If not and I blow up the back plane, I still have my drive trays and fans. I think I may pursue this, if I can make a kind of low power DAS out of this chassis, that would be interesting. If this backplane works I already have a 2U lite-on PSU out of a Dell R5500 I could use. I have hot air rework station and am pretty experienced with soldering if I was to replace the power connector, or a molex crimping tool if I decide just to make a cable adapter.
I'm not concerned about making a power adapter. From what I've read it appears the I2C cable appears to connect to the raid controller for diagnostics and communications like that. So it seems the only unknown thing at this point then is that 3.3V gray wire and if the back plane needs some sort of I2C signal to work. Most of the other connections are for the front I/O. There are 3 SAS ports (A,A1 and B) However A1 is for the flex bay (a small 2 bay backplane on the rear of the server). (the IBM X3650 7979 had a similar feature where the back plane had 6 drives but only one SAS cable out of the backplane). So it appears that the R720XD backplane has a SAS expander built in which is why it has the controller on the actual backplane. Otherwise it’s going to be difficult to rewire the backplane to work with a standard PSU. Ideally you would pick up the R720 motherboard and just use that. It should have multiple SAS connectors on it (how many depend on how many bays - with a 12-bay chassis, it’ll be either 2 or 4 SAS connectors, likely). The backplane will definitely connect to a RAID card or HBA. The more I look into it the less likely it seems but like I said there is little to no documentation online I can find for the actual electrics on this. If it works I was going to keep the chassis and could maybe make it into some sort of DAS box project. Which is a pretty sweet bonus! I was going to see if the backplane would pass through drives to a raid card. Turns out it came with the backplane also. I bought an empty R720 12 bay chassis because the drive trays and spare fans it came with (what I was after) were cheaper than buying all that stuff separately. The part number for the backplane is PGXHP