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A+ Certification SCSI Information

Interface

Connector

Clock

Maximum

Throughput

Length
(single ended)

Length LVD

Length HVD

Devices

SCSI-1

IDC50; Centronics C50

5 MHz (8-bits)

5 MB/s

6 m

NA

25m

8

Fast SCSI (SCSI-2)

IDC50; Centronics C50

10 MHz (8-bits)

10 MB/s

1.5-3 m

NA

25m

8

Fast-Wide SCSI (SCSI-2; SCSI-3 SPI)

2 x 50-pin (SCSI-2);
1 x 68-pin (SCSI-3)

10 MHz (16-bits)

20 MB/s

1.5-3 m

NA

25m

16

Ultra SCSI (SCSI-3 SPI)

IDC50

20 MHz (8-bits)

20 MB/s

1.5-3 m

NA

25m

8

Ultra Wide SCSI (SCSI-3 SPI)

68-pin

20 MHz (16-bits)

40 MB/s

1.5-3 m

NA

25m

16

Ultra2 SCSI (SCSI-3 SPI)

50-pin

40 MHz (8-bits)

40 MB/s

NA

12m

25m

8

Ultra2 Wide SCSI (SCSI-3 SPI)

68-pin; 80-pin (SCA/SCA-2)

40 MHz (16-bits)

80 MB/s

NA

12m

25m

16

Ultra3 SCSI (SCSI-3 SPI)

68-pin; 80-pin (SCA/SCA-2)

40 MHz DDR (16-bits)

160 MB/s

NA

12m

NA

16

Ultra-320 SCSI

68-pin; 80-pin (SCA/SCA-2)

80 MHz DDR (16-bits)

320 MB/s

NA

12m

NA

16

Ultra-640 SCSI

68-pin; 80-pin

160 MHz DDR (16-bits)

640 MB/s

      

16

Table 1: Parallel SCSI Versions, retrieved from http://en.wikipedia.org/wiki/SCSI

 

A+ Certification SCSI Information (Continued)

 

Figure 9: External SCSI Connectors

SCSI comes with a variety of connectors, both internal and external. The table above describes some of the connectors based on which version of SCSI you are using.

Figure 10: Internal SCSI Cable

One of the primary differences between SCSI and ATA adapters is the number of devices one port supports. SCSI supports either 8 or 16 devices, depending on the version you are using, with the SCSI adapter card being one device. Each device has a unique SCSI ID in the chain, from 0-7 or 0-15. The SCSI host adapter is typically SCSI ID 7 – the ID with the highest priority on a narrow (8 device) or wide (16 device) host adapter.

Figure 11: HD68 LVD SCSI Terminator

The other difference between SCSI and other types of drive connections is that the SCSI “chain” of devices has to be terminated on each end. A terminator tells the SCSI host adapter that it’s connection is the last in the chain of SCSI connections. Typical SCSI host adapter cards (or integrated SCSI ports on motherboards) have a built-in terminator. If you are using external SCSI devices, a lot still use a terminator device which plugs into one of the SCSI ports on the device to signal it is the last device in the SCSI chain.