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SCSI Basics

SCSI - Small Computer System Interface

SCSI is an industry standard system interface, which allows multiple peripheral devices to be installed on the system. The number of devices that can be installed depends on the type of SCSI Host adapter that you have installed in your system. Most SCSI systems have an older "Narrow" 8-bit SCSI host adapter, which have 8-bit data paths and allow for up to 7 devices to be installed on the same channel. Newer "Wide" Host adapters have 16-bit wide data paths and provide support for up to 15 devices.

Single ended SCSI

Single ended SCSI is a term applied to standard SCSI devices that are fully compliant with standard SCSI specifications. This includes SCSI, SCSI-2, Ultra SCSI, Fast Wide SCSI-2, and Ultra Wide SCSI

Narrow SCSI 2

This version of SCSI passes data through the SCSI bus along an 8-bit data path in 8 bit "computer words" and provides support for up to 7 devices and the Host adapter. This device uses a 50-pin SCSI connector, which connects via cable to the host adapter. Data transfer rates of up to 10MB/sec can be achieved with cable lengths of 3 m or less. Note: Devices should be spaced evenly on the cable, but no closer than .3 meters apart to prevent impedance mismatching.

Ultra SCSI 2

Ultra SCSI is an enhancement of the SCSI 2 specification, which when properly set up allows for up to 20MB/sec transfer rates. Cable length requirements are very important or slower transfer rates will result. Cable lengths: With 4 or less devices installed maximum cable length of 3m or less; With more than four devices, cable length would be 1.5m or less. Note: Devices should be spaced evenly on the cable, but no closer than .3 meters apart to prevent impedance mismatching.

Fast Wide SCSI

This version of SCSI passes data through the SCSI bus along a 16-bit data path in 16 bit "computer words" and provides support for up to 15 devices and the Host adapter. This device uses a 68-pin SCSI connector, which connects via cable to the host adapter. Data transfer rates of up to 20MB/sec can be achieved with cable lengths of 3 m or less. Note: Devices should be spaced evenly on the cable, but no closer than .3 meters apart to prevent impedance mismatching.

Ultra Wide SCSI

Ultra Wide SCSI is an enhancement to the Fast Wide SCSI 2 specification, which when properly set up allows for up to 40MB/sec burst data transfer rates. Cable length requirements are very important or slower transfer rates will result. Cable lengths: With 4 or less devices installed, maximum cable length of 3m or less; With more than four devices, maximum cable length of 1.5m or less. Note: Devices should be spaced evenly on the cable, but no closer than .3 meters apart to prevent impedance mismatching.

Ultra2 SCSI

Ultra2 SCSI or LVD uses a new technology called Low Voltage Differential which allows for longer cable lengths (12 meters) with up to 80Mbytes/sec burst data transfer rate. Drives and adapters utilizing this technology are backward compatible with older SCSI technology, however the capabilities will revert to the older SCSI performance and Cable length capabilities. Most of the devices that utilize this interface will utilize the 16 bit Wide SCSI format and will incorporate a high density 68 pin interface.

Differential

Differential SCSI is a special "version" of SCSI which allows for cable lengths of up to 25 meters. To utilize this technology a special "Differential" host adapter is required and is not compatible with standard SCSI devices. Differential SCSI technology has a dual data path built in and compares one line to its counterpart to interpret the data being read or written. All differential devices require external line termination as Differential SCSI specifications does not call for on board termination.

SCSI ID

This is the method by which individual devices are identified on the SCSI Bus. Narrow SCSI has SCSI IDs of 0 - 6 available for use where Wide SCSI has SCSI IDs of 0 - 6 and 8 - F (15) available. The SCSI Host adapter typically always uses SCSI ID 7. This ID has the highest priority for arbitration and therefore the Host adapter will always take priority. You probably noticed that the last number available appears to be one less than the number of IDs available, actually SCSI ID numbering uses 0 as the first device ID and counts up from there. Keep in mind when mixing Narrow and Wide SCSI on the same bus, ensure the first and last devices installed are Wide devices and that any narrow devices are occupying IDs 0 - 6 as they do not support IDs higher than 7.

SCSI ID Jumper Settings
SCSI ID 0 1 2
0 Default X X X
1 O X X
2 X O X
3 O O X
4 X X O
5 O X O
6 X O O
7 Reserved O O O

Termination

The first device and the last device in a SCSI chain require termination. Termination allows the SCSI Host adapter "see" the first and last device, which in turn will then allow the adapter to recognize the SCSI IDs of all the installed devices. Termination also helps prevent signal reflections from entering the SCSI bus. When all devices to be installed are internal, the SCSI Host adapter will be counted as an installed SCSI device. If you are installing an external device you may have to disable the SCSI Host adapter termination and terminate the last external device. (Note: most SCSI peripheral devices ship with termination in the "ON" position. If the device that you are installing is in the middle of the chain, you will need to disable termination.) When mixing narrow and wide SCSI devices on the same bus, the Wide devices must be the terminated devices otherwise the Host adapter will not see the Wide devices that are installed in the system as only half of the cable would be terminated. When terminating external SCSI 2 devices active termination may be required. Ultra2 devices require active terminators for LVD devices. Differential drives require external termination as they do not have onboard termination.

Bus mastering

Is a method by which data is moved between a peripheral device and system memory without interaction from the "Host system CPU". This allows a PCI Card (if capable) to transfer data at speeds of up to 132MB/sec (for PCI controllers).

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