SCSI V Bus Detailed Description and Connector Pin Out

SCSI-5 General Description

The SCSI-5 standard defines the electrical, physical and protocol layers for a parallel bus which operates using either 8 bits, 16 bits or 32 bits. SCSI-5, short for SCSI Parallel Interface-5 [SPI-5]; originated in 2003. SCSI-5 is backwards compatible with SCSI-4, SCSI-3 and SCSI-2 devices. SCSI-5 includes command sets for magnetic and optical disks, tapes, printers, processors, CD-ROMs, scanners, medium changers, and communications devices.
The description of different SCSI versions, and a comparison of all the SCSI revisions is located on the main SCSI Interface page.
Although the protocols are the same between versions, the physical interfaces were very different.

SCSI-5 Data Transmission Description

SCSI-5 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 V 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 the 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-5 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. SCSI-5 uses a 32 bit CRC [CRC-32], the 32-bit polynomial is X32+ X26+ X23+ X22+ X16+ X12+ X11+ X10+ X8+ X7+ X5+ X4+ X2+ X +1.

SCSI-5 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. SCSI-IV defines a clock Jitter of 0.25nS.
The minimum difference in time allowed between the rising or falling edge of a 1010 pattern on the DATA BUS or DB(P1) and its clocking signal on the ACK or REQ signal as measured at their zero-crossing points after skew compensation is applied by the receiver without allowing any error in the received data. The de-skewed data valid window shall be equal to: + [(data transfer period) - (residual skew error) - (strobe offset tolerance) - (clock jitter) - (receiver amplitude skew) - (chip noise) - (system noise at receiver) - (receiver asymmetry)] / 2.
The Physical Interconnects for SCSI-5 include:
AT Attachment with Packet Interface Extension [NCTIS.317-1998]
Fibre Channel Arbitrated Loop-2 [T11/1133D]
Fibre Channel - Physical and Signaling Interface [X3.230-1994]
High Performance Serial Bus [IEEE 1394-1995]
SCSI Parallel Interface-2 [X3.302-1998]
SCSI Parallel Interface-3 [NCITS.336-2000]
SCSI Parallel Interface-4 [INCITS.362-200x]
SCSI Parallel Interface-5 [This standard]
Serial Storage Architecture Physical Layer 1 [X3.293-1996]
Serial Storage Architecture Physical Layer 2 [NCITS.307-1998]

SCSI-5 Cable Description

The SCSI V 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 be 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 page. 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 page. 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 main Cable manufacturers page, or Cable Assembly manufacturers.
Additional component vendors may be found by the Components icon at the bottom of the page.

SCSI-5 Connector Description

SCSI-5 specifies two types of connectors: non-shielded and shielded. The non-shielded 50-Pin [25-pins x 2 rows] connector alternatives for the A cable for 8-bit transfers are; 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. An 80-pin connector is also defined, using the 'P' cable. The 80-pin connector cable combinations uses 2-rows of ribbon cables. The 'A' cable [defined in SCSI-2] used a Centronics 50 pin connector [2-rows by 25 contacts], 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. The P cable uses a 68-conductor VHDCI connector and a 68-pin ribbon cable. 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 [see Interface Topologies]. 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; however, for systems using SCSI-2 and above active termination schemes are required. 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 the TERMPWR voltage. 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.

The SCSI-4 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). The standard also added the 'P' cable.
There are a number of different transfer rates, depending on the transceiver used:
SE [Single-Ended] will operate at Async, Fast-5, Fast-10, Fast-20: all modes use Single Transition (ST)
MSE [Multimode Single-Ended] will operate at Async, Fast-5, Fast-10, Fast-20: all modes use Single Transition (ST)
LVD [Low Voltage Differential] 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 [High Voltage Differential] The High Voltage Differential [HVD] option, and the 32 bit wide bus option are obsolete in SCSI-3.
... SCSI FAST-5 runs at: 5MBps [8-bits] or 10MBps [16 bits]
... SCSI FAST-10 runs at: 10MBps [8-bits] or 20MBps [16 bits]
... SCSI FAST-20 runs at: 20MBps [8-bits] or 40MBps [16 bits], maximum bus length of 10 meters, 4 devices on the bus, 1.5 meters with 5 devices, (Ultra SCSI)
... SCSI FAST-40 runs at: 40MBps [8-bits] or 80MBps [16 bits], maximum bus length of 12 meters, (Ultra2 SCSI)
... SCSI FAST-80 runs at: 80MBps [8-bits] or 160MBps [16 bits], (Ultra3 SCSI, or Ultra160)
... SCSI FAST-160 runs at: 160MBps [8-bits] or 320MBps [16 bits], (Ultra320)
... SCSI FAST-320 runs at: 320MBps [8-bits] or 640MBps [16 bits], (Ultra640)
8-bit data bus is termed; Narrow Bus, 16-bit data bus is called Wide bus. Ultra, Ultra160, and Ultra320 are marketing terms.

SCSI-5 SCSI Standards

SCSI-5 Parallel Interface (SPI-5)
SCSI-4 Parallel Interface (SPI-4)
SCSI-2 Parallel Interface (SPI-2)
Serial Storage Architecture Physical Layer 1
Serial Storage Architecture Physical Layer 2
Serial Storage Architecture Transport Layer 1
SCSI-3 Serial Bus Protocol (SBP)

SCSI 'P' pin out SCSI 'P' pin out

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 page 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 Protocol). The data bus also carries one parity bit.

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Modified 3/05/12
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