I've been looking at the PC Engines APU line for a while. They're a line of medium size single board PCs with a DB9 serial connector, no VGA output, and multiple gigabit Ethernet ports. Because of this they're often used as firewall machines.

I want to get one and use it as either the home router, or an experimental server to mess around with. Quite a few OpenBSD folks use them and recommend them as OpenBSD router and server hardware. They aren't too expensive, have decent specs and a small physical footprint.

One thing that's always confused me was the naming scheme. Initially there was the APU, then the APU2, as described on their site, which makes sense. Then the APU model numbers get a little weird. They come in several variants, I'll list them here for context.

 apu2d0 (2 GB DRAM, 2 i211AT NICs)
 apu2e2 (2 GB DRAM, 3 i211AT NICs)
 apu2e4 (4 GB DRAM, 3 i210AT NICs)
 apu3c2 (2 GB DRAM, 3 i211AT NICs, optimized for 3G/LTE modems)
 apu3c4 (4 GB DRAM, 3 i211AT NICs, optimized for 3G/LTE modems)
 apu4d2 (2 GB DRAM, 4 i211AT NICs)
 apu4d4 (4 GB DRAM, 4 i211AT NICs)

What do the letters between the numbers mean? What is the significance of the numbers in the first place?

Let's take the apu3c2 as an example. The 3 here means it's version 3 of the APU board. The APU 1 is no longer sold, so it's left out of the list on the PC Engines website. The letter c is the revision of that board. It's pretty much irrelevant for comparison, mostly just mostly minor tweaks to the board. The final number, 2, is the number of gigabytes of RAM (in most cases).

The APU2E2 boards have a single mSATA slot, and a regular SATA connector, two mPCIe slots, and a SIM tray. It comes with 2 external USB 3.0 type A ports, 2 internal USB 2.0 ports (header only), an SD card slot, and a GPIO header. It also has 3 Intel i211AT gigabit NICs. Using the SIM tray will remove the ability to use one of the two mPCIe slots as anything other than a modem.

So what are the major differences between version numbers? They all use the same CPU, the quad core AMD Embedded G series GX-412TC running at 1 GHz. Most other components on the boards are the same as well. I'll give a description of the differences relative to the APU2 board.

The APU2D0 is the same as the regular APU2, but without a GPIO headers, and it has only 2 NICs.

The APU3 has an extra SIM tray, and the first slot, which is mSATA only on the APU2, can be used as either an mSATA or USB 3G/LTE Modem slot. The second slot can only be used for a modem. It also lets you do GPIO tray swapping and failover, so you can use only a single modem for 2 SIMs.

The APU4D2 has 4 NICs instead of 3, and the second slot is modem only. It has 2 GB of RAM.

The APU4D4 is the same as the APU4D2, except it has dual SIM trays. It has 4 GB of RAM.

I found a lot of this information wasn't really obvious to upon first reading the PC Engines website, as there's no direct comparison between the board, and the site relies on you flipping between pages to find the details. The re-seller website I looked at also didn't elaborate on the differences.