservative settings. The monitor uses 56W at full brightness and 40W at brightness setting 50 (which is not much less bright), which is very good for such a big screen.
• Not working yet: When the screen saver turns the screen black (and presumably turns off the video signal), the monitor turns on its backlight every few seconds for a few seconds to tell me that it is looking for input on HDMI1 and miniDP (if i have not locked the input), or just on miniDP (if I have locked the input). I have not found a way yet to tell it that "video off" means "standby". After some playing around I used HDMI instead of DisplayPort; the downside is that this provides only 30Hz for 4k, but for typical X-terminal uses, that's not a big problem; it was noticable at the start, but I soon got used to it.

Apollo Lake based

• ASRock J3455-ITX, again with 2x8GB DDR3L RAM (again, much less is probably enough); we put this into an old Mini-ITX case.

This board includes a HDMI 2.0 port (using a display-port->HDMI 2.0 converter chip) for 4k@60Hz. The other boards available at the time of this writing, have only HDMI 1.4 (not 4K@60Hz) and neither DisplayPort nor HDMI 2.0. If you want a Barebone PC (or a smaller PC than available in Mini-ITX format), the Intel NUC Kit NUC6CAYH also supports HDMI 2.0, but you pay about twice as much as for the board above plus a Mini-ITX case.

No Apollo Lake boards or barebones with an external DisplayPort are available at the time of this writing.

We use this board with the Samsung U32E850R monitor described above.

Experience

• One nice advantage that the ASRock board has over the Zbox is that it has PS/2 in addition to USB connectors, so I can use my old keyboard without needing a PS/2-USB converter.
• When connected to port HDMI1 of the monitor, we got only 4k@30Hz. We needed to connect to port HDMI2 to get 4k@60Hz.
• Linux kernels up to 4.7 introduce pauses of a few seconds, sometimes every few seconds; Linux 4.8 and 4.9 do not have this problem (tested both with Ubuntu 16.10 and the LTSP setup described in the following). So we built a Debian Testing/Stretch-based LTSP by first building a Debian Testing chroot setup, chrooting there, building an LTSP setup with ltsp-build-client, and finally installing it in the usual places. Earlier we tried using a backports kernel with the jessie LTSP setup, but that did not work properly (in particular, did not start X) due to some overlayfs incompatibility.
• The result works nicely for most applications; an exception is Firefox/Iceweasel: edit fields often get the wrong background colour; it's especially annoying if the wrong background colour is black, and the foreground colour is also black. Update: The problem seems to come from the (default) driver "modesetting"; adding a line

      XSERVER=intel
  

  to .../etc/lts.conf made the X server use the Intel driver which fixed this problem (and is probably also faster).
• Idle power consumption with our case is similar to the Zbox (I expect that you can get power supplies that are better); the box does not feel as warm, though, because it is larger.