💾 Archived View for mirrors.apple2.org.za › archive › ground.icaen.uiowa.edu › Faqs › R008SCSITUT.TX… captured on 2024-03-21 at 20:04:02.
⬅️ Previous capture (2023-01-29)
-=-=-=-=-=-=-
The comp.sys.apple2 Usenet newsgroup Apple II FAQs originate from
the Ground Apple II archive. Administrator: Steve Nelson
Csa2 FAQs-on-Ground resource file: R008SCSITUT.TXT
Generic SCSI Tutorial
Note: To correctly view tables and diagrams on a super-res display,
use a mono-spaced Font such as CoPilot or PCMononspaced.
____________________________
Generic SCSI (pronounced scuzzy) Tutorial
by Daniel L. Miller, MD, BSEE
This article combines two interests in that it fits into both the
advanced pc hardware series and the Texas computer firm series.
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 )
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. 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 whether 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
________________