Each parallel port, if sending data from the memory to the I/O subsystem, has a maximum bandwidth of 88 MB/s. If the parallel port is sending data from the I/O subsystem to the memory, it has a maximum bandwidth of 135 MB/s. Plug-in units (PIUs) are modular enclosures that house options. The DEC 7000/10000 AXP supported PIUs implementing the Futurebus+ Profile B and XMI, PIUs housing SCSI and DSSI drives, and a PIU housing batteries. Futurebus expansion capability was provided by the DWLAA PIU. It contains a card cage with nine usable slots and the DWLAA adapter, which implements the bus and interfaces it to the I/O controller on the I/O port module. The Futurebus PIU can be installed in PIU quadrants 2 and 4 and. Futurebus capability was optional and up to three can be installed in a system, with a maximum of two per cabinet. Futurebus capability required the system to have a XMI bus. XMI expansion capability was provided by the DWLMA PIU. It contains a card cage with 14 slots, with 12 of those slots usable by adapters. Two slots are unusable as they are reserved by the control and interface modules, the DWLMA module, which implements the XMI bus and interfaces it to the I/O controller on the I/O port module, and clock and arbitration module, which provides the XMI clock. The XMI PIU requires two PIU quadrants as they are twice as deep as the other PIUs, and can be installed only in the bottom left or right PIU quadrants. One to four XMI PIUs are supported in a system, with a maximum of two per any type of cabinet. SCSI devices are housed in the BA655 PIU, which contains two modular expansion shelves placed side by side. The left shelf can house seven drives and the right shelf can house two drives. The system cabinet can have up to two SCSI PIUs and expander cabinet up to four. DSSI devices are housed in the BA654 PIU which contains three Storage Array Building Blocks (SBBs), each housing two drives. The system cabinet can have up to two DSSI PIUs and the expander cabinet up to six. The SCSI and DSSI PIUs did not contain hardware that provides the SCSI or DSSI bus to which the drives connect to. Instead, they are connected to a KZMSA-AB adapter for SCSI, or a KFMSB adapter for DSSI, which is installed in the XMI PIU.