Q. 55. How can I tell if an unmarked
terminator is SE or differential? Answer: Back when we
had only single-ended and HVD, this could be determined by one simple
resistance measurement. Today, with single-ended, HVD, LVD and LVD/MSE it
requires a little more work to differentiate single-ended, HVD and
LVD.
One method to determine if the terminator is single-ended or
differential requires the use of a multimeter that measures low resistance
values. Using a multimeter that is capable of measuring low values of
resistance, measure the resistance between pins 7 and 9. These two pins work
regardless of the kind of SCSI connector, however, remember that while the
pins on most SCSI connectors are numbered 1, 2, 3, ... along the top row of
pins and then continued along the bottom row, the idc connector used for
50-pin internal ribbon cable connections have odd numbered pins in one row
and even numbered pins in the other row. If this resistance value is close
to zero (<a few tenths of an ohm) the terminator is single-ended. If the
value is 0.5 ohms or more, it is differential. Incidentally, this method
will differentiate single-ended from differential host adapters or
peripherals, too.
To tell a HVD terminator from an LVD terminator, you must make another
resistance measurement. Measure the resistance from pins 31 to 50 (use pins
22 to 50 on a 50-pin idc terminator). If the value is less than a few tenths
of an ohm it is a passive HVD terminator. If the value is substantially
greater than a few tenths of an ohm, the terminator is LVD or
LVD/MSE.
A rule of thumb says that if the terminator doesn't state the type on
it, it probably is a passive single-ended terminator. If it says "Active" it
is an active single-ended terminator. Otherwise, the terminator should state
the bus type it is made for or have an international SCSI symbol for the bus
type on it.
If you cannot find any information on the
device, you can try the following:
To determine if the interface is single-ended or differential requires
the use of a multimeter that measures low resistance values. Using a
multimeter that is capable of measuring low values of resistance, measure
the resistance between pins 7 and 9. These two pins work regardless of the
kind of SCSI connector, however, remember that while the pins on most SCSI
connectors are numbered 1, 2, 3, ... along the top row of pins and then
continued along the bottom row, the idc connector used for 50-pin internal
ribbon cable connections have odd numbered pins in one row and even numbered
pins in the other row. If this resistance value is close to zero (<a few
tenths of an ohm) the interface is single-ended. If the value is 0.5 ohms or
more, it is differential -- either HVD or LVD.
To determine if the device is HVD or LVD, the best way is to power the
device and measure the voltage from the DIFFSENSE pin to a GND pin. If the
voltage value is less than 0.5 VDC, the device is single-ended; if the value
is between 0.7 and 1.9 V, it is LVD; and if the value is more than 2.4 VDC,
the device is HVD. WHEN YOU ARE CONNECTING VOLTAGE TO THE UNITS, BE
ABSOLUTELY SURE YOU HAVE THE CORRECT VOLTAGE VALUES AND THE CORRECT POLARITY
OR YOU WILL TOTALLY FRY THE DEVICE.
Q. 57. How can I tell if a single-ended
terminator is active or passive? Answer: If the
terminator is not marked, try the following: With a multimeter that will
measure low resistance values, measure from a TERMPOWER pin to a GND pin. If
it reads greater than 50 ohms and you get different values with the probes
interchanged, it is an active terminator. If the resistance value is less than
50 ohms it is a passive terminator.
Q. 58. What kind of terminators does LVD
SCSI require? Answer: The LVD SCSI bus requires LVD
terminators which are a form of active terminator. If you have a multimode bus
(also called LVD/MSE), you should have multimode terminators. If you are using
multimode devices on a single-ended bus, single-ended active terminators are
okay.
Q. 59. I have heard about "high byte
termination". What is it? Answer:
Another term for high byte termination is upper byte termination. While the
Narrow SCSI bus uses only the lower byte, the Wide SCSI bus consists of both a
lower byte and an upper byte. When connecting a Narrow SCSI peripheral to a
Wide SCSI bus, you must be very careful of how the bus is terminated. If you
are locating this Narrow peripheral at the end of the Wide SCSI bus and you
are using the built in terminators of the Narrow device, you need a 68-pin to
50-pin adapter with high byte termination or the upper byte will not be
properly terminated.
Q. 60. What is "active
negation"? Answer: SCSI signals are negative true logic,
meaning that the signals are normally asserted for a "0". To change it to a
"1", the assertion was simply removed and the signal allowed to be taken to
the unasserted state by the terminators. For Ultra data throughput this method
is barely fast enough and it is not fast enough for LVD or faster data
throughputs. To overcome this, "active negation" is used to drive the signal
to the unasserted state. This allows faster transitions for LVD and faster
SCSI data throughput. The SCSI specifications recommend active negation for
Ultra SCSI devices and require it for LVD and faster.
Q. 61. How far apart should I space
peripherals on the bus? Answer: The
optimum condition is for the peripherals to be equally spaced over the entire
length of the cabling. This is attainable on internal ribbon cabling, but is
not practical when peripherals are located outside of the host. In addition,
the SCSI specifications caution against grouping of peripherals at the end of
the SCSI cable. Normally spacing peripherals at least 12 inches apart is
safe.
Q. 62. On an internal (ribbon) SCSI cable,
into which connectors do I plug my peripherals? Answer:
As long as there is a terminator or a terminated device (host adapter,
peripheral or expander) at the two extreme ends of the cabling, it does not
make any difference which connectors the peripherals are plugged into. Also,
the SCSI IDs do not have to be assigned in any particular order. Just make
sure that a given ID is not assigned to more than one device.
The SCSI
specs state that in "engineered systems" these limits may be exceeded, but
they do not define what is meant. It is best to observe these limits and, as
with any type of cabling, you should keep it to the minimum practical length
suitable for the application.
Q. 65. What are the cable length limits
for differential (HVD) SCSI? Answer: HVD SCSI can have a
total SCSI cable length (total of internal + external cabling) of up to 25
meters (82 ft). As with any type of cabling it is best to keep it to the
minimum practical length for the application.
Q. 66. What are the cable length limits
for LVD SCSI? Answer: LVD SCSI is also called Ultra 2 and
Fast-40 SCSI. Ultra 160 is also LVD. The maximum specified cable length is 12
meters (40 ft), however, in a point to point application (one initiator, one
target) you may use up to a total length of 25 meters (82 ft) of cable. As
with any type of cabling it is best to keep it to the minimum practical length
suitable for the application.
SCSI-2, also called Fast
SCSI or Fast-10 SCSI may be either single-ended or differential. Differential
may have a maximum total cable length (internal plus external cabling) of 25
meters (82 ft). Single-ended Fast SCSI may have up to a maximum of 3 meters
(10 ft) total length (internal plus external).
Q. 72. What is SCSI-3? Answer:
In common usage, the term SCSI-3, when describing a connector, refers to the
68-pin high density connector that was introduced in the SCSI-3 specifications
and was immediately implemented by many manufacturers of SCSI devices because
it eliminated the requirement of having two cables for Wide (16-bit)
SCSI.
When used to refer to the SCSI-3 specification this term is not
very definitive because the SCSI-3 specification consists of a large number of
independent documents, each with their own rev number. These various documents
specify single-ended, differential (HVD), Slow, Fast, Ultra, Ultra 2 and Ultra
3 (Ultra160) SCSI as well as SCSI Bus Expanders and several SCSI connectors.
The SPI-3 document of SCSI-3 obsoletes the HVD interface. Refer to the FAQ
"What are the differences between SCSI-2 and SCSI-3" for more information. For
a complete overview see the SCSI
Architecture Model (SAM) on the T10 Committee Website.
Q. 73. What is a SCSI-1
connector? Answer:
In general usage, this refers to the centronics type 50-pin connector that was
first defined in the SCSI-1 specifications and is used on the shielded
Alternate 2, A cable.
Q. 74. What is a SCSI-2
connector? Answer: In general usage, this refers to the
50-pin high density connector with latch fasteners that was first defined in
the SCSI-2 specifications and is used on the shielded and non-shielded
Alternate 1, A cable.
Q. 75. What is a SCSI-3
connector? Answer: In general usage, this refers to the
68-pin high density connector with thumbscrew fasteners that was first defined
in the SCSI-3 specifications and is used on the shielded and non-shielded
Alternate 3, P cable.
Q. 76. What is a SCSI-4 or SCSI-5
connector? Answer: I am not aware of the terms "SCSI-4"
or "SCSI-5" appearing in the SCSI specifications. Some cable manufacturers use
these terms for the SCA-2 (non-shielded, Alternate 4, P cable) and the VHDCI
(shielded, Alternate 4, P cable) connectors.
Q. 77. What is the VHDCI
connector? Answer:
The Very High Density Cable Interconnect (VHDCI) connector (termed the
shielded Alternate 4, P-connector) is a small connector that allows as many as
four separate 68-pin Wide SCSI connectors to be placed on one standard width
PC backplate. It has the same pinout as the 68-pin high density connector.
Some of the newer LVD host adapters include this connector.
Q. 78. What is a SCA
connector? Answer: SCA stands for Single Connector
Attachment. This connector is found in the SPI-2, SPI-3 and EPI documents of
the SCSI-3 specifications and is called the non-shielded Alternative 4, P
cable connector. It consists of 80 pins arranged as two rows of ribbon
contacts spaced on 0.05 in (1.27 mm) centers and is meant specifically for use
as the connector on devices designed to plug into hot swap backplanes. The 80
pins include all the pins of the 68-pin high density connector (shielded and
non-shielded Alternate 3, P cable) plus a number of power pins, spindle sync
and pins for the backplane to set the SCSI device ID. In order to use it in a
cabled SCSI system, you must purchase an 80-pin SCA to 68-pin (or 50-pin) high
density connector adapter. This adapter brings out the SCSI pins for the
68-pin connector, the pins for powering the drive and the pins for setting the
device ID. These adapters add a lot of capacitance and may cause problems if
you use more than two or three in a bus segment. Because most SCSI disk drives
are available with either the 68-pin connector or the SCA connector, it is
best to purchase drives with the 68-pin connector and avoid drives with the
80-pin SCA connector unless you are using a SCSI backplane.
