C.1.11 SCSI

The system unit implements a small computer system interface (SCSI) Fast-20 (UltraSCSI) parallel interface bus. The UltraSCSI provides the following:

• Efficient peer-to-peer I/O bus devices

• Mechanical, electrical, and timing specification definition that support transfer rates of 20 or 40 Mbytes per second (corresponding to the data path width of an 8-bit, or 16-bit bus, respectively)

• Peak bandwidth of 40 Mbytes per second (with implemented 16-bit bus width)

The internal SCSI bus is terminated at each end. One set of terminators is located close to the CD-ROM drive connector on the CD-ROM SCSI card. A second set of terminators is located close to the 68-pin external SCSI connector.

C.1.11.1 Host Adapter

The host adapter is a Symbios Logic PCI-SCSI I/O processor IC. The host adapter and all target devices comply with the Fast-20 single-ended drivers and receivers characteristics. The electrical characteristics of the output buffers include:

• Vol (output low) equals 0 to 0.5 VDC with Iol at 48 mA (signal asserted)

• Voh (out high) equals 2.5 to 3.7 VDC (signal negated)

• trise (rising slew rate) equals 520 mV per nanosecond maximum (0.7 to 2.3 VDC)

• tfall (falling slew rate) equals 520 mV per nanosecond maximum(2.3 to 0.7 VDC)

The Fast-20 electrical characteristics for the host adapter and target device include:

• Vil (input low) equals 1.0 VDC maximum (signal true)

• Vih (input high) equals 1.9 VDC minimum (signal false)

• Iil (input low current) equals +/- 20 mA at Vi equals 0.5 VDC

• Iih (input high current) equals +/- 20 mA at Vi equals 2.7 VDC

• Minimum input hysteresis equals 0.3 VDC

C.1.11.2 Supported Target Devices

The SCSI subsystem supports a maximum of four internal devices, including the host adapter. The CD-ROM drive is a narrow device. A unipack with one drive or a six-pack, accommodating six drives, can be used as external devices.

C.1.11.3 External Cables

External UltraSCSI-compliant SCSI cables have an impedance of 90 ohm (+/- 6 ohm) and are required for UltraSCSI interface. Sun's implementation of UltraSCSI requires that the total SCSI bus length be limited to no more than approximately 20 feet (6 meters) with up to 12 Sun compensated devices. Due to the considerably short bus length, an approximately 32-inch (0.8-meter) UltraSCSI-compliant external cable is supported (part number 530-2883) in addition to an approximate 6.5-foot (2-meter) UltraSCSI-compliant external cable (part number 530-2884).

C.1.11.4 Internal SCSI Subassembly

The internal SCSI subassembly consists of two cable assemblies and two SCSI cards. The SCSI subassembly is attached to the main logic board using an insulation displacement connector (IDC) receptacle attached to a 80-conductor cable. In addition to the SCSI signals, the 80-conductor cable carries system LED signals to the SCSI backplane card. The IDC receptacle mates with a right-angle plug that is mounted on the main logic board.

The 80-conductor cable attaches on the other end to the SCSI backplane card with another IDC connector. The SCSI backplane card incorporates two SCA-2 connectors for mounting the hard drives, a four-circuit power connector to supply 5 VDC and 12 VDC power to the hard drives, and a green, right-angle LED.

A 68-conductor cable exits the SCSI backplane card, carrying 27 SCSI signals and the Termpower to the internal CD-ROM drive (or tape drive). The SCSI backplane card houses the CD-ROM drive connector and one SCSI bus terminator. The Termpower is routed through the SCSI subassembly to connect to the terminator on the SCSI backplane card in support of the multi-host configuration. The following figure shows the functionality of the internal SCSi assembly.

C.1.11.5 SCSI ID Selection

The main logic board host adapter is assigned the SCSI identification of 7 for both ports. The two internal drives attached to the SCA-2 connectors have a SCSI identification of 0 and 1, while the CD-ROM has an identification of 6.