SCSI II Bus Detailed Description and 'B' Connector Pin Out

SCSI-2 General Description

The description of the current SCSI version, and a comparison of all the SCSI revisions is located on the main SCSI interface page.
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 SCSI-II originated in 1994 as X3.131-1994, and was superseded by SCSI-III in 1995. The SCSI-2 name stands for Small Computer Systems Interface [- Revision 2] and is obsolete, SCSI-2 should not be used for new designs.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.

SCSI-2 Data Transmission Description

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 may 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].

SCSI-2 Electrical Description

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. The SCSI-2 defines a Jitter budget of 44nS.

SCSI-2 Cable Description

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 Connector Description

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 or line. 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].

SCSI Termination Graphic
SCSI II uses Active Termination Methods

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.

SCSI-2 Connector Pinout

SCSI B Cable PinOut
Pin # Pin Function Pin # Pin Function
1 Ground 2 Ground
3 4 Data bit 8
5 6 Data bit 9
7 8 Data bit 10
9 10 Data bit 11
11 12 Data bit 12
13 14 Data bit 13
15 16 Data bit 14
17 18 Data bit 15
19 20 Parity bit P1
21 22 -ACKB
23 24 Ground
25 26 -REQB
27 28 Data bit 16
29 30 Data bit 17
31 32 Data bit 18
37 Ground 38 Data bit 19
39 40 Data bit 20
41 42 Data bit 21
43 44 Data bit 22
45 46 Data bit 23
47 48 Parity bit P2
49 50 Data bit 24
51 52 Data bit 25
53 54 Data bit 26
55 56 Data bit 27
57 58 Data bit 28
59 60 Data bit 29
61 62 Data bit 30
63 64 Data bit 31
65 66 Parity bit P3
67 68 Ground

The "A" cable is used with SCSI-1, SCSI-2, or SCSI-3 to provide "FAST" SCSI with an 8 bit [Parallel] data bus.
The pin out for the 'A' data cable is listed on the SCSI Bus 'A' connector pinout page.
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.

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Modified 3/05/12
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