SCSI II uses Active Termination Methods
The SCSI-2 standard defined the electrical, physical and protocol layers for a parallel bus which operates using either 8 bits or 16 bits. SCSI-II originated in 1994 as X3.131-1994, and was superseded by SCSI-III in 1995. The SCSI-2 term stands for Small Computer Systems Interface [- Revision 2] and is obsolete, SCSI-2 should not be used for new designs. SCSI-2 may not always run faster then SCSI-1 due to optional components added to the SCSI-2 spec. SCSI-2 is used with Personal Computers and computer Peripheral devices and includes command sets for magnetic and optical disks, tapes, printers, processors, CD-ROMs, scanners, medium changers, and communications devices. The description of the current SCSI version, and a comparison of all the SCSI revisions is located on the main SCSI interface page.
SCSI-2 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-2 may be call narrow SCSI [or simply SCSI] when the 8-bit data path is implemented, or Wide SCSI when operating with a 16-bit [or 32-bit] data bus. SCSI is an 8-bit / 16-bit [Parallel] data bus, with all data bits sent at the same time over 8 or 16 data lines. SCSI-2 will operate with a 32-bit data bus using both the 'A' cable and 'B' cable. SCSI II transactions may use either asynchronous and synchronous data transfers across the SCSI interface. SCSI-2 asynchronous transfers would operate up to 1.5 MB/s [Speed depends on slowest device]. SCSI-2 synchronous transfers would operate up to 10 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 20 MB/sec being the best case). SCSI II transfers with 8 bits operate at 10MBps (10MHz clock), using the 50 pin Centronics or 16 bits (Wide-Bus) at 20MBps (10MHz clock) using a 68 pin connector. SCSI-2 asynchronous transfers using Start and Stop bits and synchronous transfers using system timing (Hand-Shaking). The SCSI-2 data bus also used one parity bit [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 the EIA-485 interface standard. SCSI-2 defines a Jitter budget of 44nS.
The SCSI II bus uses the "A" cable to interconnect between devices and
is primarily intended for applications within a cabinet. The 'A' cable may
also used with the SCSI-1, or SCSI-3 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 below. The
differential pinout for the A Cable is shown on the Differential A Cable page. An
optional 'B' cable may also be used with SCSI-2. The 'B' cable uses a
68-pin connector. A 68-conductor flat cable or 34-signal twisted-pair cable
shall be used for the 'B' cable. The 'B' cable has a maximum length of up
to 25 meters using differential drivers and receivers. The A cable
has 50 conductors and provides an 8 bit data bus. The 'A' cable impedance is
between 90 and 132 ohms [140W maximum]. 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 bus page. Or refer to the main Cable manufacturers page, and
Cable Assembly
manufacturers
SCSI-2 defines two non-shielded 50-Pin [25-pins x 2 rows] connector
alternatives for the A cable and one non-shielded 68-Pin [34-pins x 2 rows]
connector is specified for the B cable. The 'A' cable used a Centronics 50
pin connector, with 0.10 inch spacing to allow for a 0.05 inch ribbon
cable. Connectors used Internal to a SCSI device used 50-pin IDC headers.
For Connector manufacturers refer to the main SCSI interface 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 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 28AWG. The 68-conductor cable may
be either a flat cable or twisted-pair [for Wide-SCSI] cable.
| Pin # | Pin Function | Pin # | Pin Function |
| 1 | Ground | 2 | Ground |
| 3 | Ground | 4 | Data bit 8 |
| 5 | Ground | 6 | Data bit 9 |
| 7 | Ground | 8 | Data bit 10 |
| 9 | Ground | 10 | Data bit 11 |
| 11 | Ground | 12 | Data bit 12 |
| 13 | Ground | 14 | Data bit 13 |
| 15 | Ground | 16 | Data bit 14 |
| 17 | Ground | 18 | Data bit 15 |
| 19 | Ground | 20 | Parity bit P1 |
| 21 | Ground | 22 | -ACKB |
| 23 | Ground | 24 | Ground |
| 25 | Ground | 26 | -REQB |
| 27 | Ground | 28 | Data bit 16 |
| 29 | Ground | 30 | Data bit 17 |
| 31 | Ground | 32 | Data bit 18 |
| 33 | TERMPWRB | 34 | TERMPWRB |
| 35 | TERMPWRB | 36 | TERMPWRB |
| 37 | Ground | 38 | Data bit 19 |
| 39 | Ground | 40 | Data bit 20 |
| 41 | Ground | 42 | Data bit 21 |
| 43 | Ground | 44 | Data bit 22 |
| 45 | Ground | 46 | Data bit 23 |
| 47 | Ground | 48 | Parity bit P2 |
| 49 | Ground | 50 | Data bit 24 |
| 51 | Ground | 52 | Data bit 25 |
| 53 | Ground | 54 | Data bit 26 |
| 55 | Ground | 56 | Data bit 27 |
| 57 | Ground | 58 | Data bit 28 |
| 59 | Ground | 60 | Data bit 29 |
| 61 | Ground | 62 | Data bit 30 |
| 63 | Ground | 64 | Data bit 31 |
| 65 | Ground | 66 | Parity bit P3 |
| 67 | Ground | 68 | Ground |
The pin out for the 'A' data cable is listed on the SCSI Bus 'A' connector pinout page. The
"A" cable is used with SCSI-1, SCSI-2, or SCSI-3 to provide "FAST" SCSI
with an 8 bit [Parallel] data bus.
With SCSI-2 the "A" may be used alone or with the "B" cable to provide
"WIDE" SCSI allowing a 16 or 32 bit data bus.
The "B" Single-Ended cable pin out is shown above.
On SCSI-3, the "A" cable may be used for an 8 bit data bus [pinout link
above]. However the new SCSI "P" cable 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 pinout is listed
on the SCSI Bus 'Q' Single-Ended
connector or SCSI Bus 'Q' Differential
connector page. The pin-out for the 'P' cable is listed on the
SCSI Bus 'P' 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.
Electronic Design Key words: SCSI II Physical Interface, SCSI-2,
Pinout, Bus, Small Computer Systems Interface, SCSI Pinouts,
Signal Ended
Pinout, Differential Ended Pinout, Pin Out, B Connector, Signal Names,
row, Pin Descriptions, Peripheral Computer
HardDrive Data Bus, Parallel
data bus, Description, Parallel SCSI, Direct Attached Storage [DAS],
Networked Attached Storage [NAS], Storage Area Network [SAN], Out-dated.
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