Open-source Promontory 21 card echoes ASRock’s X670 XPANSION KIT idea
A project on open-source hardware platform created by “wesd” shows a PCIe add-in card built around the AMD 600/800-series “southbridge” (Promontory 21). The idea is simple: treat the PCH as a standard PCIe device and use it as an expansion controller on any PCIe host system.
The card uses a PCIe 4.0 x4 upstream link. It breaks out two M.2 slots wired as PCIe 4.0 x4, four SATA 6Gbps ports, and a USB 3.2 Gen2x2 port (20Gbps). So the entire expansion card is limited by the uplink. According to the project page, firmware flashing is required for normal operation, and the public listing does not include firmware files. It also shows test notes stating the card is detected on an Intel platform, with USB negotiating to Gen2x2 and “normal” speed results.
The Promontory 21 chipset in this project is an ASMedia-built PCIe switch used as the AM5 PCH, and reportedly, it costs around $42 or less if you have a used motherboard around.
Source: wesd
This idea closely mirrors ASRock’s “X670 XPANSION KIT ” shown with a special B650 LiveMixer board in early 2023. That card also carries an extra Promontory 21 chipset and adds I/O such as extra M.2, USB, SATA, and 10GbE.
The main difference is how it plugs in. Reports around ASRock’s kit say it was tied to select boards and needed more than a PCIe slot, including a separate control connection and custom firmware support.
ASRock’s “X670 XPANSION KIT, Source: Levels1Tech
So the main difference is that the open-source project can even work on Intel motherboard, as shown in the header image. I couldn’t find any information on the ASRock board being available, so I guess it was some kind of reviewer-only board.
Source: Bilibili , OSHWHub , HKEPC , HXL
