Hi Mike,

On Fri, Apr 23, 2021 at 08:49:05AM -0600, Mike Jones wrote:
> Patrick,
> 
> We are using spec: Open Rack V3 48V BBU.

Got it.

I think you'll want to look here:
 - https://www.opencompute.org/wiki/Open_Rack/SpecsAndDesigns

> In the case of a PMC, since I don’t know the OCP architecture well, is there a PMC device in the rack that would manage a BBU, or do devices that plug into their rack each have a PMC?
> 
> Looking at the above spec, the functions of the MCU are quite limited, so I assume there is a PMC device that is managing these other elements like a BBU. 

If you look at the Power Shelf design you'll see mention of a PMC/PMI
connector.  This is a slot for which two different designs can fit into:
PMC or PMI (and I don't recall exactly what the "I" stands for).

With PMC, the intended design was this was a full BMC-class device with
a network interface.  This would run OpenBMC.

With PMI, there is a MCU in place of the BMC and the PMI is connected to
the top-of-rack switch using a modbus / RS-485 over RJ-45 implementation
like was available in ORv2.  I think the PMI would speak the same exact
protocol as the similar device in ORv2.

You might find details on this modbus protocol in the above link, but
there is a lot to dig through...

-- 
Patrick Williams