An Advanced Guide To SCSI

Below are the three types of SCSI Signal

SE 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 device.

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.


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 active terminator.



Although SCSI 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

Male "centronics" 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 it.

SCSI Connectors


Commonly used 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.

Centronics 50

Often called a SCSI 1 connector or a SCSI-1 A-cable connector. A very common SCSI connector.

HPCentronics 50

Similar to a Centronics 50 (above) but much smaller.


Used primarily on Macintosh PowerBook computers. Not suitable for connecting multiple SCSI devices.


Although not very common, this connector has been used on several DEC, HP, and Sun SCSI devices.


 (also called 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.


 (also called Micro/HD DB68) - Often called a SCSI-3 P-cable connector and is used both internally on ribbon cables and externally

HP Centronics 60

(also called Micro/HD Centronics 60) - This connector is typically used in IBM RS/6000 applications.

HP Centronics 68

(also called Micro/HD Centronics 68) - This connector is typically used in IBM RS/6000 applications

SCA 80-Pin

(also called 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.


 (also called 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.


SCSI Termination

SCSI terminators 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.

SCSI Terminator Types
Be sure to read this entire section before making any decisions about terminators.

Passive 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.

Active 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.

FPT Terminators ( 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.

Differential 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.

Active Negation 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.

Feed-through 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.

High Byte 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

The important 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).