SCSI Termination methods
Small Computer Systems Interface
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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. 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.
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.
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)
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 Standards List {ANSI}
SCSI Documents, Specifications and
Reference Info {T10}
SCSI Trade Association
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.
Acard {SCSI IC Chips}
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}
IC Manufacturers {All other types}
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)
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. [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.
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.
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.
Circuit Assembly Corp. {SCSI Connectors}
Comm Con Connectors Inc. {SCSI-3 Connectors}
DDK Electronics, Inc {VHDCI/SCA2 UltraSCSI}
Meritec {SCSI Adapters, RoHS compliant}
Methode Electronics Inc. {SCSI II/III Connectors}
Molex {SCSI Connectors}
NAIS {SCSI Connector Manufacturer, VHDCI 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}
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.
This listing has moved to a new page; SCSI Cable Assemblies
Engineering Design Key words: SCSI Standard, Hard Drive Bus, Small Computer System Interface Standard, SCSI Specification, SSA Spec, Ultra SCSI Personal Computer Interface, IC, SCSI Physical Interface, Description, Pinout, Pinouts, pin out, Parallel SCSI, Direct Attached Storage, DAS, Networked Attached Storage, NAS, Storage Area Network, SAN, Signal Assignment, SCSI-1, SCSI-2, SCSI-3, SCSI-4, SCSI-4.
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