SCSI Bus

Small Computer Systems Interface


[SCSI Description]
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SCSI Bus Description

The SCSI [parallel] bus width is either 8 bits or 16 bits [termed Wide bus]. The SCSI bus may also 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.
With SCSI 'initiators' start the communication while the 'targets' respond. The bus master and bus slave change between the two as communication changes direction.
SCSI may be used for asynchronous and synchronous transfers;
Asynchronous transfers using Start and Stop bits and synchronous transfers using system timing (Hand-Shaking).
As with any interface synchronous transfers operate faster and a higher data through-put.
The Handshake protocol is slower because the requested data needs to be acknowledged before a new transfer begins.
The data bus also carries one parity bit. SCSI 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 Versions:
Serial SCSI [SAS] 'Serial Attached SCSI' description my be found on the Serial SCSI page, Serial SCSI replaced parallel SCSI described on this page.
Serial SCSI uses Serial ATA [SATA] as the physical layer. The rest of this page describes the different versions of parallel SCSI which are Obsolete].
Because of the high cost of SCSI systems [compared to the IDE interface], SCSI interfaces are normally only found on Servers and workstations.

SCSI-1 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.
The SCSI I interface is out-dated and OBSOLETE.
The SCSI-1 bus used the SCSI A cable, a link to the pinout table is listed below.

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. The SCSI II interface is out-dated and OBSOLETE.
The SCSI-2 bus used the SCSI A cable or the SCSI B cable, a link to the pinout table is listed below.

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) Single-Ended Schematic
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-4 [Fast-160] (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.


SCSI Standard Organizations

SCSI Standards List {ANSI}
T10 SCSI Documents, Specifications and Reference Info [www.t10.org]
SCSI Trade Association [www.scsita.org]

{SCSI Bus Index}


SCSI Bus Interface IC Manufacturers

Single Ended (SE) Mode uses TTL logic, with a max distance of 6 meters at 10MHz (Fast SCSI), or 3 meters at 20MHz (Ultra SCSI)
Multimode Single Ended (MSE) Mode Allows operation with 'SE' devices and 'LVD' devices
High Voltage Differential (HVD) Mode uses EIA-485 Drivers and Receivers allowing a max distance of 25 meters. Used in SCSI-2
Low Voltage Differential (LVD) Mode uses EIA-644 Drivers and Receivers allowing a max distance of 25 meters.





SCSI Integrated Circuit Vendors:

Fairchild {Linear Regulator for SCSI Termination}

LOGIC Devices {SCSI IC manufacturer}

LSI Corporation {SCSI bus expanders, SCSI Controllers, Ultra 160}

Microsemi Corp. {SCSI Terminations}

National Semiconductor Semiconductor {SCSI II Disk Controller IC}

ON Semiconductor Semiconductor {SCSI II Terminator-SCSI Regulator}

Texas Instruments 'TI' {SCSI Terminators}

Zarlink Semiconductor; see Microsemi

IC Manufacturers {All other types}

{SCSI Bus Index}


SCSI Specification

SMALL COMPUTER SYSTEM INTERFACE-2 (SCSI-2), X3.131:1994:
INFORMATION TECHNOLOGY - SCSI-3 PARALLEL INTERFACE, X3.253:1995
SCSI-3 (FAST-20) PARALLEL INTERFACE, X3.277:1996
SCSI Parallel Interface-4 (SPI-4)

{SCSI Bus Index}


SCSI Bus Connector Pinout

The 'A' cable provides the interconnect for either 8 bit single ended or 8 bit differential data. [Pin-out link below]
The 'B' cable (in conjunction with the 'A' cable) provides an additional 8 bits allowing interconnect for either 16 bit single ended or 16 bit differential data, under SCSI II ~ Wide SCSI.
[The 'B' SCSI cable Obsolete after SCSI III]
The 'P' cable provides the interconnect for either 16 bit single ended or 16 bit differential data ~ under SCSI III (which does not define a 'B' cable).
[Pinout link below, residing on this web site].

SCSI A Cable Single-Ended Pinout; 50 Pin, used with SCSI-1 and SCSI-2

SCSI A Cable Differential Pinout; 50 Pin, used with SCSI-1 and SCSI-2

SCSI B Cable Pinout; 68 Pin Cable, used with SCSI-2

SCSI P Cable Pinout; 68 Pin, used with SCSI-3

SCSI Q Cable Pinout; [Single Ended Data], used with SCSI-3

SCSI Q Cable Pinout; [Differential Data], used with SCSI-3

SCSI L Cable Pinout; 32-bit Wide SCSI. Obsolete as of SCSI-3

SCSI 30-pin Apple pinout [HDI-30] for the LaserWriter 12/600, LaserWrite Pro 630, and other computers.

SCSI Bus Active / Passive Termination methods
SCSI 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 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. Forced Perfect Termination (FPT): is a more advanced form of active termination, where diode clamps are added to the circuitry to force the termination the line to the correct voltage. This virtually eliminates any signal reflections or other problems and provides for the best form of termination of a single-ended SCSI bus. However FPT is not part of the SCSI standard.
5962-94732 MICROCIRCUIT, LINEAR, VOLTAGE REFERENCE, FIXED, FOR SCSI ACTIVE TERMINATION, MONOLITHIC SILICON

{SCSI Bus Index}


SCSI bus Connector Manufacturers

SCSI-I ~ Centronics 50 pin connector, 0.10 inch spacing, with 0.05 inch ribbon cable. [CN50], or a DB37 connector.
SCSI-II ~8 bit data over a Micro D 50 pin, high density (50 mil), 16 bit data over a Micro D 68 pin (50 mil) [HD50]
SCSI-III ~ 8 bit data over a Micro D 50 pin, high density (50 mil), 16 bit data over a Micro D 68 pin (50 mil) [HD68, HPDB68].
The connector is 2 rows by 34 pins, spaced on 0.05 inch centers.
Also calls out an 80 pin connector. The connectors may be shielded or un-shielded (Plastic shrouded header).
SCSI-V ~ uses a VHDCI connector [Very High Density Cable Interconnect]; 28AWG double-shielded twisted-pair cabling, .8mm 68-pin molded connectors.
Embedded SCSI-1, SCSI-2 and Ultra-SCSI may also be found on a 50-pin male header, so the cable would be a 50-pin female IDE flat ribbon cable [IDC50].
SCSI-1 may also be found on a DB25 connector. Some Apple products may have an HDI-30 connector.





SCSI Connector Vendors:
Circuit Assembly Corp. {SCSI Connectors}

DDK Electronics, Inc {VHDCI/SCA2 UltraSCSI}

Meritec {SCSI Adapters, RoHS compliant}

Methode Electronics Inc. {SCSI II/III Connectors}

Molex {SCSI Connectors}

Panasonic Electric Works Corporation of America {SCSI Connector Manufacturer}

Power Dynamics 'PDI' {SCSI Connector}

Tyco Electronics {SCSI-1, SCSI-2, SCSI-3 Cable Assemblies, SCSI-3 VHDCI Cable Assemblies}

Yamaichi Electronics {SCSI-2, SCSI-3, SCSI-4 Connectors}

Note the acronym SCA means Single Connector Attachment.
The SCA style was the original version which was replaced by the SCA2 connector.
The SCA2 style is an 80-pin wide only connector that holds 68-pin data, 4-pin power and jumper setting.
Wide-SCSI or 16-bit SCSI is used with SCSI-2 and above.

{SCSI Bus Index}


SCSI Cable Assemblies

The A cable has 50 conductors and provides an 8 bit data bus.
The P cable allows a 68 conductor cable or an 80 pin connector and provides a 16 bit data bus.

Single-ended (unbalanced) cable length at 6 meters.
Differential (Twisted-pair) SCSI cable length at 25 meters. A 1 foot minimum cable shall be used between devices.
Cable types allowed under SCSI III (30 AWG min.): Un-shielded Flat Ribbon/Flat Twisted Pair Ribbon/Round Twisted Pair Ribbon/Shielded Round Twisted Pair Ribbon.

Engineering Design Note: The electrical layer and mechanical layer of this interface, referenced on this page is obsolete and should not be used for new designs.
The upgrade path that would still use the SCSI protocol layer is called Serial SCSI [Serial Attached SCSI Description].

{SCSI Bus Index}


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Modified 2/26/12
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