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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 .
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 Ultra160 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 Specification requires that SCSI devices „running“ in differential mode automati-
cally 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 Ultra160 SCSI. LVDS is treated completely independently 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
explicitly designed for LVDS. Very often all GND wires are put together in the connector.
These kinds of cables must not be used for LVDS.