SCSI-1
SCSI-1 is a complete document with all the
physical and protocol Layers, it is obsolete. 8 bit SCSI, Single-ended open collector
drivers with Asynchronous transfer for all commands and data transfers. Bipolar technology
for the drivers and receivers, bus DC loads a major consideration, passive termination.
The connectors are the low density 50 pin internal and external connectors, the external
low density connector is also known as the Centronics connector, in Cable terms the SCSI-1
Connector.
SCSI-2
SCSI-2 is a complete document with all the
Physical and protocol layers. SCSI asynchronous commands with Synchronous data transfer
rates up to Fast 10 megatransfers per second or 20 megabytes per second. Bipolar
technology with Open collector or active negation single ended bus transceivers, passive
or active termination, and Differential (Now called HVD – High Voltage differential)
bus transceivers based on EIA 485 allowed in SCSI-2. The A connectors are the 50 pin low
density internal connector, low density Centronics external connector and the High density
external connector. Cable terms the SCSI-2 connector is the high density 50 pin. The B
connectors for wide bus was not practical because it required a second cable 68 pin High
density.
SCSI-3
SCSI-3 change the document structure,
SCSI-3 is not one document with all the different layers and electrical interfaces, but a
collection of documents that cover the physical layer, the basic protocol specific to that
electrical interface, the primary command set layer (SPC) and the specific protocol layer.
The specific protocol layer contains the Hard Disk interface Commands in the Block
Commands (SBC), Steam Commands for tape drives(SSC), Controller Commands for RAID arrays
(SCC), Multimedia Commands (MMC), Media Changer Commands (MCC) and enclosure services
commands (SES) for example. There is an overall architectural model (SAM). Each document
has its own revision level, these are normally referred to as SCSI, the –3 has been
dropped.
The SCSI Parallel interface (SPI)
defines the Parallel bus electrical connections and Signals. SCSI Interlock Protocol
(SIP) defines the parallel command set. SIP is include in the SPI-2 and SPI-3
revisions. SPI define the P connector 68 pin High density primary cable for a wide bus in
a single cable. This 68 pin High density cable is referred to as the SCSI-3 connector. SPI
includes speeds up to Fast SCSI (Wide Fast SCSI is 20 MegaBytes per second), electrical
interfaces are defined in CMOS terms, with the termination specified at 0.2 volts instead
of 0.5 volts. The A cable is referenced to SCSI-2 but not included in SPI.
Fast-20 commonly known as Ultra SCSI
is a speed doubling from the SPI document allowing Fast-20, 20 megatransfers per second or
on a wide bus 40 megabytes per second. This is not complete standard, it only has the
sections required for the Fast-20, all other sections use the SPI standard.
SPI-2 doubles the
speed again to Fast-40 commonly know as Ultra2 SCSI, 40
megatransfers per second or on a wide bus 80 megabytes per second with a new electrical
interface, low voltage differential, LVD SCSI. Single ended can not be used for speeds
above Fast-20. The SPI-2 standard included SIP, the Single Connector Attachment (SCA-2) 80
pin Host swap connector and the 68 pin Very High Density Connector (VHDCI). The SCSI-2 A
and SPI P connectors are include in SPI-2.
SPI-2 is a complete document and does not
refer to older documents, one reference of all parallel interfaces up to Fast-40 speeds.
SPI-3 doubles the speed again to
Fast-80DT commonly know as Ultra160 SCSI, 80 megatransfers per second or on a wide
bus 160 megabytes per second. There are a lot of changes with SPI-3; High voltage
differential (HVD SCSI) and the 32 bit data bus with the Q cable are obsolete, clocking is
defined for LVD on both the rising and falling edges of the REQ/ACK clock – Double
Transition (DT) defined for wide bus only, Cyclic Redundancy Check (CRC) defined, Domain
Validation defined part of the domain validation will be in SPC-x, Packetized Commands and
Messaging defined and Quick Arbitration Defined.
SPI-3 is a complete document and does not
refer to older documents, one reference of all parallel interfaces up to Fast-80DT speeds,
but it does obsolete HVD and 32 bit data buses.
EPI (Enhanced Parallel Interface)
Technical Report describes how to design SCSI systems. The cable lengths and loads are
defined by the electrical parameters, allowing system designs to take advantage of faster
cables and reduce loads. The technical report describes how to work with wide (68 Pin) and
narrow (50 pin) devices on the same system. EPI includes documentation of Expanders,
Bridging expanders, Switches and common connectors not documented in the standards.
Quick reference
for cables
SCSI-1 External A cable is the 50 pin
Centronics connector cable. (C50M) 85 mil (2.16 mm) spacing
SCSI-2 External A cable is the 50 pin
High Density connector cable. (MiniD50M) 50 mil (1.27mm) Spacing
SCSI-3 External P cable is the 68 pin
High Density connector cable. (MiniD68M) 50 mil (1.27mm) Spacing
SCSI SPI-2 very high density connector
(VHDCI) external P cable is some times incorrectly called the SCSI-4 or 5
cable uses the 68 pin VHDCI connector cable. (.8 mm 68M) *
SCSI-1, SCSI-2 Internal A cable is the
50 pin connector Ribbon cable. 100 mil (2.54mm) spacing
SCSI-3 Internal P cable is the 68 pin
High Density ribbon Cable. 50 mil (1.27mm) Spacing
SCSI-3 divided the standard into separate standards, each a stand alone document.
