SCSI
What is
SCSI?
Small Computer System Interface.
Introduced by ANSI in 1986.
An I/O bus interconnecting computers and
peripheral devices.
Why SCSI?
Industry standard interface.
Generic interface for different devices from
different vendors.
Easy addition of new SCSI devices.
Allows for faster data input and output.
Ability to process
multiple overlapped commands.
Various
SCSI standards .
Initial version as SCSI-1 .
Introduced in 1986.
Defined the basics of SCSI Bus.
Talked about cable length, signaling
characteristics.
Defined various Commands and Transfer modes.
Revised and upgraded to SCSI-2 .
Approved in 1990.
Defined the new features as enhancements to
SCSI-1.
Incorporated the backward compatibility
with SCSI-1.
Recent version is SCSI-3
Much more features added to SCSI- 2.
SCSI-1
Defined the basic 8 bit data bus.
Maximum transfer speed was limited to 5
MB/s.
Had difficulties in acceptance, as many vendors
implemented only subsets of this
protocol.
SCSI-2
Introduced Fast SCSI
Doubling of the speed of data transfer to
10MB/s with regular SCSI.
Introduced Wide SCSI
Increasing the width of the SCSI bus
from 8 bits to 16/32 bits.
This increases the data throughput
of the devices.
More number of devices per bus
On buses with Wide SCSI, 16 devices are
supported, as opposed to 8 for regular
SCSI.
Improved cables and connections
Incorporated new higher density connections.
Active termination which provided the reliable
termination of the bus.
Command queuing
Introduced the concept of having multiple
outstanding requests on the bus at any
point of time.
Additional command sets
Included new command sets to support the use
of more devices like CD-ROMs, scanners
and removable media. SCSI-1 focussed
more on supporting hard disks.
Command set enhancements
SCSI-3
Ultra SCSI
Further doubling of system bus speed
resulting in data transfer rates up to 20MB/s for regular and 40MB/s for Wide SCSI.
Serial SCSI
Uses Serial SCSI as one of its new protocol
standards (also called FireWire).
Improved Cabling
Improvements over SCSI-2 for tuhe use of Wide
SCSI.
SCSI - Electrical Signal
types
Single Ended
Conventional signaling as used on other buses.
A positive voltage indicates a
logical TRUE and no voltage FALSE.
Each signal is carried on one wire.
Flexible and cost effective, as is
very common.
Cable length, however, is highly limited due to
the effect of noise.
Differential
Uses differential signaling.
Each signal is carried by two wires,
each the mirror image of the other.
A positive voltage on one wire and an equal
negative voltage on the other indicates
a TRUE.
Zero voltage(electrical ground) on both the wires
indicates a FALSE.
Use of two conductors makes this scheme more
resilient and immune to electrical
noise.
Longer cable lengths possible.
Cost is much more higher than single
ended.
Back to the technical jargon!!
Two different classifications
SPEED
BUS WIDTH
Speed of SCSI Bus
Regular
5MB/s data transfer rate.
Fast
10MB/s data transfer rate.
Ultra
20MB/s data transfer rate.
Bus width of SCSI Bus
Narrow
8 bit wide data bus.
Wide
16 bit wide data bus.
All in a nutshell
SCSI ID
Unique identification number assigned to
each device on the bus.
Used to identify devices on the bus.
For Narrow SCSI, ID ranges from 0 to 7.
For Wide SCSI, ID ranges from 0 to 15.
ID determines the priority of the device during
arbitration.
Larger the ID, higher the priority.
Generally the host adapter is at ID 7 (highest
priority).
Termination
Mandatory for SCSI devices.
Used to prevent signal reflections from entering
the SCSI bus.
Provided by using a terminator.
SCSI Bus
Communication on the SCSI bus is allowed between
only two SCSI devices at any given time.
Of the two devices on the SCSI bus, one acts
as the initiator and the other as target.
Initiator originates an operation.
Target performs the operation.
A device usually has a fixed role as an initiator
or target Some devices can assume either
role.
Certain SCSI bus functions are assigned to the
initiator and certain to the target.
Data transfer takes place through the mechanism
of handshaking signals.
SCSI Bus Signals
Total of 47 signals.
11 signals for control, 36 for data.
Important signals:
BSY - Bus being used.
SEL - Used by initiator/target to select/reselect
a target/initiator.
C/D - Control or Data information on
the bus.
I/O - Indicates the direction of
data transfer w.r.t the initiator.
MSG - Used during message phase.
REQ/ACK – Handshaking signals.
REQ – request for an ACK handshake
ACK- acknowledgement of a REQ.
DB - Data bus.
ATN- signal driven by an initiator to indicate to
a target that the initiator has a
message ready.
RST – Reset condition
SCSI Bus Phases
SCSI Architecture has 8 distinct phases.
BUS FREE Phase.
ARBITRATION Phase.
SELECTION Phase.
RESELECTION Phase.
COMMAND Phase.
DATA Phase.
STATUS Phase.
MESSAGE Phase.
BUS FREE Phase
Indicates that there is no current I/O process on
the bus.
SCSI bus is available for connection.
Devices detect this phase after the BSY and SEL
signals are both false.
Generally this phase is entered, when a target
releases its BSY signal.
ARBITRATION Phase
Allows one SCSI device to gain control of the
SCSI bus to initiate or resume an I/O
process.
Following steps occur while a SCSI device
gains control of the SCSI bus:
Device waits for the BUS FREE phase to occur.
After the Bus Free phase occurs, the
device waits for a specified amount of time before driving any signal.
Device then arbitrates for the SCSI bus by
asserting the BSY signal and its own SCSI ID bit on the data bus.
Device waits for an arbitration
delay, before examining the data bus.
If a higher priority SCSI ID bit is found on the
data bus, the device has lost its
arbitration and releases the BSY signal
and returns to step 1.
Else, the device has won the arbitration. It
then asserts the SEL signal.
SELECTION Phase.
Allows an initiator to select a target to
carryout target functions (READ, WRITE).
I/O signal is negated during this phase to
differentiate it from RESELECTION phase.
Selection is done by the following mechanism:
Initiator sets the data bus to a
value that is the OR of its SCSI ID and the target SCSI ID bit.
It then asserts the ATN signal indicating a
MESSAGE OUT Phase is to follow.
Then the initiator releases the BSY signal.
The target shall determine that it
is selected when the SEL and its SCSI ID are high and BSY and I/O lines are low.
It then scans the data bus to find
the initiator and asserts the BSY signal.
If more than two SCSI ID bits are on
the data bus, the target shall not respond to selection.
RESELECTION Phase
Optional Phase
Allows a target to reconnect to an initiator to
continue some I/O operation that was
previously started by the initiator
and suspended by the target.
Reselection follows the mechanism
stated below:
After Arbitration phase, the winning
device asserts the BSY and SEL lines.
The winning SCSI device becomes a target by
asserting the I/O signal.
It then puts its SCSI ID on the data
bus OR ed with the ID of the initiator.
The device then releases the BSY signal.
The initiator, on seeing its ID on
the data bus along with SEL and I/O being true and BSY being false, determines that it is reselected.
It then finds the target by scanning the data bus.
The reselected initiator shall then assert the BSY
signal.
After the target finds that BSY
signal is asserted, it releases the SEL signal.
Upon this, the initiator releases
the BSY signal and the target asserts the same.
The I/O then continues normally until the
target drops the SEL signal.
INFORMATION TRANSFER Phases
Consists of the Command, Data, Status and
Message Phases.
C/D, I/O, MSG signals are used to distinguish
between the different phases.
The information transfer phases use one or more
REQ/ACK handshake signals to control
the information transfer.
During the information transfer phases, the BSY
signal shall be True and the SEL signal,
False.
Two modes of information transfer.
Asynchronous Information Transfer.
Synchronous Information Transfer.
Asynchronous Information
Transfer
Target controls the direction of data transfer
through the I/O signal.
I/O signal true indicates data transfer from
target to the initiator.
If I/O is true, target puts data on the data bus,
asserts REQ signal.
Target puts data on the data bus and
drives the REQ signal to true.
Initiator reads the data when it
finds the REQ signal true, then asserts the ACK signal.
When ACK is true, target may change the data
on the data bus and then release REQ.
When REQ becomes false, initiator releases the
ACK signal.
After this, the target then can
either continue the data transfer or stop it.
I/O signal false indicates data transfer from
initiator to the target
Initiator requests for data transfer
by asserting the REQ signal.
Initiator puts the data when it
finds the REQ signal true, then asserts the ACK signal.
Target reads the data when it finds the ACK signal
true, then negates the REQ signal.
When REQ is false, initiator may change the data
on the data bus and then negate ACK.
After this, the target then can
either continue the request for data transfer or stop it.
Synchronous Information Transfer.
Used generally for Data Transfer Phase.
Used only if a prior agreement is reached using
messaging phases.
The offset for REQ/ACK signals is also reached
upon then.
Offset specifies the number of REQ signals the
target sends in advance of the number of
ACK signals received from the initiator.
For each REQ signal, one byte of data is
transferred and an ACK pulse is sent.
Different phases during Information Transfer
Command Phase
Allows the target to request command
information from the initiator.
Data Phase
DATA IN - Allows the target to request that
data be sent to the initiator from the
target.
DATA OUT - Allows the target to request that
data be sent to the target from the
initiator.
Status Phase
Allows the target to request that status
information be sent to the initiator
from the target.
Message Phase
MESSAGE IN - Allows the target to request that
messages be sent to the initiator from
the target.
MESSAGE OUT - Allows the target to request that
messages be sent to the target from the
initiator.
Target invokes Message Out phase in response to the
Attention condition from the initiator
by ATN signal.
SCSI Bus Conditions.
Two conditions, which can cause the SCSI device
to perform certain actions and alter the
phase sequence.
Attention Condition - Allows the initiator to
inform the target that it has a message
ready. Target reads it by performing a
Message Out Phase.
Reset Condition - Immediately clears
all SCSI devices from the bus. This condition takes precedence over all the other phases on the bus.
SCSI bus phase
sequences.
Brief description about messages
Allows communication between target and
initiator for interface management.
Messages may be of one, two or multiple bytes
in length.
One or more messages may be sent during a single
MESSAGE Phase.
Some of the generally used messages:
Abort - Sent from initiator to
target for clearing any I/O process.
Command Complete - Sent from target to
initiator to indicate the completion of
an I/O process and that valid status has
been sent to the initiator.
Disconnect - Sent from T to I indicating that
the present connection is being broken
and will be later resumed by invoking
the RESELECTION phase.
SCSI Commands
SCSI Commands follow the pattern of the
Command Descriptor Block (CDB) to
the target.
First byte in any SCSI Command should contain
the valid opcode.
Opcodes are classified into Optional, Mandatory
and Vendor specific.
Some fields in the CDB are reserved and
are set to zero.
If a target receives a CDB with the reserved fields
not being set to zero, shall terminate
the command with a CHECK CONDITION status.
Three groups – 0- 6 byte
1,2 -10 byte
5- 12 byte
3 and 4 reserved . 6 and 7 are vendor
specific.
Format of a 6-byte CDB
The transfer length field specifies the amount of
data to be transferred, usually the
number of blocks.
Commands that use one byte for the transfer
length allow up to 256 blocks of data to
be transferred by one command.
SCSI Status
Shall be sent from the target to the
initiator during the STATUS Phase at the completion of each command.
General status conditions:
Good - Target successfully completed the
command.
Check condition - Indicates that contingent
allegiance occurred.
Busy - Indicates that target is
busy.
Command Terminated - Returned whenever the
target terminates the I/O process.
SCSI Commands
Test unit ready : Queries
device to see if it is ready for
data transfers (disk spun up, media loaded, etc.). Opcode = 0x00
Inquiry : To obtain basic information
from a target device. Opcode =12h
Request sense : Returns any
error codes from the previous
command that returned an error status.
3h.
Send diagnostic :requests
the target to perform a self test. Test
is standardized and response is GOOD if all is well or CHECK CONDITION status if test fails. 1Dh
Start/Stop unit : Spins disks up
and down, load/unload media. 1Bh
Read capacity : Returns storage
capacity. 25h
Format unit : Sets all sectors to all zeroes, also allocates logical blocks avoiding defective sectors.
ftp
Read (four variants): Reads data from a device.
Write (four variants): Writes
data to a device.
Log sense : Returns current information
from log pages.
Mode sense : Returns current
device parameters from mode
pages.
Mode select : Sets device parameters in a
mode page.
