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| The SCSI [parallel] bus width is either 8 bits or 16 bits, termed Wide bus.
Also the bus may be either Single ended or Differential; however the two are
mutually exclusive SCSI is a chained parallel bus, cables start at the Host
and run from device to device in a chain. SCSI may be used for asynchronous
and synchronous transfers; Asynchronous transfers using Start and Stop bits
and synchronous transfers using system timing (Hand-Shaking). The data bus also
carries one parity bit. Serial SCSI [SAS] "Serial Attached SCSI" description
my be found on the Serial SCSI page. Serial SCSI uses Serial ATA [SATA]
as the physical layer. The rest of this page describes the different versions
of parallel SCSI.
SCSI-1 Which defined the physical and protocol layers over an 8 bit asynchronous
Single-ended (unbalanced) interface using a 50 pin Centronics connector. Both
the data and commands are transferred asynchronous at 5MBps (5MHz). A total
of 7 devices may be connected on the bus. This interface is out-dated.
SCSI-2 [FAST-SCSI] Which defined the physical and protocol layers. The
bus used asynchronous commands and synchronous data. Transfers with 8 bits at
10MBps (10MHz clock), using the 50 pin Centronics or 16 bits (Wide-Bus) at 20MBps
(10MHz clock) using a 68 pin connector. The number of devices on the bus increased
to 15. Another form; Differential (Balanced) twisted-pair SCSI also used. SCSI-2
may not always run faster then SCSI-1 due to optional components added to the
SCSI-2 spec.
SCSI-3 specification defines the mechanical, electrical and protocol
layers of the interface. Data transfers of 8 bits at 20MBps over a 50 pin connector,
and 16 bits at 40MBps over a 68 pin connector. The number of devices on the
bus increased to 16 (for Fast-10), Fast-20 allows 8 devices maximum, with a
number of other combinations. Differential (Balanced) twisted-pair SCSI "SPI"
(SCSI Parallel Interface) also used; in addition Serial SCSI via P1394 (Firewire),
and SSA (Serial Storage Architecture). Also added the "P" cable.
There are a number of different transfer rates, depending on the transceiver
used:
SE will operate at Async, Fast-5, Fast-10, Fast-20: all modes use Single
Transition (ST)
MSE will operate at Async, Fast-5, Fast-10, Fast-20: all modes use Single
Transition (ST)
LVD will operate at Async, Fast-5, Fast-10, Fast-20, Fast-40 (using ST),
or Fast-10 to Fast-80 in Double Transition (DT).
HVD The High Voltage Differential [HVD] option, and the 32 bit wide bus
option are obsolete in SCSI-3.
... SCSI FAST-20 runs at: 20MBps [8-bits] or 40MBps [16 bits]
... SCSI FAST-40 runs at: 40MBps [8-bits] or 80MBps [16 bits]
... SCSI FAST-80 runs at: 80MBps [8-bits] or 160MBps [16 bits]
... Ultra SCSI [Fast-20] runs at either 20MBps (8 bits) or 40MBps using
wide bus (16 bits). Normally 10 meters maximum for 4 devices on the bus, 1.5
meters with 5 devices.
... Ultra2 SCSI [Fast-40] runs at either 40MBps (8 bits) or 80MBps using
wide bus (16 bits). Normally cable lengths up to 12 meters (39,3 ft).
... Ultra3 SCSI (Ultra160) runs at either 80MBps or 160MBps using wide
bus. Includes CRC.
SCSI-4 (Ultra320). runs at either 320MBps using wide bus.
SCSI-5 uses the Very High Density Cable Interconnect (VHDCI) 68 pin,
0.8 mm connector. How ever I don"t think they use "-#" any longer to designate
SCSI versions, but I see it used on many web sites.
15 June 2004 there were 18 pinouts of SCSI connectors listed at pinouts.ru.
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