SCSI II uses Active Termination Methods
The SCSI-3 defined the electrical, physical and protocol layers
for a parallel bus which operates using either 8 bits or 16 bits.
SCSI-III originated in 1995 as X3.131-1995, and was superseded by SCSI-IV
in 2002. The SCSI-3 term stands for Small Computer Systems Interface [-
Revision 3] and is obsolete, SCSI-3 should not be used for new designs.
SCSI-3 includes command sets for magnetic and optical disks, tapes,
printers, processors, CD-ROMs, scanners, medium changers, and
communications devices. In addition to the SCSI-3 Parallel Interface
[SPI], an enhancement called SCSI-3 Fast-20 was released.
The description of the current SCSI version, and a comparison of all the SCSI revisions
is located on the main SCSI interface bus
page.
This reference applies to the older parallel version of SCSI, not the newer Serial SCSI.
In other words this version of SCSI is out-dated, and in fact any specification released in 1995 is dated.
SCSI-3 may be either a Single-Ended data bus transmitting data referenced
to ground [unbalanced], or a balanced differential bus. Single-ended and
differential devices are electrically incompatible and can not be mixed
on the same physical bus [refer to the electrical description]. SCSI is
an 8-bit / 16-bit / 32-bit [Parallel] data bus, with all data bits sent
at the same time over 8, 16, or 32-bit data lines. SCSI III transactions
may use either asynchronous and synchronous data transfers across the
SCSI interface. SCSI-3 asynchronous transfers would operate up to 1.5
MB/s [Speed depends on slowest device]. SCSI-3 synchronous transfers
would operate up to 40 MB/s. SCSI commands are sent at the asynchronous
rate. SCSI data is transferred either at the asynchronous rate (worst
case) or a negotiated synchronous rate (with 40 MB/sec being the best
case). The SCSI-3 Parallel Interface [SPI] defaults to the 8-bit
asynchronous rate. SPI may use Slow or Fast transfers operating with a
maximum transfer speed of 10MBps [8-bits], 20MBps [16-bits] or 40MBps
[32-bit transfers]. Two cables are required for 32-bit transfers using
both the 'P' and 'Q' cables. Fast-20 [SCSI-3] allows operating with a
maximum transfer speed of 20MBps [8-bits], 40MBps [16-bits] or 80MBps
[32-bit transfers]. Fast-20 only operates in a synchronous mode.
SCSI-3 asynchronous transfers use Start and Stop bits and synchronous
transfers using system timing (Hand-Shaking). The SCSI-3 data bus uses
one parity bit [DBP] for an 8-bit wide data bus; DB[7-0]. The SCSI-3 data
bus also used two parity bits [DB0, DBP1] for a 16-bit wide data bus;
DB[7-0], DB[15-8]. SCSI-3 data bus also used four parity bits [DB0, DBP1,
DBP2, DBP3] for a 32-bit wide data bus; DB[7-0], DB[15-8], DB[23-16], and
DB[32-24]. Parity bits were not required in SCSI-1.
The single-ended lines use IC's having standard TTL logic levels, VOL = 0v to 0.5v [48mA sink], and VOH = 2.5v to 5.25v. IC input drive is VIL = 0v to 0.8v, and VIH = 2.0v to 5.25v. The differential lines use VOL = 1.7v and VOH = 2.7v [with sink/source of 55mA]. The differential characteristics conform to EIA-485 interface standard. SCSI-2 defines a Jitter budget of 44nS.
The SCSI III bus uses the 50-Conductor "A" cable [defined in SCSI-2] to
provide 8-bit data and as an interconnect between devices and is
primarily intended for applications within a cabinet. The 'A' cable may
also used with the SCSI-1, or SCSI-2 buses to provide "FAST" SCSI. The
Single-ended 'A' cable interface used a 50 pin Centronics connector, with
a single-ended driver and receiver configuration which defined the
maximum cable length of 6 meters. The cable used may be either a
50-conductor flat cable or 25-signal twisted-pair cable.
The pin out for the 'A' Single-ended data cable is listed on the Single-Ended A Cable. The
differential pinout for the A Cable is shown on the Differential A Cable
page. SCSI-3 also defines a 'P' cable to allow for 16-bit transfers. The
'P' cable uses a 68-pin conductor cable, Single-Ended A Cable. A
68-conductor flat cable or 34-signal twisted-pair cable shall be used for
the 'P' cable. The 'P' cable has a maximum length of up to 25 meters
using differential drivers and receivers. The pin out for the 'P'
Single-ended data cable is listed on the Single-Ended Q Cable.
The differential pinout for the Q Cable is shown on the Differential Q Cable
page.
The A cable has 50 conductors and provides an 8 bit data bus. The
'A' cable impedance is
between 72 and 96 ohms [84W nominal]. The
differential cable impedance is
between 115 and 160 ohms [122W nominal]. The
minimum conductor size [for terminator power] is 28 AWG solid or stranded, other
wires may be smaller. The cable used may be either a 50-conductor flat
cable or 25-signal twisted-pair cable.
For wire and Cable Assembly
manufacturers refer to the main SCSI page.
Or refer to the Cable manufacturers page, and
Cable Assembly manufacturers.
Keep in mind that the older SCSI becomes, the fewer companies supplying products remain.
SCSI-3 specifies a two non-shielded 50-Pin [25-pins x 2 rows] connector
alternatives for the A cable for 8-bit transfers, one non-shielded 68-Pin
[34-pins x 2 rows] connector is specified for the 'P' cable to handle
16-bit transfers, and a 68-pin 'Q' cable to allow for 32-bit transfers.
The 'A' cable [defined in SCSI-2] used a Centronics 50 pin connector,
with 0.10 inch spacing to allow for a 0.05 inch ribbon cable. Connectors
implemented Internal to a SCSI device used 50-pin IDC headers. For
Connector manufacturers refer to the main SCSI bus page, or refer to the main Connector manufacturers page.
SCSI is a chained parallel bus, cables start at the Host and run from
device to device in a chain. A total of 16 devices may be connected on
the bus. The two devices at each end of the chain require terminations,
either added external to the devices or provided internal to the devices.
SCSI II devices only use passive terminations [see below].
Terminations reside on both sides of the bus, and define the ends of the
bus. Some SCSI devices have internal terminators. The voltage is normally
provided by the bus line: 'TERMPWR', and also requires a Schottky
diode to handle reverse currents.
Decoupling capacitors (ranging between 2.2uF
and 10uF) should reside on the 'TERMPWR' line at each termination point.
Passive [Resistor] Termination provided
reliable operation in SCSI-1 systems, how ever for systems using SCSI-2
and above require active termination schemes. The primary problem is
double clocking on the Strobe lines, which may occur because of a
reflection. Of course
the passive approach also has a constant resistive path from TERMPWR to
ground, and is not regulated so varies with TERMPWR. SCSI I devices only
used passive terminations. The minimum conductor size for the TERMPWR is
28AWG.
The SCSI B cable uses a shielded 68 conductor connector, using 2 rows of
34 male contacts (0.05" spacing). The pin out for the B cable is shown below. The pinout for
the 68-pin connector differs from the cable pinout provided below. The
signal-ended B cable has a maximum length of 6 meters. The cable impedance is
between 90 and 140 ohms. The minimum conductor size is 30AWG. The 68-conductor cable may
be either a flat cable or twisted-pair [for Wide-SCSI] cable.
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 [Fast-80] (Ultra160) runs at either 80MBps or
160MBps using wide bus. Includes CRC.
SCSI-3 Parallel Interface (SPI) [X3T10/855D]
SCSI-3 Interlocked Protocol (SIP) [X3T10/856D]
SCSI-3 Fiber Channel Protocol (FCP) [X3T10/993D]
SCSI-3 Serial Bus Protocol (SBP) [X3T10/992D]
SCSI-3 Generic Packetized Protocol (GPP) [X3T10/991D]
SCSI-3 Architecture Model (SAM) [X3T10/994D]
SCSI-3 Primary Commands (SPC) [X3T10/995D]
SCSI-3 Block Commands (SBC) [X3T10/996D]
SCSI-3 Stream Commands (SSC) [X3T10/997D]
SCSI-3 Graphic Commands (SGC) [X3T10/998D]
SCSI-3 Medium Changer Commands (SMC) [X3T10/999D]
SCSI-3 Fast-20
On SCSI-3, the "A" cable may be used for an 8 bit data bus. However the new "P" cable [shown above] is used to provide a 16 bit data path (wide SCSI). To allow 32 bits of data the "P" cable is combined with another new cable called "Q" (wide SCSI). The "Q" cable pin-out is listed on the SCSI Bus 'Q' Single-Ended connector or SCSI Bus 'Q' Differential connector page.
Back to the main SCSI Interface Bus page
The SCSI [parallel] bus width is either 8 bits or 16 bits [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.
Engineering Editor Note; The SCSI III interface was released in 1995 and there for some what out-dated.
The SCSI III interface has been replaced both by updates to the parallel SCSI interface, SCSI IV and the serial SCSI interface.
The description of the SCSI III interface is presented here to explain the operation of SCSI III.
But at the same time a mention has to be made in regards to the interface bus being obsolete.
That's not to say that some large number of computers or devices are not using the SCSI III cabling standard.
Just to make note that the interface it self is obsolete and should not be used for new designs.
Navigation: Engineering Home > Interface Buses > Cabled Interface Standards > SCSI Bus Description > SCSI III
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