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3.3 Physical Configuration Winchester disk drives The Winchester hard disk drives (one, two, or four can be present) are mounted on two Winchester Drive Trays, resident in the Winchester Disk Assembly that occupies the upper two circuit board slots (slots 4 and 5) of the module as illustrated in Figures 3-2 and 3-3. SCSI bus interconnections Dual Small Computer Systems Interface, (SCSI) bus ribbon cables connect the Smart Peripheral Controller I/O (SPCII/SPC2) circuit board, located at the rear of the Smart Peripheral Controller (SPC) circuit board (slot 2), to the Winchester Drive Interface I/O (WDI I/O/WDI) circuit board, located at the rear of the Winchester Disk Assembly (slot 5). The Winchester Drive Interface (WDI) circuit board, located in the Winchester Disk Assembly (WDA), extends the SCSI bus through two printed flex-circuits to the Winchester disk drive(s) mounted on the Winchester Drive Tray(s) as illustrated in Figure 3-5. The bus “splits” on the WDI I/O circuit board, providing the bus interface to each Winchester Drive Tray. The Smart Peripheral Controller (SPC) circuit board interconnects by the SCSI bus ribbon cable and printed flex-circuit with one or two drives on each tray present. The SCSI bus is terminated by termination modules installed on the last (end) drive. SCSI bus termination When a tray has one Winchester disk drive mounted on it, the drive is mounted in the front position and must have three bus termination modules installed on the drive if it is a 210 megabyte or 445 megabyte drive. The 875 megabyte and 1.8 gigabyte drives do not have bus termination modules. Instead, internal bus terminators are enabled electronically on the drive by jumper block selection. If a second Winchester disk drive is present on the tray, the second drive is mounted in the rear position on the tray without SCSI bus termination modules installed on it as illustrated in Figure 3-5. The WDI circuit board, resident in the Winchester Disk Assembly, has two sets of SCSI bus resistor terminators, one set for each tray. The set of terminators is activated when power is removed from the individual Winchester Drive Tray by a power switch at the front of the tray. This arrangement allows a failed drive on a tray to be removed and replaced without disturbing its redundant partner that is mounted on another tray, interfacing the same SCSI bus.