The I/O architecture of the 20xx series KL machines is based on a DEC bus design called the Massbus. While many attributed the success of the PDP-11 to DEC's decision to make the PDP-11 Unibus an open architecture, DEC reverted to prior philosophy with the KL, making Massbus both unique and proprietary. Consequently, there were no aftermarket peripheral manufacturers who made devices for the Massbus, and DEC chose to price their own Massbus devices, notably the RP06 disk drive, at a substantial premium above comparable IBM-compatible devices. CompuServe for one, designed its own alternative disk controller that could operate on the Massbus, but connect to IBM style 3330 disk subsystems.