Q. 79. Where can I find SCSI connector
pinout information? Answer: The various SCSI connector
pinouts can be found in the SCSI specifications in section 5 of the SPI
documents. Many of the SCSI websites have the information in an easier to use
format. Try 50-Conductor Pinout
for 50-pin high density and centronics connectors or 68-Conductor Pinout for the 68-pin
high density and VHDCI connectors. These pages not only have the SCSI
connector pinouts, they also include the names of the SCSI signal on each pin
and the correct selection of cable pairs on which to place the signal pairs in
shielded external round cabling in order to correctly assemble a SCSI cable.
Do not expect to find the cable wire color coding as many manufacturers of
SCSI cable use different color codes.
Q. 81. What is a SCSI
expander? Answer: A SCSI expander is a device that allows
the user to expand the capabilities of the SCSI bus for certain applications.
These applications include mixing of single-ended and differential interfaces,
extending cable lengths to greater distances, isolating bus segments, adding
more devices than the usual 8 or 16 and dynamic reconfiguration of SCSI
components.
Per Section 10 of the EPI document of the SCSI-3
specifications, Simple Expanders do not occupy a SCSI ID and are invisible to
the protocols. Simple Expanders are "devices" in the SCSI sense. Bridging
Expanders have SCSI IDs on all ports and participate in SCSI arbitration and
messaging. They are "SCSI devices" in the SCSI sense.
Again per the EPI,
there are homogeneous expanders and there are heterogeneous expanders.
Homogeneous expanders have the same type of bus segment on both sides - in
other words, they do not perform a bus conversion such as SE to HVD. Used to
expand the total length of SCSI cabling allowed in the SCSI domain or to
provide isolation between SCSI bus segments.
A heterogeneous expander
has a different type of bus segment on both sides, such as a SE to HVD
converter. They are used for signal conversion and provide isolation between
bus segments. Any expander having the multimode LVD/MSE interface may become
either a homogeneous or a heterogeneous expander.
Q. 82. Where can I find books on
SCSI? Answer: The SCSI Trade Association has a list of "Good SCSI Reads".
Making SCSI Work is written for the individual who needs to put SCSI
systems together and make them work correctly and who needs a more thorough
understanding of SCSI. Books by Ridge and Schmidt are written at a more
technical level.
Q. 85. Can I locate peripherals more than
25 meters (82 ft) from the host? Answer:
The "old" differential (HVD) offered maximum SCSI cable lengths of 25 meters
(82 ft). HVD was rendered obsolete in the SPI-3 document of SCSI-3. The
current LVD SCSI offers 12 meter (40 ft) maximum cable lengths except in point
to point applications where you may use up to 25 meters (82 ft). To extend
these distances you need a SCSI expander. Some expanders extend the copper
SCSI cabling distances two to four times the usual length. Some put the SCSI
signals on coaxial or fiber optic cables to extend the distances to many
miles.
Q. 86. Is there anything I can do if I
need to locate a peripheral more than the SCSI recommended cable length from
the host? Answer: There are several things you can do.
Your choice depends on the distance you need and your budget. SCSI expanders
called repeaters or RegeneratoR™ can increase copper SCSI cable distances by a
factor of two or four. SCSI extenders convert the parallel SCSI signals to
serial, place them on coaxial or fiber optic cable and then convert them back
to parallel SCSI. These devices allow 300 feet to several miles of cable.
Another choice is the use of a SCSI to fibre channel bridge, however, at the
time of this writing this approach is expensive and problematic (refer to FAQ
"Is Ultra 160 SCSI
better than fibre channel?").
Q. 87. How can I locate SCSI backup
devices in another building or another part of this building? Answer:
If the same building location is within approximately 250 feet, SCSI repeaters
or RegeneratoR™ will allow full data throughput over standard copper SCSI
cabling. Beyond that you will need a SCSI extender that converts the parallel
SCSI signals to serial, places them on coaxial or fiber optic cable and then
converts them back to parallel SCSI signals. These extenders allow distances
of 300 feet to several miles between devices. Another choice is the use of a
SCSI to fibre channel bridge, however, at the time of this writing this
approach is expensive and problematic (refer to FAQ "Is Ultra 160 SCSI
better than fibre channel?").
Q. 88. What effect does SCSI cable
impedance mismatches have and what are the
symptoms? Answer: Any kind of impedance mismatches in the
SCSI cabling, including device interfaces, terminators, adapters, connectors,
or cabling can cause SCSI signals to be reflected back to the source. These
reflected signals cannot be distinguished from the actual SCSI signals but, of
course, are not what is expected by the receiving device. This results in
requests for "SCSI re-sends" and in extreme cases, shutdown of the SCSI bus.
These problems are exacerbated by faster SCSI signaling.
Symptoms vary from the system recognizing that drives are there, but not
being able to read or write to them, to reading (or writing) to some drives
but not to all of them, all the way to bus shutdown. Sometimes the symptoms
can be eliminated by changing SCSI cable length by five or six feet. This
changes the time that reflected signals arrive and may actually eliminate
the symptoms. If this works, and you are getting the data throughput that
you expect, leave it. The system will run reliably until you change
something - like adding a new drive or changing cable length.
To actually eliminate the problem, you must make sure you have correct
termination (use active terminators on single-ended systems), reduce the
number of adapters and connectors to a minimum, pay attention to the
impedance of all the cables (ribbon as well as round, shielded) in the
system, make sure that you are leaving sufficient cabling between all
peripherals and observe all the cautions referenced in the SCSI
specifications.
Q. 89. What causes impedance mismatches in
SCSI subsystems and what can I do to reduce them?
Answer: Impedance mismatches occur in a SCSI bus segment
at any place where the SCSI signals encounter an impedance change. This can
occur at the host adapter or peripheral device interface, the terminators,
connectors, adapters or any change from one cable to another. It is important
to reduce the number of connectors to a minimum. Also pay attention to the
characteristic impedance of each of the SCSI cables, whether they are ribbon
or round, shielded cables. Even cables from the same manufacturer may have
different impedance. To reduce the effect of stubs, first reduce them to a
minimum in terms of length, then space peripherals a minimum of 12 inches (300
mm) apart on the cabling. Reflections caused by impedance mismatches become
increasingly important with each increase in SCSI data throughput.
Also, many SCSI cables are built by cable people, not SCSI
people.
They do not understand that SCSI cables are rf transmission lines and
consequently some of the shortcuts they take to save manufacturing cost
cause problems for the installer. SCSI is, at a minimum, a 50-wire
bus.
We have found so-called SCSI cables with 50-pin connectors on each end,
but only 37 or even as few as 25 wires in the cable. This cable may work on
a single-ended system if they are very short and the bus is running Slow
SCSI, but they absolutely will not work in an HVD or LVD system and can
cause problems in a single-ended system that are very difficult to trouble
shoot.
Also, leaving wires out of the cable changes the characteristic
impedance of the cable, increases crosstalk and reduces the noise rejection
capability of the system.
To be on the safe side, buy only cables that are termed "differential
SCSI cables" as they will work on either single-ended or differential
systems. This does not assure, however, that they are properly assembled
cables. It is very important that SCSI signal pairs be assigned to cable
pairs and that certain signal pairs be assigned to cable pairs in the core
of the cable with control signals on the second layer and data signals on
the outer layer.
This emphasizes the importance of buying SCSI cables from a supplier
that uses quality components in his cable assemblies including cable that is
designed to meet the SCSI specs which includes characteristic impedance,
twisted pairs and shielding and knows which signal pairs to assign to which
cable pairs.
Q. 91. Must I use twisted pairs in my
ribbon cables or round SCSI cabling? Answer: The SCSI
specifications strongly recommend the use of twisted pairs in both ribbon and
round, shielded SCSI cables. Twisted pairs help in noise rejection and in
maintaining the characteristic impedance of the cable and become more
significant with increases in SCSI data throughput. Although cabling without
twisted pairs will work in many applications, why take the chance. Problems
caused by cables are very difficult and time consuming to trouble
shoot.
Q. 92. What are the rules for SCSI
terminators? What kind of terminators should I use and where should I put
them? Answer: Each SCSI bus segment must be terminated at
the two extreme ends. There must be two terminators, and only two terminators.
On an internal ribbon cable the first and last connector on the ribbon cable
must have a terminated device (host adapter, peripheral or expander) or a
terminator plugged into it. On external round, shielded cabling, the last
device must be terminated. If the last device does not have built in
terminators, you can use a pass through terminator or, if the last device is
in an enclosure with two SCSI connectors, you can use one connector for
connection to the bus and the second connector for a
terminator.
Passive single-ended terminators should be used only for
Slow SCSI systems (5 Mbytes/sec max). Use active single-ended terminators for
any bus that runs faster than 5 Mbytes/sec. HVD SCSI has no need for anything
other than passive (HVD) terminators. LVD terminators are different than
multimode (LVD/MSE) terminators. If you are plugging a multimode LVD device
into a single-ended bus, a single-ended terminator is sufficient. Note that
single-ended and differential terminators are different and HVD terminators
are different than LVD terminators.
When you are
placing a Narrow device or devices on a Wide SCSI bus that already has some
Wide peripherals attached, you must be very careful that the upper byte of the
Wide bus is properly terminated. Refer to FAQ on attaching Narrow devices to a
Wide bus for more information.
Q. 94. How do I assign SCSI IDs to various
devices on the bus? Answer: IDs are assigned to devices
through software or by setting hardware jumpers or switches. A given ID may be
assigned to only one device in the SCSI domain. Regardless if the bus is a
Wide or a Narrow bus, ID 07 has the highest priority and the SCSI host adapter
normally is assigned to it. ID priority order from highest to lowest is 07,
06, 05, 04, 03, 02, 01, 00, 15, 14, ..., 09, 08. If you are placing Narrow
devices along with Wide devices on a Wide bus, it is best to assign only IDs
in the range of 07 to 00 if you can, because Narrow devices cannot recognize
or exchange data with a device in the range of 15 to 08. The order in which
IDs are assigned makes no difference and whether the devices are internal or
external mount has no significance. Some operating systems do prefer that hard
drives be at IDs 00 or 01 or CDs at ID 02.
In most home or small
computer systems, you need not pay much attention to priority assignments, but
if you have very fast hard drives assigned to high IDs and they are very busy,
they can "hog" the bus and keep slower devices assigned to lower priority IDs
from ever getting control of the bus. SCSI "fairness" is designed to combat
this potential problem.
Remember that peripherals with the 80-pin SCA
connector are designed to have their ID assigned by the backplane. If you want
to use such a peripheral in a cabled SCSI system, you need an adapter that
converts the 80-pin connector to a 68-pin (or 50-pin) and also brings out the
power and ID assignment pins.
Q. 95. What is the priorty sequence of
SCSI IDs? Answer: SCSI ID 07 has the highest priority
regardless of whether the bus is Wide or Narrow SCSI. The host adapter is
normally assigned ID 07. The order of priority from highest to lowest is 07,
06, 05, 04, 03, 02, 01, 00, 15, 14, ..., 09, 08. Narrow SCSI uses only the IDs
from 07 to 00.
Q. 96. In what order do SCSI IDs have to
be assigned? Answer: SCSI IDs do not have to be assigned
in any particular order. The host adapter is normally assigned to ID 07
because it has the highest priority. Probably the most important thing to
remember is that a given ID may be assigned only to one peripheral. If you are
placing Narrow devices along with Wide devices on a Wide bus, it is best to
assign only IDs in the range of 07 to 00 if you can, because Narrow devices
cannot recognize or exchange data with a device in the range of 15 to 08. The
order in which IDs are assigned makes no difference and whether the devices
are mounted internally or externally has no significance. Some operating
systems do prefer that hard drives be at IDs 00 or 01 or CDs at ID 02.
Q. 97. What is a SCSI
"domain"? Answer:
From the EPI document of SCSI-3: "A SCSI domain is a logical bus with at least
one bus segment, at least one initiator, and at least one target. Domains with
multiple bus segments are enabled through the use of bus expanders. Domains
consist of the set of SCSI devices that are addressable from an initiator or
target. Wide domains are limited to a maximum of 16 initiators and/or targets
without the use of bridging expanders. Narrow domains are limited to a maximum
of 8 initiators and/or targets without the use of bridging
expanders."
In most cases, this means all the peripherals, expanders,
terminators and cabling connected to one SCSI host adapter. Also refer to the
FAQ "What is a bus segment" for related information.
"A SCSI bus segment consists of all the conductors and connectors
required to attain signal line continuity between every driver, receiver,
and two terminators for each signal. It is not necessary that a SCSI bus
segment contain any initiators or targets but it has at least two devices
attached. Drivers and receivers may be part of expanders as well as part of
initiators and targets."
In practice, a SCSI bus segment is
the cabling and devices (if any) between two terminators, including the
terminators. A SCSI domain may consist of one or more bus segments.
Q. 99. What is a
"device"? Answer: From the EPI document of SCSI-3:
"Devices include targets, initiators and bus expanders. The term "SCSI device"
is limited to targets and initiators."
Q. 100. What is a
"stub"? Answer: A stub is an unterminated section of
transmission line. The SPI-3 document of SCSI-3 defines a stub as: "Any
electrical path connected to the bus that is not part of the bus
path".
An example of a stub on a drive enclosure that is externally mounted is
the short length of ribbon cable between the drive's connector and the
connector on the outside of the drive enclosure. This stub may not be longer
than 0.1m (4 inches) on a single-ended or LVD bus or 0.2 m (8 inches) on a
HVD bus. Even the conductive paths on a drive's pcb from the SCSI connector
to the electronics is a stub. Stubs are unavoidable and should be kept to a
minimum length. Stubs tend to accumulate, so peripherals should be separated
from one another on the cabling. It is best to equally space peripherals
over the length of a SCSI bus segment. As a minimum, drives should be
separated by at least a foot of cabling.
Q. 100. What is
"SCAM"? Answer: SCAM stands for SCSI Configured
Auto-Matically. The last specification for SCAM is found in the SCSI-3 SPI-2
document in Annex B. It is an optional protocol designed to reduce problems
with the configuration of SCSI IDs on single-user systems. It is obsoleted in
SPI-3.
Q. 101. Does it hurt to have more than one
device supplying term power? Answer: It normally does no
harm to have more than one device supplying term power (TERMPWR). In fact, it
is very helpful in systems with long SCSI cables to prevent voltage "droop"
caused by IR loses through the cable or in systems that mix many Wide and
Narrow SCSI devices. It is best that TERMPWR be supplied from all the
initiators, expanders and device enclosures using only a single source per bus
segment per enclosure. Theoretically, having too much TERMPWR current
available (could be up to 32 Amps with 16 devices sourcing term power) could
cause things to melt in an unusual defect situation.
Q. 102. I have a hard disk connected to
the PCMCIA host adapter card in my laptop but it does not work. What is
wrong? Answer:
Although the SCSI specifications require that the host adapter provide power
for the terminators, many PCMCIA and PC-Card host adapters do not include it.
Maybe it has something to do with battery life, the space available or heat
dissipation, but it is a problem. To make your system run, you have to provide
term power from a source other than the host adapter. Many devices have a
jumper option to provide termination power.
