Below are the three types of SCSI Signal
SCSI (also called
Single-Ended SCSI) - Most SCSI devices use SE SCSI
signalling. In SE SCSI each signal is carried by a single
wire. SE SCSI is very susceptible to noise and has a
rather short distance limitation, a maximum of 6 meters.
Unless a device says otherwise, it is probably a SE SCSI
Differential SCSI (also called HVD or High Voltage
Differential SCSI) - Differential SCSI is completely
incompatible with SE SCSI above because it uses
"differential" signalling rather than "single-ended"
signalling. The benefit of using differential SCSI is that
it works well in noisy areas and can reach up to 25 meters
in distance. Caution: Unless a SCSI device, controller, or
cable adapter specifically says "differential" or "HVD",
it will probably not work with other differential SCSI
devices. Because of the benefits of LVD SCSI (below),
differential SCSI is becoming less popular. Note: Never
mix Differential SCSI with SE or LVD SCSI.
(also called "Low Voltage Differential" SCSI) - LVD is the
newest type of SCSI cabling, and LVD SCSI specifications
offer distances up to 12 meters and legacy support if
LVD/SE which offer LVD mode or SE mode. Most LVD SCSI
devices are LVD/SE, however you can only run in SE mode or
LVD mode. If one device on your SCSI bus is SE, all
devices will be limited to SE limitations. All devices
must be set to LVD to achieve LVD distance and speed
capabilities. Note that LVD SCSI cabling requires "Twist
and Flat" ribbon cable and an LVD/SE terminator or a
"Twist and Flat" ribbon cable with built-in LVD
termination. If operating in SE mode, you only need an
SCSI CAPABILITIES AND LIMITATIONS:
devices are often referred to as SCSI 1 devices, SCSI 2
devices, SCSI 3 devices, and SCSI 5 devices, the definitions
of each of these standards is very confusing. This is because
technical standardization terminology gets mixed up with
market terminology. The result is that very few people truly
understand the difference between these definitions. When
trying to understand SCSI connectivity, it is often better to
understand the meaning of the terms listed below, especially
the term Narrow SCSI (which means 8 bits) and Wide SCSI (which
means 16 bits).
Identifying Male or Female SCSI Connectors
type SCSI connectors have an "island" that sticks up in the
middle of the connector with flat pins around it. The female
"centronics" type connector has a hole in the middle of the
connector with flat pins against the side of the hole. Male
"DB" type connectors have "pins" that stick up inside the
connector. The female "DB" type connectors have holes inside
the connector. The male IDC50 is often called IDC50S ("S"
stands for "socket"), and this is the end that has the little
pins inside. The female IDC50 is often called IDC50P ("P"
stands for "plug"), and this is the end with the holes inside
on SCSI scanners, zip drives, and Apple/Mac computers.
This is a
commonly used internal SCSI connector. The male IDC50 is
often called IDC50S ("S" stands for "socket"), and this is
the end that has the little pins inside. The female IDC50
is often called IDC50P ("P" stands for "plug"), and this
is the end with the holes inside it. Also called an
internal SCSI-1 or SCSI-2 A-cable connector.
Often called a
SCSI 1 connector or a SCSI-1 A-cable connector. A very
common SCSI connector.
Similar to a
Centronics 50 (above) but much smaller.
on Macintosh PowerBook computers. Not suitable for
connecting multiple SCSI devices.
very common, this connector has been used on several DEC,
HP, and Sun SCSI devices.
Micro/HD DB50) - Most 8-bit "Fast SCSI" (see above)
controllers use this connector. Often called a SCSI 2
connector or SCSI-2 A-cable connector.
called Micro/HD DB68) - Often called a SCSI-3 P-cable
connector and is used both internally on ribbon cables and
Micro/HD Centronics 60) - This connector is typically used
in IBM RS/6000 applications.
Micro/HD Centronics 68) - This connector is typically used
in IBM RS/6000 applications
an HP Centronics 80, Micro Centronics 80, or HD Centronics
80) - Note that LVD drives with and SCA interface need LVD
SCA adapters. SCA drives can operate in SE mode, but they
still require SCA 80 adapters. The extra pins are used to
provide power. Often called a SCSI 3 connector.
0.8mm or Very High Density Centronics/Connector Interface)
- This is a very small connector that is very tightly
pinned. Also called an Ultra2 SCSI-P-cable connector or a
SCSI 5 cable connector.
must be placed at each end of the SCSI bus. The SCSI
controller card can be at one end of a SCSI bus or in the
middle of a SCSI bus. Most SCSI controller cards have built-in
terminators that are turned on by default but can be disabled
if you place your SCSI card in the middle of your SCSI bus. If
you turn your SCSI card's terminator off, you must then attach
a terminator at each end of the SCSI bus (two terminators
instead of one). See sample SCSI buses below.
NB: Can a SCSI Bus have four terminators? Yes. See
section on High-Byte Terminators.
Be sure to read this entire section before making any
decisions about terminators.
Terminators - If you have a DB25 or Centronics 50 (CHAMP)
connector anywhere on your SCSI bus, you are probably using
SCSI-1 cabling standards ( Narrow 8-bit SCSI ) and can often
use a passive terminator. However, a passive terminator is not
recommended when using any Fast SCSI, Wide SCSI, or Ultra SCSI
devices. Passive terminators are not recommended when there
are more than two SCSI devices on your SCSI bus (besides the
SCSI controller). It uses simple resistors to terminate the
bus Passive termination is fine for short, low-speed SCSI
buses but is not suitable for Fast SCSI or any other advanced
applications. Uses resistors for each signal line as voltage
dividers. Signal reflection is blocked out but not controlled.
Terminators - Active terminators are the most common type
of SCSI terminators and usually used in SE SCSI cabling (see
SE SCSI section) with Fast SCSI devices on your SCSI bus.
Active terminators use voltage regulators to produce a desired
termination voltage and control impedance which allows for
more reliable and consistent termination of the SCSI bus.
Better than passive termination, active termination is the
minimum required for any of the faster-speed SCSI buses. Note:
Active termination = Passive Termination + Voltage Regulation.
( Forced Perfect Terminators ) - FPT Terminators are better
than active terminators in one useful way - They allow extra
distance to your SCSI bus when using Fast SCSI devices. It is
highly recommended that you use an FPT terminator when you
have a high speed SCSI system that has many different types of
devices and cables that might produce impedance mismatches and
degrade the signals. FPT alters its impedance to actively
compensate for these variations by means of diode switching
and biasing and usually feature troubleshooting LEDs. Diode
clamps are added to the circuitry to force the termination to
the correct voltage. This virtually eliminates any signal
reflections or other problems and provides highly effective
termination of a narrow SE SCSI bus.
Terminators - Differential terminators ( also called HVD
terminators or High Voltage Differential terminators ) are not
compatible with Single-Ended SCSI ( SE SCSI ) cabling or Low
Voltage Differential SCSI ( LVD SCSI ) cabling. When using
differential SCSI devices, you must use differential
Terminators - Used with 8-bit Ultra SCSI devices in SE
mode ( single-ended SCSI mode ), not in LVD mode. LVD/SE
terminators (see below) have become more popular when
terminating these devices because LVD SCSI terminators are
backwards compatible ( downwards compatible ) with older
versions of SCSI cabling and single-ended SCSI ( SE SCSI )
cabling. However, LVD SCSI cannot be mixed with HVD SCSI (
also called differential SCSI or High Voltage Differential
SCSI ). See section on Differential SCSI Terminators.
Terminators ( also called Pass-through Terminators ) -
This terminator is usually used when you have no connectors
available on the end of a SCSI bus to attach a terminator.
This type of terminator is placed between your last connector
and your last SCSI device on your SCSI bus.
LVD Terminators ( or LVD/SE Terminators ) - LVD SCSI
terminators ( or Low Voltage Differential SCSI terminators )
are commonly used for Ultra Wide SCSI devices and above (see
chart). LVD/SE terminators automatically sense SE SCSI mode or
LVD SCSI mode on the SCSI bus. Because LVD/SE SCSI is
backwards compatible, LVD/SE terminators can be used for SE
SCSI devices instead of active terminators. This will help
future-proof your SCSI growth.
Terminators (50-Pin / 68-Pin SCSI Adapters) - High byte
terminators are used when attaching Narrow SCSI devices (
8-bit SCSI ) to the end of a Wide SCSI bus ( 16-bit SCSI ). If
adding Narrow SCSI devices in the middle of a Wide SCSI bus,
you can simply attach a 50-pin/68-pin SCSI adapter without
built-in high-byte termination. For example, assume you have a
Wide SCSI controller card which goes to a Wide SCSI hard drive
and then to another Wide SCSI drive and then to a Narrow SCSI
hard drive. You must attach a high-byte terminator between
your 16-bit Wide SCSI bus and your Narrow SCSI hard drive to
terminate half of the 16-bits so that only the remaining
8-bits go to your Narrow hard drive(s) ). Doing this allows
your wide SCSI controller (16-bit) to operate with your wide
SCSI drive at 16-bit speeds and your narrow (8-bit) drive to
operate at 8-bit speeds properly (on the end of your SCSI
bus). Note: 8 bits equals 1 byte. See sample SCSI bus below.
Problems with Mixing 50-Pin and 68-Pin SCSI Devices
thing to understand when mixing 50-pin SCSI (Narrow SCSI or
8-bit SCSI) and 68-Pin SCSI (Wide SCSI or 16-bit SCSI) is
this: If you put a narrow SCSI device in the middle of a Wide
SCSI bus, you can use a regular 50-pin-to-68-pin SCSI adapter
(an adapter without built-in high-byte termination) and still
have all wide SCSI devices transferring data at wide speeds.
However, if you are attaching a narrow SCSI device to the end
of a Wide SCSI bus, you must use a high byte terminator. For
more details, see section on High Byte Terminators. Below are
two examples of a SCSI bus with a 68-pin-to-50-pin SCSI
adapter with a high-byte terminator and a SCSI bus with a
regular 68-pin-to-50-pin SCSI adapter (without a high-byte
terminator). Note: The first example is typically an external
SCSI situation, and the second example is typically an
internal situation (a ribbon cable situation).