Q. 29. What is SCSI "packetization" (also
called information units)? Answer: Information units is a
form of SCSI packetization or encapsulation that is designed to reduce
overhead and speed data transfers. The SCSI command and data phases are
reduced from a seven to a three phase process and then further combines the
status phase and message phase that occur after a data transfer. Also, the
timing on the command phase, status phase and message phase is changed from
asynchronous to synchronous which results in another reduction in
overhead.
An "SPI information
unit" is a data structure that packetizes data, status, command, task
attributes and CRC into various formats. An "information unit transfer" is a
transfer of data, status, commands, task attributes, task management, and CRC
using only SPI information transfers.
Q. 30. What are the five optional features
of Ultra 160 (Ultra 3) SCSI? Answer: As defined in
the SPI-3 document of SCSI-3, they are: Double Transition Clocking, CRC,
Domain Validation, Quick Arbitration and Select (QAS), and Information Units
(Packetization). In order to be compliant with the SPI-3 U160 specification,
at least one of these features must be implemented. A group of industry
leaders agreed to incorporate three of these features in order to speed up
introduction of U160 products and to simplify backward compatibility. These
three features are Double Transition Clocking, CRC and Domain Validation. U160
devices with these three features are called U160/m.
Q. 31. All the new hard disk drives have
the LVD/MSE interface. Can I use them on a Narrow single-ended SCSI
bus? Answer: Yes. Although the LVD/MSE SCSI interface is
a Wide (16-bit) interface, you can connect it to a Narrow (8-bit) single-ended
bus. The LVD/MSE interface will switch to the single-ended mode with the
single-ended limitations on data throughput and cable length. Remember that
LVD peripherals do not have built in terminators. Also, remember that it is
best to terminate the unused upper byte data bits on the LVD drive.
Q. 33. Can I attach Narrow devices to a
Wide SCSI bus? Answer: Yes. But, it is a little more
involved than placing a Wide peripheral on a Narrow bus and there are several
considerations that must be observed. For purposes of discussion, let's say
that you have a Wide host adapter and a Narrow drive. Both are single-ended or
both are differential.
If you have only a Narrow peripheral (or multiple Narrow peripherals)
with no Wide devices connected to the Wide host adapter, simply cable
everything together with 50-wire cables. Do not use 68-wire
cables.
If you already have a Wide SCSI bus with Wide peripherals and you want
to add a Narrow peripheral, it gets a little more complicated as you must
make sure that all 68 wires of the SCSI cabling is carried all the way
through to the end of the bus.
The cleanest, easiest and most reliable, but not the cheapest way to
place Narrow peripherals on a Wide bus is to use a Wide SCSI expander such
as a SCSI RegeneratoR™. A Wide RegeneratoR™ has 68-pin connectors and
terminators on both connectors. They are single-ended in, single-ended out
or differential in, differential out (note that if it is single-ended in and
differential out, it is a converter) (Refer to FAQ "What is a SCSI
expander"). The RegeneratoR™ terminates one Wide bus segment at one
connector and regenerates another Wide bus segment on the other connector.
You can use this second Wide bus segment as a Narrow bus simply by using a
cable with a 68-pin connector on one end for the RegeneratoR™ and a 50-pin
connector on the other end for the Narrow peripheral with only 25 pairs of
wires between the connectors. Using this RegeneratoR™ assures that the Wide
bus is properly terminated and that the Narrow bus is properly
terminated.
If you are adding only one Narrow peripheral to a Wide SCSI chain, it is
probably easiest and cheapest to place the Narrow peripheral at the very end
of the chain. Use a 68-pin female to 50-pin male adapter with "upper byte
termination" and plug it directly into the Narrow drive. Make sure the
Narrow drive is terminated. If it does not have built in termination, you
will have to get a 68-pin to 50-pin adapter with termination for all the
lines. Make sure that the total length of conductors from the
terminator/adapter to the electronics in the peripherals is less than 0.1 m
(4 inches) for single-ended and LVD or 0.2 m (8 inches) for differential
(HVD) SCSI or you will create a "stub" with its associated problems.
A third method that works for connecting a Narrow peripheral in the
middle of a Wide SCSI chain is to use an enclosure for the peripheral that
has two Wide (68-pin) SCSI connectors with 68-wire twisted pair ribbon cable
between them and a 50-pin high density idc type connector in the middle of
the ribbon between the connectors. This 50-pin idc connector must be placed
in exactly the correct position on the ribbon in order to get the correct
signals to the Narrow peripheral, but with a little care, it is easy to do.
By the way, this will only work with the high density 50-pin SCSI connector
because the "pitch" (distance between pins) on the centronics type and the
idc type of connectors is 0.1 inch while the pitch on the 68-wire SCSI
ribbon cable is 0.05 inch.
Remember, when connecting 8-bit (Narrow) devices to a 16-bit (Wide) SCSI
bus, the Narrow devices with SCSI IDs in the range of 00 to 07 cannot
recognize or communicate with Wide devices having IDs in the range of 08 to
15. If possible, assign IDs in the range of 00 to 06 (07 is usually used by
the host controller) to all devices whether Wide or Narrow. If you have more
than 7 devices, think about which devices may have to exchange data before
assigning IDs.
You should also know that adding Narrow devices to a Wide SCSI bus adds
skew to the lower byte that is not added to the upper byte because the
Narrow devices are not connected to the upper nine bits. This can cause data
errors in the upper 9 bits but is not normally a concern unless there are
several Narrow devices placed on the Wide bus. The worst case is with a Wide
device at each end of the bus with 6 Narrow devices between. The risk of
using mixed width devices increases with increased data throughput. It is
possible to compensate for this added skew. Refer to the EPA document of
SCSI-3.
Q. 34. I have heard the term "idc" applied
to internal SCSI ribbon cables. What does it
mean? Answer: The term "idc" stands for insulation
displacement connector and it refers to the manner in which the connector is
placed on the ribbon cable. The connector and ribbon cable are placed in a
press and squeezed together. The pins on the connector are designed to
penetrate (displace) the insulation and make contact with the wire conductor.
This connector has two rows of 25 pins on 0.1 inch centers, but that is too
long, so the term "idc" is frequently used. It is not required that this
connector be an idc type. The term idc is really a misnomer when applied to
the male SCSI connector on the board.
Q. 35. Can I attach Wide peripherals to a
Narrow SCSI bus? Answer: Sure, it's easy. If you have
only Narrow peripherals to attach to a Wide SCSI host adapter with no Wide
devices, you can cable them together with 25 pair (50-wire) cabling. You will
need a cable having a 68-pin connector on the host end and a suitable 50-pin
connector on the peripheral end. This cable must have only 25 pairs of wire in
it. As usual, make sure there are terminators at the two extreme ends of the
cabling.
If you have a Wide peripheral to connect to a Narrow bus that
already has Narrow peripherals on it, you can use a 50-pin female to 68-pin
male adapter and plug the adapter into the Wide peripheral. If it is at the
end of the bus, make sure it is terminated.
Although we have not
experienced any problem with it, if you are conservative, it is best practice
to either terminate the upper byte on the Wide device(s) or provide other
circuitry to assure that the 9 bits plus parity of the upper byte are biased
to a false state in order to prevent failure to complete the arbitration phase
or to prevent attempts to execute a Wide transfer.
Q. 36. I note that in several of your FAQs
you recommend the use of 68-pin to 50-pin adapters. Aren't they "impedance
lumps" in the SCSI transmission line that can cause reflections and associated
problems? Answer: Absolutely. Like any connectors,
adapters add capacitance and impedance anomalies that can cause problems that
are difficult to trouble shoot. These problems are worsened as SCSI data
throughput increases. However, they do provide a solution to certain
connection problems. Adapter cables that have a 68-pin connector on one end
and a 50-pin connector on the other end are a better solution as there are
fewer connections and thus fewer impedance lumps. By the way, you may find
that problems associated with reflections caused by impedance mismatch can be
"cured" by changing the length of your SCSI cable by five or six feet. This
does not really cure the problem, but it will make the SCSI bus perform
reliably and at full data throughput until you add another peripheral or
change cable lengths again.
Q. 37. I had a problem with my SCSI bus. It
was much slower than I thought it should be. When I used a new SCSI cable that
was six feet longer than the one it replaced, the system performance returned
to what I think it should be. Why? Answer: You have
experienced the pleasures of reflections. It is usually indicative of cable or
termination problems but can also be caused by poorly designed peripheral
interfaces, a poorly designed SCSI backplane or placing too many peripherals
too close together on the cabling. At the speed of operation of today's SCSI
systems the cabling is actually an rf transmission line. According to
transmission line theory, when a signal (the incident signal) travels down the
cable and encounters an impedance change, part of the signal is reflected back
down the cable (the reflected signal). The amount that is reflected depends on
the amount of impedance change. If that reflected signal arrives at a device
that is expecting to receive a signal, it cannot tell the reflected signal
from the signal it is looking for. This reflected signal is not the
information the device is expecting, so it issues a "resend" or shuts down the
bus. Changing the length of the SCSI cable changes the time at which the
reflected signal arrives at this device and the peripheral ignores it. As you
can imagine this is easier to get by with on a system with a few peripherals
rather than one with lots of peripherals.
Q. 38. How can I connect a HVD device to my
single-ended bus? [or vice versa] Answer: You need a SCSI
expander called a Converter that converts the single-ended signals to HVD. A
correctly designed Single-ended to Differential Converter will allow the full
single-ended cable length, the full HVD cable length and will not decrease the
data throughput.
Q. 39. How can I connect a HVD device to my
LVD bus? Answer: If you use a SCSI expander called an LVD
to HVD Converter, the LVD devices will continue to pass data at LVD throughput
and the HVD devices will pass data at HVD throughput. If the LVD bus is
LVD/MSE you can use a single-ended to HVD Converter. However, remember that
connecting the single-ended converter to the LVD/MSE bus will cause the entire
LVD bus to switch to the single-ended mode with its limitations on data
throughput and cable length. If you have no LVD devices connected to it, this
is usually not a problem.
Q. 41. How can I connect a LVD peripheral
to my single-ended SCSI bus? Answer: If the LVD
peripheral has a multimode interface, simply connect it as if it were a
single-ended peripheral. All the LVD devices we have encountered so far have
the multimode interface, but better check the manual for the peripheral to be
sure.
Q. 42. Why does my LVD/MSE bus slow down
when I connect even one single-ended peripheral? Answer:
In order for the LVD bus to be compatible with single-ended peripherals, it
must switch to the single-ended mode whenever a single-ended peripheral is
connected to it. The maximum data throughput for single-ended SCSI is only 20
Megatransfers/sec, whereas Wide Ultra 3 is 80 Megatransfers/sec.
Q. 44. How can I connect a LVD peripheral
to an HVD SCSI bus? Answer: There are a great number of
installed higher-end computers in the workstation to mainframe range that have
HVD SCSI ports and in some cases, only HVD SCSI ports available. Now that HVD
disk drives are not being manufactured how can these newer LVD disk drives be
connected to these HVD ports. The key is a SCSI expander that provides LVD to
HVD conversion. This device allows any HVD devices to remain connected to the
HVD port of the computer and LVD devices to be connected to the LVD port on
the Converter. There is the added advantage that you can have up to 25 meters
(82 ft) of HVD cabling and up to 12 meters (40 ft) of LVD cabling. The HVD
host, of course, limits communication to the LVD devices to that of the HVD
interface.
Q. 45. What are
LUNs? Answer: Logical Unit Numbers are a method available
in SCSI to considerably expand the number of peripheral devices that can be
attached to a SCSI bus. In fact, on a Narrow bus it can expand the number of
devices on each address by a factor of seven and by fifteen on a Wide bus!
Except for RAID systems, LUNs are not widely used.
Q. 46. Can I have more than one computer on
my SCSI bus? Answer: SCSI requires that each bus have at
least one initiator and one target. It can have more than one initiator and
more than one target. The problem in having more than one host is the
operating system. Consider what happens if one host takes some information
from a drive thinking it is the latest data and immediately thereafter, the
other host updates that information. Or if one host writes information on a
drive and the other host does not know about it. The other host could
conceivably write some new information over the information the first host
wrote. Windows does not do well with a multi-initiator SCSI bus. Depending on
the SCSI drivers, UNIX can be better.
Q. 47. What is the difference between SCSI
and IDE (or EIDE or ATAPI)? Answer: SCSI is superior to
IDE in nearly every application except for very simple, one disk drive and
maybe a CD or scanner system. IDE is less expensive than SCSI. In other words,
IDE is probably the best choice in most home applications. If you have a
system with compute-intensive or storage intensive applications, SCSI is a
better choice. Here are some reasons:
Speed: The latest EIDE systems are nearly as fast as Wide Ultra SCSI for
single disk drive applications. However, in systems with a "busy" bus and a
multitasking OS, SCSI will win the race. During a data transfer, when data
is not actually being moved between devices, SCSI will free the bus for use
by other devices. This makes much more efficient use of the bus. IDE will
hold the connection between the two devices until the transfer is complete.
Wide Ultra SCSI is about two times as Fast as EIDE. LVD and Ultra160 SCSI
are many times faster than EIDE.
Number of Devices: An IDE bus can have two peripherals attached (your
PCI bus computer has two IDE buses for a total of four peripherals). Simple
Narrow SCSI, without the use of LUNs, can have six devices attached. Wide
SCSI can have up to 15 devices. The use of LUNs can multiply these numbers
by 7 or 15 respectively.
Loading of CPU: IDE requires the CPU to manage every aspect of the
transfer of data between two devices, so the CPU is tied up while data
transfers are taking place. On the other hand, SCSI is an intelligent bus
that manages data transfers and does require the intervention of the CPU.
Once the task is complete, the SCSI host adapter notifies the CPU. Command
queuing in SCSI allows the host adapter to list up to 256 tasks for a
peripheral to perform. The peripheral keeps working off each task until the
list is completed. These features free the computer for other tasks.
Variety of peripherals: SCSI has a wide variety of peripherals
available: hard disk, floppy disk, tapes, CDs, optical, RAID, scanners,
printers, etc. IDE is pretty much limited to disks, CDs and an occasional
tape drive.
Locating peripherals: SCSI is designed for both internally and
externally mounted drives. Internal SCSI drives are interconnected with 50-
or 68-wire ribbon cables. External SCSI drives are interconnected with
round, shielded 25- or 34-pair cables. The total length of cabling that can
be connected to a SCSI host depends on the version of SCSI being used.
Single-ended SCSI can have up to 6 m (20 ft) for Slow SCSI; 3 m (10 ft) for
Fast SCSI; and 1.5 m (5 feet) for Ultra SCSI. Differential (HVD) SCSI can
have up to 25 m (82 ft). LVD SCSI can have up to 12 m (40 ft). IDE
peripherals cannot be mounted outside the enclosure and allows something
like 0.5 m (18 in) of cabling. In addition, various SCSI expanders offer
methods of extending SCSI cable length to miles.
Data integrity: For years SCSI has had a parity check to minimize the
possibility of undetected errors in data transmission. Ultra160 SCSI has
added a CRC check that results in a tremendous increase in the ability of
SCSI to detect data transmission errors of many different types. Refer to
"What is
CRC?" FAQ for more information.
Q. 48. Is SCSI or IDE better for
me? Answer: This is definitely determined by the manner
in which you use your computer. If you have a single drive and maybe a CD, IDE
is probably better as it is less expensive and in simple applications, almost
as fast as SCSI. However, if you have compute or storage intensive
applications, SCSI is probably a better choice.
Q. 50. What is termination? Answer:
SCSI passes signals between devices over SCSI cables that are transmission
lines. The transmitting device, the cabling and the receiving device must all
be impedance matched in order to accomplish maximum transfer of energy
(signals) from end to end. If not, upon encountering a change of impedance
part of the signal well be reflected back to the transmitting device. If those
reflected signals arrive at the transmitter after it has switched to the
receive mode, it will think the reflected signal is the signal it is looking
for. Not recognizing the reflected signals will cause that device to issue a
SCSI re-send or perhaps even shut down the bus. Termination or terminators are
used to provide a better impedance match to reduce these reflections to a
minimum.
Q. 51. SCSI requires terminators. Where
should they be placed? Answer: Terminators must be placed
at the extreme ends of the cabling for each SCSI bus segment. There must be
two, and only two, terminators for each bus segment. This means that the last
connector at the end of a ribbon cable must have either a terminator or a
terminated peripheral plugged into it. If the host adapter is at the end of
the cabling, it too must be terminated. Higher end host adapters do a fairly
good job of automatically terminating when needed.
Q. 52. What is the difference between
passive and active terminators? Answer:
Passive terminators consist of a resistor network. Single-ended passive
terminators are suitable only for Slow SCSI. You should use active terminators
on all single-ended SCSI applications that run Fast SCSI or faster. There is
no real need for active terminators on the HVD SCSI bus. There are some
available but are used primarily in SCSI devices because they can be enabled
or disabled with a jumper change.
Active single-ended terminators
include a voltage regulator in addition to a resistor network. This voltage
regulator increases the stability of SCSI signals by isolating them from
fluctuations in the 5 volt termpower that would otherwise show up on the SCSI
signals.
Q. 53. Should I use passive or active
terminators? Answer: Passive single-ended terminators are
okay on a Slow SCSI bus (max data throughput of 5 Megatransfers/sec), but you
are better off to use active terminators on all SCSI single-ended
applications.
Differential SCSI has no need for anything other than passive
terminators.
LVD and LVD/MSE SCSI must have active
terminators.
