I am, of course, talking about the Power Macintosh 6400/6500. The last Apple computer (and probably the last computer period) to ever use a backplane mainboard.
The board attatches to a forward bus slot upon which sits the ATA controller for the HD. The SCSI circuits are in the mainboard, however, but the connections are on the bus board.
3 PCI slots are provided by a riser board.
In addition to the 3 PCI slots, the 6400/6500 mobo has a Comm II Slot, one of the last times you'll ever see a custom bus slot on a computer. Comm II is made for modems and ethernet. Unfortunately, you can only have one or the other.
I'd like to get an ethernet card, but everybody's too cheap to make Mac drivers, especially for OS 9.
Oh, and of course, nobody has ever made Mac drivers even for old network cards. Why? Didn't need 'em! Power Macs have had ethernet built-in since the first PCI models, save for a few oddities, like the 5x00/6x00 series, which were consumer-aimed models.
Introduced in February of 1997, the Power Macintosh 6500 represents the last consumer-aimed Power Macintosh, and the last Apple consumer machine before the iMac.