Appendix 1 Scsi Questions and Answers

LaCie d2 SCSI Hard Drive	Appendix - SCSI Q & A
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8. Appendix 1 – SCSI Questions and Answers

For the new user, SCSI terminology can be a little daunting. The table below was designed to help you get a clearer understanding of what the different SCSI names mean and their performance characteristics.

Before you read the chart, let’s define what we mean by Standard, Protocol and Industry names:

•Standard - The broadest category, featuring general guidelines given by the various industry groups and standards associa- tions. There are three SCSI standards: SCSI-1, SCSI-2 and SCSI-3.

•Protocols – Sub-categories of SCSI standards with defining specific characteristics and features. "Narrow" and "Wide", for example, tell us about the bus width of a particular type of device.

•Industry names – Names used by computer hardware manufacturers to describe SCSI devices to potential buyers.

Standard	Protocols	Industry Names		Bus Width	Signaling	Throughput
SCSI-1	"Regular" SCSI	SCSI-1, SCSI		Narrow (8-bit)	SE/HVD	Up to 5MB/s

SCSI-2	Wide SCSI	Wide SCSI-2		Wide (16-bit)	SE/HVD	to 10MB/s
	Fast SCSI	Fast SCSI-2		Narrow (8-bit)	SE/HVD	to 10MB/s
	Fast Wide SCSI	Fast Wide SCSI-2		Wide (16-bit)	SE/HVD	to 10MB/s
						to 20MB/s

SCSI-3	Ultra SCSI	Fast-20	SCSI	Narrow (8-bit)	SE/HVD	to 20MB/s
	Wide Ultra SCSI	Fast-20	Wide SCSI/Ultra	Wide (16-bit)	SE/HVD	to 40MB/s
		Wide SCSI
	Ultra2 SCSI	Fast-40	SCSI/Narrow	Narrow (8-bit)	LVD	to 40MB/s
		Ultra2 SCSI
	Wide Ultra2 SCSI	Wide Fast-40 SCSI/Ultra2		Wide (16-bit)	LVD	to 80MB/s
		SCSI/Ultra2 Wide SCSI
	Ultra3 SCSI	Fast-80	SCSI	Wide (16-bit)	LVD	to 160MB/s
	Ultra 160 SCSI	Ultra 160/m SCSI		Wide (16-bit)	LVD	to 160MB/s
	Ultra 160+ SCSI			Wide (16-bit)	LVD	to 160MB/s
	Ultra320 SCSI	Ultra4 SCSI/Fast-160 SCSI		Wide (16-bit)	LVD	to 320MB/s

What issues are involved when mixing Narrow SCSI and Wide SCSI devices on the same SCSI chain?

The simplest SCSI configurations use only Narrow devices, or only Wide devices. In this type of configuration, you choose the correct cable, connect all of the devices in a bus topology, and place the appropriate type of termination at both ends of the bus. In this case, termination is relatively straightforward because all of the devices have the same width.

Sometimes, however, you may need to mix Narrow and Wide devices on a single SCSI channel. This is becoming increasingly common as the newest hard drives are Wide only, but many other kinds of devices are still produced for the Narrow interface.