ECC memory support is only one of many reasons that someone building a workstation might base it on a Xeon processor. The specialized motherboards that they require include other features, such as formal support for hardware virtualization and advanced remote management features. If you've been looking for all of that in cherry flavor instead of blueberry, you might have been struggling. However, ASRock would like you to know that it has just the motherboard for you in the ASRock Rack X470D4U.
This is a micro-ATX Socket AM4 motherboard that takes second-generation Ryzen CPUs and up to 64GB of unbuffered ECC memory. It's based on AMD's X470 chipset just like your usual Socket AM4 overclocker boards, but it's a pretty far cry from something like ASRock's own X470 Taichi. Besides the official ECC memory support, this board also has an Aspeed AST2500 baseboard management chip that can handle IPMI and graphics duties.
The AST2500 has a Realtek-powered gigabit Ethernet connection all to its own, while the host system has access to a pair of Intel-powered GbE ports. Storage accommodations include eight 6 Gbps SATA connections and a pair of M.2 sockets, both of which supports PCIe drives, and one of which additionally hooks up to SATA.
External I/O is a bit of a weak point for the board; a VGA port, two USB 3.0 ports, the three RJ-45 connections, and an old-school serial port are all you get. There is at least a front-panel header for another pair of USB 3.0 ports. Given the extensive networking and management features available on this board, you'd presumably be using it over the network, so the lacking local I/O isn't cause for much concern.
ASRock doesn't actually provide a CPU support list, but the handy-dandy block diagram above (taken from the manual for the X470D4U) notes that the board supports CPUs rated for thermal design power of 105W. By our reckoning, that's everything on up to the Ryzen 7 2700X. In theory, this board should support the third-generation Ryzen CPUs when they launch later this year.