SCSI Technology

GUIDE: SCSI Technology

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SCSI stands for Small Computer System Interface.
A standard for high-speed connections to peripherals.

[Source and Reference] [History] [Usage] [SCSI Hard Drives] [APPLE SCSI] [Hardware] [Single Ended or Differential] [Cabling] [SCSI Termination][SCSI Connectors]

Source and Reference


SCSI has it’s roots in the mainframe world, but it’s first implementation in the PC world came soon after the first PC. Shugart Associates devised an interface that they designated the SASI, or “Shugart Associates Standard Interface” They proposed that SASI be adopted by ANSI for small computers, but during the work required for ratification, they discovered the process would take too much effort, and that the IPI groups were already well into their effort. (which had many features the same as SASI) A decision was made to take features of both interfaces, and put forth a new specification for a new interface, SCSI was born, and ratified in 1986 by ANSI. Since then, many have said that the original spec. was not tight enough, and that it allowed Manufacturers to make drives that met the ANSI spec., but would not talk to each other. Recently, the ANSI SCSI committee has proposed newer, tighter, more extended specs., for SCSI-2, and now SCSI-3.


SCSI is used to connect peripherals to an computer. It allows you to connect harddisks, tape devices, CD-ROMs, CD-R units, DVD, scanners, printers and many other devices. SCSI is in opposite to IDE/ATA very flexible. Today SCSI is most often used servers and other computers which require very good performance. IDE/ATA is more popular due to the fact that IDE/ATA devices tend to be cheaper.

SCSI Hard Drives

The normal internal cable for SCSI is a 50 conductor ribbon, with all odd numbered conductors grounded. Two conductors, numbers 25 & 26, are often left not-connected, as they deal with Terminator power, and can be easily shorted by cable reversals. There are no twists in this cable, and it’s length may be a maximum of 6 meters. But one is advised to use minimum lengths to improve timing. Up to seven drives, or devices may be attached to an SCSI cable. Each is daisy-chained on the cable, or, when a device has two connectors, another cable may be “spliced” into the chain starting at the second connector, and continued on. Care must be taken to insure that cables and connectors are not reversed, as this would short pin 26 (TERMPWR) to ground, and likely damage the drive or controller. Also, as explained earlier, the terminating resistors should remain only on the controller (Host Adapter) and the LAST drive on the cable, regardless of it’s address. The Differential SCSI requires balanced lines, and is used mostly on high-end workstations.


Unlike in the PC world, the Apple standardised on one drive interface, SCSI. Also, Apple standardised on a 25 pin connector for external connections. However, Apple decided not to implement the complete ANSI spec., so one must be careful that peripherals used are certified to work with Apple’s SCSI bus. Apple also developed it’s own pin-configuration. The Apple and Future Domain 25-pin SCSI connectors are as close to “Standards” as there are in the world of PCs. But the real ANSI Standard called for a 50 pin connector commonly referred to as a “Centronics” type (made popular by the Centronics printer company). Instead of the 25 staggered pins of the Apple & Future Domain type connectors, the Centronics type uses 2 parallel rows of 25 pins. This arrangement allows the use of extra grounds for better isolation.


The basic SCSI bus can connect up to seven devices and 1 host adapter on it, the bus uses 3 address lines which gives you a possibility of 8 addresses. Each device is assigned a SCSI address from 0 to 7 where 7 is reserved as the host adapter. The remaining 7 addresses are used for any device connected anywhere on the bus. Each address is assigned a priority, where 7 (the host adapter) has the top priority and 0 beeing the lowest priority.

Each device can be assigned any address (except for address 7) and is user selected via DIP switches or jumpers. No two devices can have the same address on the same SCSI chain, internal nor external. Usually you can assign any address number to a device as long as the software or drivers for the specific device allow it. Some devices will not let you the freedom of selecting the address, in these cases you must make sure that the predifined address on the device does not conflict with other devices already in use. For example some systems will force you to assign the address 0 on the boot hard drive, this is true for most host adapters that emulate the Western Digital WD1002 controller.

Single Ended or Differential

The original design of the SCSI standard allowed two different types of buses, the single ended and differential. Single ended SCSI uses a single ended electrical signal (also refered to as unbalanced), this means that a single wire is used as a common ground return path for all signal lines. As of in the differential SCSI (refered to as balanced) each signal line has it’s own return path and is also isolated from the reference ground. The cable uses twisted pair wiring, this makes it less prone to electrical interference and to picking up noise. This means that differential SCSI needs more wires and more electronics, and consequently also means higher prices, so as you guest most users systems use single ended SCSI because their cheaper and work good in most applications, like small systems and where electrical noise is not a problem.

Since differential SCSI is less prone to electical interference the cable lengths are greatly improved. On single ended SCSI, the cable length is limited to 6 meters (around 20 feet) and with differential SCSI the cable length can span up to 25 meters (about 82 feet). The shortest cable must not be less than 30 centimeters (12 inches) between each device. For example, if you have 3 device connected to the host adapter, the minimum lenght of the cable should be 1 meter (3 feet).


Internal devices use a 50 pin dual inline connector and a flat ribbon cable. Multiple connectors are usually attached to it in increments of around 12 inches (yet they might be farther apart). You can use any connector on the cable and in any order.

External devices use a different cable and connector for obvious reasons. The cable is usually consisted of 25 twisted pair wires, so you get a total of 50 wires. Each external device has two connectors on it’s back side, they are connected to each other pin for pin and are used to connect the next device in the daisy chain or to connect the terminator. This means that you need a cable for each additional external device.

The standard SCSI connector has 50 pins in two rows and is termed as the type A connector from the SCSI specifications, this connector looks like a Centronics printer connector yet has 50 pins instead of 36. Some devices use a regular 25 pin D-shell connector, this connector is good, but can only handle single ended SCSI because they don’t have enough connections.

SCSI-2 specified a new cable for wide SCSI, since the original Type A cable does not have enough connections on it. The SCSI-2 specifications indicates the use of a second cable described as the B cable, yet most developers did not follow this path and use a similar cable known as the type P cable. This cable has a 68 pin connector arranged in two rows of 34 pins just like the B cable. The difference is in the pin assignments on the connector. The B cable has since been abandoned and the standard is now the P cable.

SCSI Termination

[Passive Termination] [Active Termination] [Differential Termination] [Forced Perfect Termination] [Termination Power]

On SCSI drives, the Host Adapter also has resistors. These are needed to terminate both ends of the bus. Since the SCSI bus can have up to 7 devices attached to it, only the Host Adapter and the device farthest from it will retain the resistors. All devices in-between should have theirs removed. With the advent of increased use of SCSI for peripherals comes the chance that one day you’ll need an SCSI terminating resistor. Prepare for a shock, because you might be very surprised at the prices charged, for what you get. Many Manufacturers still have SCSI peripheral hardware priced for the Workstation market, not the PC market. We may see these prices erode as more PCs adopt SCSI as their disk interface of choice, but for now be prepared to pay a premium for anything to do with SCSI. So here you are, with a disk drive mounted internally, and a CD-ROM hanging off the back of the PC. Everything looks great, but it just doesn’t work… Maybe it doesn’t even recognise the CD-ROM. You’ve checked the connectors, and everything looks good… So what’s the problem? Well, did you check the terminators? (Say Whaaat??) Improper termination of an SCSI bus can raise havoc with the Host Adapter’s interface circuit, and result in missing peripherals, or intermittent operation and possible loss of data. Well, here’s a way to build an inexpensive terminator that will connect to the second SCSI connector on many SCSI peripherals.

SCSI termination is very important, there must be two terminator on each SCSI chain, one at the beginning and one at the end device chain. The terminator must be within 10 centimeters (4 inches) of each end of the chain on the bus. Take a look at figure 1 to see what is acceptable and what’s not when terminating SCSI devices.

The following ARE acceptable:

     |            |          |           |           |         |
  DEVICE A     Unconnected Unconnected Unconnected DEVICE B   DEVICE C
 Terminated                                       (adapter-Terminated)
     |            |          |           |           |         |
  DEVICE A  Unconnected  DEVICE B  Unconnected  Unconnected  DEVICE C
 Terminated              (adapter)                        Terminated
     |            |          |           |           |         |
  Terminated   DEVICE A  DEVICE B  Unconnected  Unconnected  DEVICE C
               (adapter)                                  Terminated

The following ARE NOT allowed:

     |            |          |           |           |         |
  DEVICE A     DEVICE B   DEVICE C    Unconnected Unconnected Unconnected
 Terminated    (adapter)  Terminated
     |            |          |           |           |         |
 Unconnected   DEVICE A     DEVICE B   DEVICE C   Unconnected Unconnected
              Terminated    (adapter)  Terminated

Figure 1: Correct and wrong way of connecting SCSI devices

Note: When putting the host adapter in the middle of the chain, you must disable the host terminators. If this is impossible you MUST put the host adapter at either end of the cable.

Passive Termination

Termination of the SCSI chain is achieved by using resistors. The termination will give you an electrical equivalent of a 3 volt voltage source in series with a 132 ohm resistor. This voltage is achieved by connecting a 220 ohm resistor to the TERMPWR line and through a 330 ohm resistor to ground. All signals must terminated at both ends of the bus in this fashion.

 TERMPWR --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  220 ohms
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
 SIGNALS   O  O  O  O  O  O  O  O  O  O  O  O  O  O  O
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
           R  R  R  R  R  R  R  R  R  R  R  R  R  R  R  330 ohms
           |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
 GROUND  --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Figure 2: Passive termination

Active Termination

Active termination is achieved by using a voltage regulator to source a 2.85 VDC level in series with a 110 ohms resistor to the signal line. This assures a constant voltage and helps eliminate noise in a greater way than with passive termination, where variations in TERMPWR affects greatly the voltage actually being put on the signal lines. An example of how an active terminator looks like is presented in figure 3.

            2.85 Volt Regulator
                +-----------+    +2.85V   110 Ohms
 TERMPWR   -----| in    out |------+-----/\/\/\/----SCSI signal
                |   gnd     |      |
                +-----------+      |
                     |             +-----/\/\/\/----SCSI signal
                     |             |
 GND  ---------------+             |
                                   +-----/\/\/\/----SCSI signal

Figure 3: Active termination

This type of termination is critical when using many devices and when using long cables. If you have many external devices connected on the bus this would also be good alternative.

Differential Termination

When using differential SCSI, the termination is different. They use a network composed of 3 resistors (2x 330 ohms and 1x 150 ohms), these resistors are connected as follows in figure 4. This type of termination is needed since each signal has it’s own return path.

  TERMPWR  ----+----------------------+---------------+
               |                      |               |
               R  330 ohms            |               |
               |                      |               |
  SIGNAL - ----+                      |               |
               |                      |               |
               R  150 ohms           === 0.01 uF     === 2.2 uF
               |                      |               |
  SIGNAL + ----+                      |               |
               |                      |               |
               R  330 ohms            |               |
               |                      |               |
  GROUND   ----+----------------------+---------------+

Figure 4: Differential termination

Forced Perfect Termination

Another method used for termination is FPT (Forced Perfect Termination). This method uses diode clamps to eliminate over and undershoot. Instead of clamping to +5 and GND they clamp to the output of two regulated voltages. This allows the clamping diodes to turn on earlier and is therefore better at eliminating overshoot and undershoot. Although this method would probably very good, it is not widely used.

Termination Power

Termination power is usually supplied by the host adapter, but any device is allowed to provide this power. To make sure that if another device on the chain is giving power to the TERMPWR signal line, they usually supply the power through diodes to prevent termination power from other devices from flowing back into the device. They usually also provide for a current limiting device so not to overload the signals and risking damage to devices connected on the bus.

SCSI Connectors

External Connector Types (not to scale)

Number of
Devices (A,B)
Max Total
Length of Cable
Uses Comments
50-pin low-density 50-pin low-density (SCSI-1, Centronix, A-Cable)

Apple DB-25>Apple DB-25C
SCSI-1 5 8 6 mDScanners, Zip drives, hard drives, etc. The DB-25 version is standard on Macintosh® computers.C
Many peripherals are available.
SCSI-2 (Fast SCSI, Fast Narrow)
50-pin low-density 50-pin low-density (SCSI-1, Centronix, A-Cable)

50-pin high-density 50-pin high-density (SCSI-2)

10 8 ²4 devices: 3 m
5+ devices: 1.5 m
Desktop hard drives, scanners, CD-ROM drives, CD-Recordable drives, and removables Backward-compatible with SCSI-1. Many products available.
“Wide” products use 16-bit instead of 8-bit data paths.
Fast Wide SCSI (Wide SCSI) 68-pin 68-pin

20 16 ²4 devices: 3 m
5+ devices: 1.5 m
Faster hard drives, workstations, servers and RAID systems Backward-compatible with SCSI-1. Many products available.
“Wide” products use 16-bit instead of 8-bit data paths.
50-pin low-density 50-pin low-density (SCSI-1, Centronix, A-Cable)

50-pin high-density 50-pin high-density (SCSI-2)

68-pin 68-pin
Ultra SCSI
(SCSI-3, Fast-20, Ultra Narrow)
20 8 1.5 mD,LVDNewer and faster hard drives, performance desktop, workstations, servers, RAID systems Backward-compatible with previous SCSI versions.
SCSI-3 specification still under consideration, but products are available that meet current specifications. “SCAM” specification allows for devices to set own ID automatically.
Some variations of SCSI-3 are hot-pluggable.
Ultra2 devices available now.
Ultra3 devices under development.
Wide Ultra SCSI
(Fast Wide 20)
40 16 1.5 mD,LVD
Ultra2 SCSI 40 8 12 m
Wide Ultra2 SCSI 80 16 12 m
Ultra3 SCSI 80 8 12 m
Wide Ultra3 SCSI 160 16 12 m

(A): Including Host Adapter
(B): Per channel
©: The Apple DB-25 is not an official standard. It uses SCSI-2 protocols, but at SCSI-1 speeds.
(D): Differential allows for cable lengths of up to 25 meters.
(E): The DB-9 Fibre Channel connector is the industry standard, but only the four outer pins are live.
(LVD): Low Voltage Differential allows for cable lengths of up to 12 meters.
(F): Standard systems carry 2 channels.


© 1998-2017 – Nicola Asuni - - All rights reserved.
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