PC Engines APU 4D4

I recently decided to replace a couple of Banana Pi R1 routers with something rather more standard (the R1 has a pretty strange networking design, with four network ports based on a layer 2 switch instead of multiple actual network interfaces) and also maintainable (the Bananian Linux distro which I installed on these things when I started using them has not been updated for years, and it's far from straightforward to install a standard Linux distro on them, mainly due to the strange hardware design).

I was recommended to look at PC Engines' APU 4D4 systems, and I liked what I saw.

They contain a standard AMD64 CPU (so, no out-of-the-ordinary installer is required to get Linux onto these things), they have 4 completely standard and independent Gigabit network ports, and they're designed to take mSATA SSDs, so they're completely silent in operation. There is no video output.

My only real niggle with them is that they have a standard SATA port on the motherboard (plus a power connector), but there is insufficient space in the casing to mount any standard SATA disk (HDD or SSD). It is just possible if you don't mind gluing a thin SSD to the lid of the case, or actually dismantling an SSD and using the PCB without the plastic casing.

However, the installation process for getting standard Devuan onto them was incredibly easy (provided you have a PC with an RS232 serial port, or a USB to RS232 serial converter, and a null modem cable).

  1. Create a completely standard Devuan installer USB stick (AMD64 architecture)
  2. Insert the USB stick into one of the two USB3 ports on the APU
  3. Connect your null modem cable to the APU and to the serial port on your PC (or the USB to serial converter)
  4. Run a terminal emulator on ttyS0 or ttyUSB0, speed=115200, no parity, 8 bits
  5. Power on the APU
  6. No need to press F10 at the prompt; the default boot order is:
    1. SD card (internal)
    2. USB device
    3. mSATA internal SSD
  7. When you get the Grub menu, edit the standard "install" option (press Tab to do this) and add "console=ttyS0,115200n8" to the kernel boot line
  8. Press F10 to start the installer boot loader
  9. Proceed with the Devuan installer in exactly the same way as you would on any standard system which has a display screen and a keyboard.

I used the network installer (which I always do), so connect a network cable from the left-most port (which is eth0) to a switch on your LAN. The installer will as usual attempt to get an IPv6 address, and then an IPv4 address by DHCP, or allow you to specify an address and a gateway manually.

Once the installer is complete, remove the USB stick and let the system reboot - it will then boot from the (completely standard) Grub system on the mSATA drive, and if you added "console=115200n8" to the /etc/default/grub variable GRUB_CMDLINE_LINUX_DEFAULT, you will get all the standard boot-time messages output on the serial port (followed by a login prompt, which works).

The system should by that stage also be available by SSH over the network.

This must be one of the simplest systems with no video output that I've ever come across to get Linux installed onto.

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