Xpeed 300 is one of the more unusual pieces of SDSL CPE. It is not a standalone unit, but a plug-in PCI card for SDSL. It was only for Copper Mountain and other Flavor B DSLAMs, not for SDSL/ATM. There is also an IDSL sibling called Xpeed 200.

A friend of mine has recently managed to obtain one and examine it for me. It turns out to be very simple and straighforward. The device is basically a juxtaposition of a bog-standard PCI synchronous serial port adapter and an SDSL or IDSL DSU on the same card. The host OS drivers (of which an open source one is available for Linux) are concerned only with the bog-standard PCI sync serial part and don't contain anything interesting with regard to SDSL.

Here is the interesting part: on the SDSL version of the card the DSU section is an RS8973 bitpump controlled by an 8051! The 8051 is obviously a direct carryover from the Brooktree/Rockwell/Conexant reference design — apparently Xpeed wasn't bothered by the 8051 as much as Larscom was. The firmware for the 8051 is in a flash chip; user utilities which configure the SDSL section do so by patching the 8051 firmware image and reprogramming the flash.

Xpeed 300 is definitely one of the CopperCompatible CPE devices which CM had supported without CMCP.