The Tyan M3289 SMDC
Overview of how I think it works.
First, a couple of definitions: BMC = Baseboard Management Controller, TSO = Tyan System Operator, SMDC = System Management Daughter Card, which is Tyan's BMC.
The Tyan M3289 is built around a QLogic Zircon BMC and implements IPMI 1.5, with a proprietary serial over ethernet extension (this requires TSO). It doesn't include any ethernet parts, so relies on one of the motherboard's NICs for ethernet access. Only one NIC will relay data between the SMDC and the ethernet, see image here. The firmware for the onboard NICs have to be IPMI aware, and redirect IPMI traffic to the SMDC. Supposedly, the SMDC and the OS can share access to the NIC, with the NIC differentiating and handling appropriately.
A utility is run on the local machine to configure the BMC (DOS and linux versions available; DOS version has more features, ie sensors monitoring), this util is used to: set basic IP info, passwords for 4 levels of users (see IPMI 1.5 docs), enable ARP broadcasting (not sure what purpose this serves), update firmware on BMC. Info regarding the SMDC can be found here and a local copy of the pdf of docs can be found here.
Once configured, the SMDC should be listening and when soft powered down respond to remote "out of band" commands (power on, etc). And when powered on respond to both "out of band" (power off, reset) and "in band" commands and queries.
The "nifty" features of the Tyan solution are supplied by the TSO-agent, which (in linux) ties into the "openipmi" supplied kernel modules, and several services supplied when installing Tyan's "TSO-Agent".
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