Chatted Dell sales support, they couldn't make any headway using the 311-2236 number.  Chatted tech support and they cross-referenced that number to X8157.  Not too sure that is correct.  That is a PE backplane riser itself.  Besides the fact that it looks nothing like the SCSI Backplane Daughter Card show in the documentation link below, my configuration already shows that I have an X8157.  Time to pry the cover off and peek inside.  
I have a PE2850 with 6 drive bays and a PERC 4 card.  Bays are currently occupied by 4x 147GB and 2x 38GB SCSI drives.  I want to run my OS on the 38GB SCSI drives in a RAID 1 configuration on one channel, and put the other 4 in a RAID 5 configuration on the other channel.  Without a split backplane card, of course, they all show up on Channel 0.  I've seen reference in various threads to a PN 311-2236 but can't find any reference to that part otherwise on dell.com. Is 311-2236 still a valid PN?  Has it been superceded by another number, or is there a different number I should be using? I need to split the backplane in a 2+4 configuration.  Can the 311-2236 do that, or does it just do a 2x3? Thanks! Chris
Split Backplane Daughter Card (311-2236?)