SCSI Concepts and Configuration

2 SCSI Transmission Methods SCSI Buses and Termination SCSI Data Path Sizes and Bus Speed SCSI Cables, Connectors and Cable Length SCSI Bus Adapters SCSI Devices and Device IDs Lesson Topics

3 SCSI Transmission Methods Single Ended One data lead and one ground lead Prone to noise problems and strict adherence to maximum cable lengths Inexpensive Differential Neither lead is at ground potential Has immunity to noise, allowing longer cables and faster bus speeds More expensive than Single-Ended transmission Only one type of transmission method is used on a single SCSI bus segment

4 SCSI Transmission Methods Single-ended devices connect to single-ended buses Differential devices connect to differential buses Use signal converters (e.g. DWZZA) to mix single- ended and differential devices on a shared bus

5 Possible SCSI Bus Connections Differential device Differential device Differential device Single-ended device Single-ended device Single-ended device DWZZA 25 meters maximum Differential bus 25 meters maximum3 m Single-ended bus Differential bus 3 m25 m3 m

6 Possible SCSI Bus Connections Shared SCSI Bus refers to the total interconnection between the cluster servers The Shared SCSI Bus usually consists of multiple bus segments, each electrically complete Each bus segment must be correctly terminated

7 SCSI Bus Termination Remove internal termination from devices not at the end of the SCSI bus To disconnect a component and not disrupt the bus use Y-cables or Tri-link connectors to implement termination

8 SCSI Bus Termination Example Server with Differential SCSI adapter and NO internal termination Differential bus Terminator Server with Differential SCSI adapter and NO internal termination DWZZA Storage enclosure with single ended SCSI connection Single-ended bus Tri-link connector with terminator 2 internal terminators - one on each side of DWZZA Y Cable

9 SCSI Data Path Sizes Narrow 8-bit data path usually single-ended devices, although narrow differential devices do exist Wide 16-bit data path Either single-ended or differential Both narrow and wide devices can exist on a single shared SCSI bus

10 SCSI Bus Speed SCSI devices can operate in multiple speeds: Slow 5 MB/sec 5 Narrow only Fast 10 MB/sec Wide or narrow Ultra 20 MB/sec Wide or narrow Fast 40, Fast 80, etc.

11 SCSI Bus Speed Wide SCSI devices can operate up to 2 times the data rate for narrow devices Devices on SCSI bus can be set to different speeds (can mix Slow and Fast devices on same SCSI bus) Using devices set to Fast speed decreases maximum cable length of a single-ended bus segment

12 SCSI Cables and Connectors Type of cable depends on width of data path and transmission method of bus Data path capacity is reflected by its connector: 50-pin connectors always have a narrow data path, and most are single ended 68-pin connectors are narrow (usually single-ended) or wide (usually differential)

13 SCSI Cable Length SCSI bus segments must adhere to strict cable length limit Length is determined by transmission method and bus speed: Type of BusBus SpeedMax. Length Single-endedSlow6 meters Single-endedFast3 meters Single-endedUltra1.5 meters DifferentialFast or Slow25 meters

14 SCSI Cable Length Total cable length for a bus segment is calculated from one terminated end to the other Total cable length must include internal cables Internal cable lengths vary, depending on the device

15 SCSI Bus Adapters Unused port of a dual-port adapter must be terminated If using a dual-port adapter, use the external port for the shared bus SCSI bus reset must be disabled for adapters on the shared bus

16 Number of SCSI Devices 8-bit SCSI bus can have up to 8 devices, with SCSI IDs in the range of 0 to 7 16-bit SCSI bus can have up to 16 devices, with SCSI IDs in the range of 0 to 15, but cannot mix more than 8 devices if any device on the bus is narrow

17 SCSI IDs Each device on a SCSI bus must have a unique SCSI ID Signal converters are not considered devices and are not assigned SCSI IDs IDs 6 and 7 have the highest priority and should be used for the adapters on the servers which connect to the shared bus SCSI ID priority list is: 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8