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Hardware configurations The 210 MB, 445 MB, 875 MB, or 1.8 GB drive is easily identified by its small size, with the size of the disk only 3 1/2 inches. An adapter plate is used to adapt the drive directly to the mounting holes on the tray assembly that are normally used for the WREN III drive. No shock-mounts are used. The Winchester Drive Modules and the electronics module are always mounted one above the other as illustrated in the Figure 2-5. A single SCSI bus ribbon cable from the SPC I/O circuit board in the I/O cage at the rear of the electronics module enters a slot at the rear of the lower Winchester Drive Module, routes over the slide tray assembly, and connects to the drive(s) on the tray assembly. The cable then exits the module and enters the upper Winchester Drive Module where it connects to the drive(s) on the tray assembly. In the standard nonredundant configurations, the SCSI (Small Computer Systems Interface—often nicknamed “scuzzy”) bus cable interconnects to one or two drives. The end drive on the interface has termination resistor modules installed on it. The SCSI bus ribbon cable is a group of transmission lines that must be terminated at each end, on the SPC I/O circuit board, and at the drive end of the SCSI bus cable. The actual routing of the SCSI bus ribbon cable is illustrated elsewhere in this manual. If the drives are installed in a nonredundant History Module, three SCSI bus termination resistor modules are located on the last drive on the cable. When drives are installed in a redundant History Module, a bus terminator card provides the termination at the physical end of the SCSI bus ribbon cable, separate from the last drive on the bus. The bus terminator card is fastened to the top of the upper Winchester Drive Module chassis. This arrangement permits a failed drive to be disconnected while power is applied and not disturb its redundant partner on the same SCSI bus.