Scuzzy Moderato
- From: "UZnal" <unalz-at-mail333-dot-com>
- Date: Mon, 26 Dec 2005 19:56:34 +0100
Ref. EPRMx.HTML
The following technical letter was found on INTERNET (Adaptec)
SCSI Terms and Definitions
There are 2 handshaking modes on the SCSI bus, used for transferring data:
ASYNCHRONOUS and
SYNCHRONOUS. Asynchronous is a classic Req/Ack handshake. SYNCHRONOUS is
'sort of' Req/Ack, only it
allows you to issue multiple Req's before receiving Ack's. What this means
in practice is that SYNCHRONOUS
transfers are approx 3 times faster than ASYNCHRONOUS.
SCSI-1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec and
synchronous transfers at up to 5.0
Mbytes/Sec.
SCSI-2 had some of the timing margins 'shaved' in order that faster
handshaking could occur. The result is that
asynchronous transfers can run at up to 3.0 bytes/Sec and synchronous
transfers at up to 10.0 Mbytes/Sec. The
term 'FAST' is generally applied to a SCSI device which can do syncrhonous
transfers at speeds in excess of
5.0 Mbytes/Sec. This term can only be applied to SCS1-2 devices since SCSI-1
didn't have the timing margins
that allow for FAST transfers.
Differential SCSI
For each signal that needs to be sent across the bus, there exists a pair of
wires to carry it. The first in this pair
carries the same type of signal the single-ended SCSI carries. The second in
this pair, however, carries its
logical inversion. The receiver takes the difference of the pair (thus the
name differential), which makes it less
susceptible to noise and allows for greater cable length.
Single-ended SCSI (normal SCSI)
For each signal that need to be sent across the bus, there exist a wire to
carry it.
Wide SCSI
SCSI may now transfer data at bus widths of 16 and 32 bits. Commands,
status, messages and arbitration are
still 8 bits, and the B-Cable has 68 pins for data bits. Cabling was a
confusing issue in the closing days of
SCSI-2, because the first project of SCS1-3 was the definition of a 16 bit
wide P-Cable which supported 16-bit
arbitration as well as 16-bit data transfers. Although SCSI-2 does not
contain a definition of the P-Cable, it is
quite possible that within the year, the P-Cable will be most popular
non-SCSI-2 feature on SCSI-2 products.
Fast SCSI
A 10 MHz transfer rate for SCSI came out of a joint effort with the IPI
(Intelligent Peripheral Interface commitee
in ASC X3T9.3. Fast SCSI achieves 10 Megabytes/second on the A-Cable and
with wider data paths of 16 and
32 bits can rise to 20 Megabytes/second and even 40 Megabytes/second.
However, by the time the market
starts demanding 40 Megabytes/second it is likely that the effort to
serialize the physical interface for SCSI-3 will
attract high-performance SCSI users to the Fiber Channel.
A word of caution. At this time the fast parameters cannot be met by the
Single Ended electrical class, and is
only suitable for Differential. One of the goals in SCSI-3 is to identify
the improvements needed to achieve 10
MHz operation with Single Ended components.
SCSI Termination (PASSIVE-termination)
The Single Ended electrical class depends on very tight termination
tolerances, but the passive 132 ohm
termination defined in 1986 is mismatched with the cable impedance
(typically below 100 ohms) Allthough not a
problem at low speeds when only a few devices are connected, reflections can
cause errors when transfer rates
increase and/or more devices are added. In SCS1-2, an active terminator has
been defined which lowers
termination to 110 ohms and is a major boost to system integrity.
Command Queueing
In SCSI-1, initiators were limited to one command per LUN e.g. a disk drive.
Now up to 256 commands can be
outstanding to one LUN. The target is allowed to re-sequence the order of
command execution to optimize seek
motions. Queued commands require Tag messages which follow the Identify.
Active / Passive - Terminator sample
An active terminator actually has one or more voltage regulators to produce
the termination voltage, rather than
using resistor voltage dividers.
This is a passive terminator:
TERMPWR - - - - /\/\/\/ - - - - - + - - - - - - /\/\/\/ - - - - - GND
|
|
SCSI signal
Notice that the termination voltage varies with the voltage on the TERMPWR
line. One voltage divider (two
resistors) is used for each SCSI signal.
This is a active terminator:
An active terminator looks more like this (supply filter caps omitted):
+---------+
| |
TERMPWR - - - -|in out| - - - - + - - - - - - /\/\/\/ - - - - SCSI
signal
| gnd | |
+---------+ + - - - - - - /\/\/\/ - - - - SCSI
signal
| |
| + - - - - - - /\/\/\/ - - - - SCSI
signal
GND - - - - - - + |
etc.
You MUST use Active-termination if an ULTRA or RAID SCSI adapter is
installed !
Assuming that the TERMPWR voltage doesn't drop below the desired termination
voltage (plus the regulator's
minimum drop), the SCSI signals will always be terminated to the correct
voltage level.
Server 325/330 recommendations for ULTRA-SCSI (SCSI-3) support are:
NOTE: The following are guidlines, and many setup which break this rules
will still work. Setups which stick to the
below rules will have the best chance of working.
Maximum of up to 7 devices (plus the controller) if the cable length is
1.5m (4.5-ft) or less.
Maximum of up to 3 devices (plus the controller) if the cable length is
3.0m (9-ft) long.
8-bit and 16-bit devices can be mixed (ensure 16-bit at end of chain).
SCSI-3 and legacy devices can be mixed. Beware of capacity loading of
older devices though.
To support more devices (up to 15) or to support external SCSI devices,
ULTRA-SCSI support should be
disabled in the SCSI Select utility.
This restrictions basically say that ULTRA-SCSI is an internal solution
only.
Mixing SCSI Types.
The prime concern when intermixing devices operating with differen SCSI
configurations is the data bus width.
The 50-pin connector used for external SCSI-1 and SCSI-2 FAST devices will
not support greater than 8 bits.
So to use WIDE devices, they must be placed physically before the 8-bit
devices. Also highlighted is that the
bus lines that are cut off, must be terminated as usual.
Mixing Wide and Narrow External SCSI Devices
68-pin SCSI-2 Wide or 50-pin SCSI-2 Fast or
Fast & Wide device SCSI-1 device
+----+ +----+
----| |------------------------| |------
| | Control Bus | |
----| |------------------------| |------
from previous | | | | to next SCSI-1
--------> | | | | -------> or
Wide device | | | | SCSI-2 FAST (only)
or ----| |------------------------| |------ device
Controller | | Data (0-7) | |
----| |------------------------| |------
----| |------------------+ | |
| | Data (8-15/32) ||T | |
----| |------------------+ | |
+----+ +----+
Wide SCSI Cable Wide (68-pin) to SCSI-1 (50-pin)
(68-pin) SCSI-1 (50-pin) Cable
with termination (T)
For internal use the bus cable will be the same for its whole length,
therefore, all signal lines can be terminated
at the end. In such cases the data bus width reduction can be performed
without termination.
Term-Power Rules
When SCSI-I Adapter/Devices are used, the Adapter only should supply
termination power.
When SCSI-II Adapter/Devices are used each Adapter and/or Device can supply
termination power. As a
general rule, (for SCSI-II, F/W, Ultra..) only the Adapter should supply
termination power. If the SCSI cable is
very long (>1.2m), Term-Power also should/can be supplied from the last
device (in chain) on this cable, but all
other devices in such a chain, should not have Term-Power enabled.
.
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