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B.4 SCSI - Basics

Whoever has been involved with the subject of SCSI will have noticed that the "Small Com-

puter System Interface" is an extremely interesting technology, which has become widely

accepted in comparison to other interface models and has constantly adapted to the needs

of customers. This builds up confidence and (investment) security. Last but not least, SCSI

has remained the de facto I/O interface for smaller to mid-sized mass storage systems.

The innovating factors are and were; downward compatibility (connector, protocols), the

bandwidth (10MB/sec., 20MB/sec., 40MB/sec., 80MB/sec., 160MB/sec.) and the cable length

(3 Meters to 12 Meters). The following table gives you an overview on the various develop-

ment steps:

"Ultra SCSI"

(FAST-20)

8 Bit 16 Bit

Narrow Wide

"Ultra2 SCSI"

(FAST-40)

8 Bit 16 Bit

Narrow Wide

"Ultra160 SCSI"

(FAST-80)

16 Bit

Wide

Max. Transfer-

rate [MB/sec.]

20 40 40 80 160

Max. Number

Devices

7 7 (15) 7 15 15

Bus-Type

SE SE LVDS LVDS LVDS

Cable Length

[Meter]

1.5 1.5 12 12 12

Connector

50 pin 68 pin 50 pin 68 pin 68 pin

We differentiate between Single Ended (SE) and differential buses .

(Note: The above listed table does not show differential Bus-Types for Fast-SCSI.)

Single ended buses have the disadvantage that they are highly susceptible to distortion

and therefore have to be shorter than differential buses. In SE environments, logical infor-

mation is represented in terms of voltage levels on a wire. Therefore, small distortions can

change the voltage level significantly which results in transmission errors. The current lev-

els on the SE bus are reasonably higher than on a differential bus.

If the differential data transfer method is used, two wires with separate voltage levels are

used per signal. Only the voltage difference between the wires represents the logical infor-

mation. Because distortion has the same effect on both wires, it is automatically deleted

during calculation of the difference. In this way, much longer cables with higher transfer

rates are possible than in SE systems.

Differential SCSI has existed for a long time. However, it has always played a somewhat ex-

otic role as the SCSI Controller and also the disks had to be differential and could only be

operated in this way. Single Ended devices could and were not allowed to be used.

The Wide/Ultra2 SCSI and Ultra 160 SCSI ICP Controllers are able to work with one and the

same connector in SE-Mode as well as in differential mode without the need for additional

hardware. However, a SCSI Bus can be operated either in SE or differential mode, but not

both at the same time. The SCSI Spec requires that SCSI devices „running“ in differential

mode automatically switch to SE mode if they recognise that a pure SE device is connected

to the cable.

The so-called LVDS - Low Voltage Differential Signaling - was chosen as the differential

transfer technology for Ultra2 SCSI and Ultra 160 SCSI. LVDS is treated completely inde-

pendently from SCSI and is defined via an ANSI and IEEE Norm. At present LVDS is also

used for transferring video data etc.

In LVDS-Mode, an SE signal wire and its associated SE ground wire (GND) build a differen-

tial pair. Therefore, please take care when choosing an external round cable. It has to be

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