Transcript The SCSI Interface - Pioneer Pacific College

The SCSI Interface
Richard Goldman
April 2000
SCSI
• Small Computer System Interface
– A bus system that allows many different types
of devices to operate simultaneously from a
single SCSI controller card (Host Adapter).
• 1 to 4 SCSI host adapters (ISA or PCI)
may be used per PC.
• Each SCSI host adapter can support up to
7 Standard or 15 Wide SCSI devices.
SCSI Communication
• Each SCSI device has its own controller
which includes a SCSI interface circuit.
• The SCSI device communicates with the
SCSI host adapter through the SCSI bus
• The host adapter communicates with the
system through the system bus.
SCSI
Communications
Hard Drive Read/Write Heads
Hard Drive Controller
SCSI
Interface
Hard Drive
SCSI Bus
SCSI Host Adapter
System Bus
Mother Board
CPU
Standard SCSI
(50 Pin)
Wide SCSI
(68 Pin)
Host Adapter
Host Adapter
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
Hard Drive
SCSI ID Number
• Each SCSI device must be set to a different
SCSI ID number. (0-7 or 0-15; number 7 or
15 is used for the host adapter)
– SCSI ID number may be set with either a set
of jumpers or a switch.
– Some SCSI hard drives are mounted in hotswap cages that allow the SCSI ID number to
be set remotely.
SCSI Signaling
• Standard SCSI uses Single-ended (SE) signaling.
(Signals are transmitted and sensed by a voltage
transition.)
• SCSI devices that transmit at speeds greater then
20 MHz use Low Voltage Differential signaling
(LVD). (Signals are sensed by comparing the
difference between two voltages.)
• All devices on a SCSI bus must support LVD or
it will automatically drop back to SE signaling
and lower transmission speeds.
SCSI Bus Speeds
Bit Transfer Width, Cable Width, & Devices per Controller
8-bit, 50-pin,
7-devices
Standard
MB/s
MB/s
5
16-bit, 68-pin,
15-devices
Standard/Wide
Fast
10
Fast/Wide
20
Ultra
20
Ultra/Wide
40
10
(7 devices)
Ultra2
40
(LVD)
Ultra3
(LVD – 2X)
Ultra2/Wide
80
(LVD)
80
Ultra3/Wide
(LVD – 2X)
160
Internal SCSI Cables
• Internal SCSI cables (ribbon cables) usually
come with a SCSI device when you buy it.
However they often only support a few devices.
• They come in either 50 pin (standard) or 69 pin
(wide).
• To accommodate more devices you must order
special or custom made cables.
• You may also purchase the materials and tools to
make your own cables.
• Cable length nor cable quality are usually not a
big problem for internally mounted SCSI
devices.
External SCSI Cables
Cable
Connector
“A”
Centronics 50-pin
“A”
High Density 50-pin
“P”
High Density 68-pin
Hot-Swap Cage
80-pin
Apple & some others
DB-25 (same as printer
connectors)
External SCSI Cables
• Purchase high quality cables that match or
exceed your specific requirements.
• Observe overall cable length requirement
for your equipment.
• Avoid using the DB-25 SCSI cable. They
have poor transmission quality and are
susceptible to damage by connecting to the
wrong connector.
Passive SCSI Bus Termination
• Each end of the SCSI bus must be
terminated.
– The host adapter is usually preset to terminate
one end of the SCSI bus. (or designed to
automatically provide termination if needed)
– The other end of the bus may use a
terminating resistor – used on standard SCSI
external devices – it plugs into the open SCSI
bus connector of the last device in the bus.
Active SCSI Bus Termination
• Newer and faster SCSI devices require active
termination circuitry to ensure that the SCSI signals
are terminated at the proper voltage levels.
• Whenever Fast or Wide SCSI devices are used the
SCSI bus must be terminate with the appropriate type
of active termination circuitry.
• Active terminator, Forced Perfect Termination (FPT),
and LVD Termination may be set by setting a switch
or jumper on the last device on the bus.
• Special terminators are required for SCSI devices
that do not have built in active termination circuitry.
Plug-and-Play SCSI
• SCSI host adapters, devices, and operating
systems that comply with the SCSI
Configuration AutoMatically (or
AutoMagically) (SCAM) standard can be
Plug-and-Play.
• SCAM compliant host adapters and device
can dynamically assign SCSI ID numbers
at startup and automatically terminate the
SCSI bus.
SCSI BIOS
• The SCSI host adapter BIOS can load into
system memory and controls the operation of the
SCSI bus.
• Most system BIOS's can access a SCSI drive,
execute the load program in the MBR, and load
the SCSI device drivers stored on the hard drive
into memory.
• If an IDE drive is present in the system the
system BIOS will usually want to boot from it.
Other SCSI Setup Options
• Start On Command - Starts devices in order by SCSI
number, starting with 15. (reduces load on power
supply) - set to ON for internal devices
• SCSI Parity - Provides error checking - set to ON*
• Termination Power - Supplies power to active
termination devices (normally supplied by the host
adapter - OK to have more than one supply on) - set
to ON
• Synchronous Negotiations - Allows devices to select
best type data transfer mode. - Enable*
* Some very old devices may need to be off or disabled.
Why use SCSI?
• Expandability:
– Can operate up to 15 devices on one SCSI host
adapter. 4 host adapters will operate 60 devices per
PC.
– Allows computers (such as older Macintoshes) that do
not have expansion slots to add additional peripheral
devices.
– Used for data servers, internet servers, application
servers, etc. that need to access large amounts of data.
– Required for RAID. (Redundant Array of Inexpensive
Disks) A method used on servers to insure that data is
not lost in the event of a hard drive failure.
• (IDE RAID may be in limited applications)
• Speed
Why use SCSI?
– Currently high end SCSI devices have a max transfer rate of 160
MB per Second for Ultra-3/Wide vs. a max transfer rate of 100
MB per second for the IDE ATA-5.
– Slower hard drive speeds will limit data transfer rate despite
higher SCSI bus speed.
– SCSI devices can operate simultaneously in a multitasking mode
and support command queuing.
– Used for workstations that must be able to read and or write large
amounts of data very quickly.
• Computer Aided Design (CAD) workstations
• Graphics/Animation workstations
• Video Production Workstations
– SCSI devices operation may all be operating at the same time on
the SCSI channel. (This will result in slower individual devise
transfer rates if several devices are using the SCSI bus at the
same time.)
Why use SCSI?
• Flexibility
– Allows more types of devices to be connected to the
computer.
• Hard Drives, CD-ROM’s, Scanners, Printers, Tape Drives,
Etc.
– External SCSI devices can easily be moved form
computer to computer.
• Reliability
– Many server are configured with two or more SCSI
host adapter in a RAID. If one of the host adapters
should fail in this type of configuration the server can
continue to operate without interruption.
Why Not To Use SCSI?
• Expensive
– Requires the purchase of a host adapter card.
– Requires special cables and termination devices.
– The price of SCSI devices are more expensive than
other devices.
• More Work
– Usually requires special drivers and additional steps to
setup SCSI devices.
– Usually requires additional knowledge/skill to setup
SCSI devices.
• Cost/benefit
– System performance improvement may not even be
noticeable in many ordinary uses.