Cyrus64 posted this news:

After doing a bit of googling, I recently found a Connexant/Rockwell pdf relevant to the Dreamcast modem 🙂
For anyone who wants to have a hack-about, modem registers 0x00-0x1f are mapped to 0xA0600004 and following on the dreamcast, with 0xA0600000 being a general modem id / enable h/w register.
The modem is designed to have programs run on its dsp, but can also seemingly be merely controlled via direct register writes. Already, NickK has made some great progress tinkering with it, and this would be a very cool piece of hardware to have supported both for developmental purposes (connecting it to direct to pc) and for other (online) projects.

Download the Pdf file from Boob!