SCSI Solutions White Paper - HP-UX

3
• Single-Ended (SE) – an electrical signal protocol that transmits information through changes
in voltage. Single-ended SCSI uses standard TTL signal-to-ground pairs to transmit
information over the SCSI bus. It is a low cost, low power solution, but the signal quality
degrades rapidly which limits cable length to a maximum of 6 meters.
• High-Voltage Differential (HVD) – an electrical signal protocol that transmits information
through a current loop rather than by changes in voltage, thereby reducing the
susceptibility to electrical interference. As a result, cable lengths can be much longer, up
to a maximum of 25 meters.
• Low-Voltage Differential (LVD) – uses a differential scheme like HVD, but at a lower voltage
level and hence at a lower cost and power level but at a shorter maximum cable length
(12 meters with more than one target and initiator, as shown in Table 1).
HVD signaling is expensive, uses more power, and is electrically incompatible with either SE or
LVD signaling. Mixing LVD/SE devices with HVD devices on the same bus can result in actual
physical damage because of the higher voltages that might be sent to the SE or LVD devices.
LVD signaling is compatible with SE and can be mixed on the same bus, but the maximum
speed and cable lengths reduce significantly when SE devices are used, as shown in Table 1.
Note: Use of a Single-Ended (SE) device on a bus will result in the entire SCSI bus running in SE mode
at a maximum of 40 MB/s.
The following table outlines the various features that have evolved with SCSI over time.
Table 1: Key Features
Bus Speed
Max
Data
Rate
(MB/s)
Bus
Width
(Bits)
Signaling
Method
Max Bus
Length
(meters)
5
Max #
of SCSI
IDs Term
3
Cables
4
SCSI-1
Slow (5 Mhz)
5 8 SE 6 8 P/A/F
8 SE 3 8 A/F
8 HVD 25 8 HVD
16 SE 3 16 A/F
16 HVD
1
25 16 HVD
8 SE 1.5 / 3 8 / 4 A/F
8 HVD 25 4 HVD
16 SE 1.5 / 3 16 / 4 A/F
16 HVD 25 4 HVD
40 8 LVD
2
8 / 2 "A"
80 16 LVD
2
SPI-3
Ultra160 (40 Mhz)*
160 16
†
LVD
SPI-4
Ultra320 (80 Mhz)*
320 16
†
LVD
12 / 25
16 / 2
10
20
20
40
"A"
"P"
"A"
LVD
"P"
"P"
Fast (10 Mhz)
Ultra (20 Mhz)
Ultra2 (40 Mhz)
SCSI Specification
SPI
‡
SPI-2
SCSI-3
SCSI-2
‡
SPI = SCSI Parallel Interface.
1
Also known as Fast Wide Differential (FWD).
2
Only LVD is shown here with Ultra2; HVD was also officially supported for Ultra2 SCSI, but was not
generally used.
3
Termination: P/A/F = Passive/Active/Forced-perfect, HVD=HVD termination, LVD=LVD termination
4
Cables: "A" cable (50 pin), "P" (68 pin)
5
Max bus length: 1.5 / 3 = max of 3 meters if no more than 4 SCSI IDs are used, otherwise 1.5 meters
12 / 25 = max of 25m if no more than 2 SCSI IDs (point-to-point), otherwise 12 meters
†
Ultra160/320 buses are wide-only.
* Ultra160 and Ultra320 use double-transition (DT) clocking – sampling data on both the rising and
falling edges of the clock – to double the MB/s throughput per Mhz.