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SCSI (pronounced scuzzy) TUTORIAL By Daniel L. Miller, MD, BSEE
11 West Wedgewood Glen
The Woodlands, TX 77381
copyright 9-5-87 daniel l. miller
HAL PC MEMBER
This article combines two interests in that it fits into both the
advanced pc hardware series and the Texas computer firm series.
The article format is an expanded outline to save printed space.
1 Operational Description
The Small Computer Serial Interface is an input/output channel
bus specification. The specification allows up to eight bus
controllers to exchange control packets and bytes of data between
any two pairs of controllers.
2 Examples of Data Exchange
Workstation to hard disc, optical disc and high density floppy.
Workstation is the bus master the peripheral devices are
individually accessed. One initiator , 3 targets.
Several workstations accessing a bank of hard discs for fast
data retrieval with interleaved seeks and queed commands.
Multiple initiators, multiple targets. Each drive disconnects
from the bus during long seeks, while other drives transfer
data and then reconnects when ready to transmit data.
3 Rates of data exchange
Data rate of readily available bus interface chips 12 - 15
megabits per second. NCR 5380 / NCR 5385
Logic Devices L5380 32 megabits / sec
4 Controller Selection
Bus controllers are free to access the bus to try to gain
control to initiate a data exchange. Bus arbitration is
according to a fixed priority 0 - 7 rating. If two devices
request access the lower priority device must wait.
5 How many devices. The specification allows 8 controllers
to be selected from 8 data lines -- one line per each device.
Two additional lines select up to 4 devices for each controller.
Up to eight bus controllers each controlling up to 4 devices
totals 32 devices.
A proposed extension allows binary decoding of the selection
lines to allow 64 controllers. ( Ampro SCSI+ protocol, Ampro advertises in
Byte )
6 Accessing the Bus
The scsi interface protocal may be implemented with a dedicated
cpu and 4 input and 4 output data latches. ( Novix Beta Board )
However, scsi bus interface chips with data latches are readily available
both in TTL and CMOS configuration. NCR, Toshiba 2nd source and
similar purpose chips are available from Adaptec and Western digital.
Emulex has announced a vlsi chip scsi processor which they claim can
implement bus sequences without external microprocessor intervention.
The NCR chip is programmed by addressing register select lines and
storing a byte in the controll register and if appropriate in the
data register. A programming manual lists the possible commands.
The chip will perform aribitration logic and drive the bus and
detect protocol errors ( called phase errors in the NCR literature )
and parity errors. Data ready and error conditions are flagged
by an inerrupt line. A register holds a code to identify the type
of error detected to be read by the host cpu for error interrupt
handling routines.
The interface chip has handshake signals to operate efficiently
with a DMA controller to move blocks of data into or out of a
section of memory. The dma is a dedicated cpu chip that generates
addresses for moving data sequentially into or out of memory.
6.5 Terminology
Initiator - the device that begins an interchange
Target - The device that is selected by the initiator
Host - In the configuration of a computer sending data to
one or more disk drives, "host" refers to the main workstation
computer.
Controller - any device that connects to the scsi bus
connects through a controller.
Device - Each controller on the bus can control up to 4 devices.
Each device can be individually addressed, so a device connected
to one controller can specifically talk with a device on another
controller. Devices connected to the same controller cannot
talk to each other over the scsi bus. The devices are also
called "Logical Units" or LUN's because they are addressed and
specified
by 2 bits in each command sent over the bus, rather than being
selected by a hardware bus line.
SASI - sasi stands for Sugart Associates System Interface, a single
host multiple target ( non arbitrating ) interface which was the
precursor to the SCSI specifications.
SCSI - Small Computer System Interface, the current standard
specification is the ANSI (American National Standards Institute)
version X3.131, 1986.
Embedded Controller- some drives have a scsi interface built into
the drive electronics mounted in or on the drive itself. This
will be referred to as a "scsi drive."
SCSI controller - a scsi interface board that then connects with up
to four devices like esdi drives or st412/506 drives or optical drives.
( The scsi controller is made to talk to regular devices, like the
drives in a xt or at computer.) Scsi controllers are made for
optical drives, tape systems, ESDI and st412/506 hard discs.
7 Data Transfer Protocol
The initiating controller first gains control of the bus and
gets the attention of its target controller. The target then
takes control of the data exchange and asks the initiator what
it wants to do. ie give a command, send or receive data. The
initiator responds and the target aknowleges receipt of command
and transmits or recieves data, under control of the target controller.
The target will indicate with a message that it has successfully
completed its mission.
Bus phases
BUS FREE phase
SELECTION phase
COMMAND phase --
DATA phase |-----These are the information transfer
STATUS phase | phases.
MESSAGE phase --|
ARBITRATION phase (optional) | |-for multiple hosts
RESELECTION phase (optional) |---optional-| for multidrive high
bus performance
7.5 DATA EXCHANGE WALKTHROUGH
An example of a simple data interchange between an initiating host
PC and a target hard disc drive.
To send a block of data to a disk drive a pc first checks the bus
busy line. When the line indicates the bus is free, the pc will
send its identification code over the data lines and pull the bus
control lines. If there is more than one device desiring to use
the bus at the same time, the device with the highest priority
always wins. There is no inherent mechanism to insure fairness
or to force a round robin to prevent one device from tying up the
bus.
After gaining control of the bus the pc will select its target
device by activating one of eight lines. The device selected
then takes control of the data interchange, and will remain in
control until the end of the exchange. The target asks the pc
for a command. The pc responds with a command block to read
10 blocks beginning with block 5. The target acknowledges and
translates the logical block numbers to absolute sectors, seeks and
reads the sectors, performs error correction and transmits the
data byte by byte with a handshake acknowlegement after each byte.
When it is finished it requests to send its status and transmits
a status report, if the status is good and the data does not have
to be retransmitted the drive then requests to send a message and
reports completion.
8 SCSI Command Format
When the target requests a command, the initiator responds by
sending out 5 bytes ( 40 bits ) of command information. These
bits are used to specify the command and identify the logical
device. There is a SCSI COMMON COMMAND SET specification
The purpose of the specification was to
"Allow true second sourcing of SCSI hardware." However,
because there are still multiple ways to do certain commands
and vendor specific functions, hard disc interchangeability
has not yet been achieved. From the end user point of view,
one can't unplug one scsi drive and replace it with one
from another manufacturer and expect it to work without
rewriting the device driver software for formats and the message
handling scsi driver routines.
The set of 40 bits is called a command block. The first byte
specifies the class of commands or opcode.
Some Common Command Set Opcodes are the following:
Opcode 00 (hex) TEST UNIT READY
03 FORMAT UNIT
08 READ
0A WRITE
0B SEEK
Since the drive controller electronics is on the drive side of
the SCSI BUS the data is organized by the drive controller and
requested by block number. The controller translates the block
number to the track and sector information used to step to the
data. This permits flexibility for the drive manufacturer. A
drive with an embedded SCSI controller can reallocate bad blocks
internally and substitute blocks automatically from a spare track.
The meaning of the remaining bits varies for each opcode.
The Write Command, opcode 0A, for example
BYTE 0 opcode 1A (00001010)
BYTE 1 Logical unit number in bits 5-6, bits 1-4 specify logical
block address
BYTE 2 logical block address
BYTE 3 logical block address
BYTE 4 Bits 2 to 5 specify the transfer length
BYTE 5 Bits 7 and 6 are vendor unique, bit one is a flag, bit 0 a link
9 SCSI Hardware Bus Specification
25 signal lines; 25 ground lines
Signals may assume true or false values. Signal assertion (true)is 0 v.
Signal negation (false) is 2.5 to 5.25 V dc
( Bus driver chips typically invert the signal so bus voltage
levels are typically the reverse of normal logic levels input to the
on chip or input to the bus drivers where
+5 V is true.)
SCSI BUS SIGNALS
Busy indicates the bus is in use
Select used by the host to select the drive
Control/Data This signal is driven by the target (like a disc drive
and indicates whetehr CONTROL or DATA information is
on the DATA bus. A true value indicates a control
signal is on the bus.
Input/Output
This signal is driven by the target (disc drive) and
controls the direction of data movement on the DATA
bus with respect to the host.
A true value indicates input to the host.
Message This signal is driven by the target (disc drive)
during the message phase.
Request This signal is driven by the target (disc drive) and
indictes a request for a data transfer handsake acknowlegement.
Acknowledge This signal is driven by the host and acknowleges a
a data transfer handshake.
Attention This signal is driven by the host to indicate a
condition needing attention.
Reset This forces a reset of devices on the bus.
Data lines (8)
Parity line (1) This line may be used by the devices to signal a
parity error in the received data or command.
Bus Power lines
10 SCSI CATALOG
HOST BOARDS for the IBM PC, XT, AT, ... IBMetc...COMPAC... CLONES...
Drop in boards for the pc,xt,at and 386 IBM, Intel, Compac and close
clones are available from Advanced Storage Concepts
and Hudson and Supinger and Future Domain.
Hudson and Supinger is of interest in
that they supply an
example dos driver program in C language although their prom routines
remain proprietary.
ADVANCED STORAGE CONCEPTS (The Houston based company in this
article.)
sources a board that just drops into the pc
XT AT or 386 and uses an included self installing device driver
to configure the board and install itself as a DOS device
driver. I have had one operating in my XT for a year with
no problem. Included software functions set up drives of any
size and get around the dos 32 megabyte max drive size "limit".
Operation is transparent. And from my keyboard and application programs I
can't tell that I'm using a scsi disc. The short slot size
board has a eeprom that remembers the configuration so there
are no jumpers or switches to set. The proms and driver source
code are proprietary. Many ( 30+ ) scsi drives are supported.
The installation program has the data on file for the drives
like the number of logical blocks and the message codes returned
and the format commands so the user only has to pick the drive
name and model from a menu and the program does all the work.
The installation program has nice colors and jazz and looks like
something well done in Turbo Pascal and has a good feel to it.
Advanced Storage Concepts does systems integration for large
oem's and drive manufacturers and systems suppliers, but did not
seem to mind my bothering them. At one time they had advertised
in PC Tech magazine.
National Memory Systems sells scsi drive systems and boards
primarily for minicomputers and super micros and IBM
comptible scsi systems.
SCSI drives are available from a variety of commercial
electronics suppliers including Arrow and Hallmark and
Dallas Digital and Quality Components in Richardson.
Integrated scsi controllers built into disc drives are made
by many drive manufactureres including
Quantum, Maxtor, Rodime, Teac, Kodak, Fujitsu,
C. Itoh, NEC, Seagate, Shugart Associates (who originated the
SCSI specifications in conjunction with NCR and Ampro), and
Connor Periferals.
A partial list of devices with scsi interfaces include
5 1/4 inch drives up to 760 meg from Maxtor, Quantum, NEC
Toshiba, Fujitsu, Connor Periferals, plus about 10 others.
5 1/4 inch floppy drives up to 12 (twelve is correct !) megabytes
from Verbatim (now a Kodak data division).
3 1/4 inch drives up to 170 meg from Maxtor.
Miniature lightweight Connor Periferal 40 and 80 meg drives.
Optical Write once read many time discs from ( 5 1/4) Optotech in Denver
200 meg per side and Kodak at 1.2 gigabytes in a 12 inch disc.
SCSI controllers made to connect a scsi bus to up to 4 drives
using the esdi or 502 interface are available. These controller
boards would be used to hook the hundreds of cheap drives
available for the pc xt or ps2 computers to a scsi bus.
Manufactureres of these controller boards include Adaptec,
Emulex, and OMTI.
SCSI computer motheroards with a built in
scsi chip set and the system
proms or operating system has the software, interrupt handlers,
and message handlers to talk to at least one brand of scsi drive.
These computers include the Macintosh Plus, The Amiga,
Steve Ciarcia's Micromint's SB180FX, PC TECH SUPER
10 MHZ AT clone and their 386 machine, The Slicer, The Semidisc
64180, Chicago Computer's 68000 machine.
Drop me a note if you need a commercial address or telephone
number.