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Small Computer System Interface (SCSI)

Click here to jump down to Terminating information
Click here to jump down to SCSI symbols
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SCSI (1) was first released in 1986. It allows for communications at 5MHz speed using a cable carrying 8 data bits (1 Byte) simultaneously, so it gave an effective rate of 5 MBps. The standard cable is 50pin, allowing for the data, and other handshake & timing signals.
Connectors used with the original SCSI (SCSI 1): DB25, CN50, DB50, and ID50.

SCSI-2 has added more commands, most of them for CDROMs and Scanners. In addition, SCSI 2 has allowed for the doubling of the clock rate to 10MHz giving a 10 MBps transfer rate (it was called "fast" SCSI at that time). SCSI-2 also introduced the "wide" or 16 bit SCSI (using the 68 pin connector). The wide SCSI allows 20 MBps transfer rates with a 10MHz clock speed. SCSI-2 controllers are backwards compatible with SCSI (1) equipment. If the controller is a wide or 68 pin version, a 68 to 50 pin adapter would be needed to connect each older or "narrow" SCSI device. In addition to the Single Ended (SE) used with the original SCSI (1) and SCSI-2, there came a Differential version of SCSI-2. The single ended is the most frequently used; it uses a 5 Volt signal for the SCSI cable. The differential model uses a higher voltage, which allows for longer cabling lengths. NEVER plug a differential drive or controller directly onto a single ended drive or controller. The higher voltage will damage the Single ended unit's electronics. To mix HVD with Single Ended equipment, you can use a LVD/SE to HVD converter.
Connectors used with the SCSI-2: CN50, HDi30 (PowerBook), DM50, DM68, and ID50.

SCSI-3 provides for additional enhancements. With it comes Ultra SCSI which doubles the clock speed again to 20MHz. This means 8 bit or "Narrow" SCSI transfers at 20 MBps, and Ultra Wide at 40 MBps. With the gain in speed comes the disadvantage of shorter cable lengths. Ultra SCSI cabling is limited to 1.5metres with up to 4 devices attached, and 0.5metres with 7 devices.
Connectors used with the SCSI-3: DM68, and CU68.

SCSI-3, Ultra2 introduces Low Voltage Differential (LVD). This allows the clock speed to be doubled again to 40MHz, so transfer rates are now 40 MBps for Narrow Ultra2 SCSI and 80 MBps for Wide Ultra2 SCSI. Normally this would have resulted in the maximum cable length being reduced again. To avoid this, the signal voltage level has been doubled (the signal is now +5 V or -5V) as well allowing for 3 meter cables. The change in voltage levels is what makes this LVD. NOTE that LVD is different to the older DIFFERENTIAL (now called HVD). You cannot connect Differential (HVD) and Low Voltage Differential (LVD) Drives together as the higher voltage in the HVD components will damage the LVD components. However, you can use a LVD/SE to HVD converter to make the two types work in the same bus. If you attach older SCSI-1, SCSI-2 or Ultra SCSI drives to the controller all LVD drives will revert to Ultra mode, and you will be limited in cable length again.
Connectors are the same here as are used with SCSI-3 above.

Ultra/160 basically increases the data transfer rate to 80MHz (160 MBps) while using a 40MHz clock. In effect, you get twice the data transfer rate while not having to change the Ultra2 LVD cabling, although specifications for the LVD cabling have been "tightened", so some poorer quality LVD cabling may not work reliably.
Connectors are the same here as are used with SCSI-3 above.

Ultra/320 has doubled the transfer rate again. Although cabling between Ultra160 and 320 may appear similar the primary difference is in the specification of the cabling. As the data speed has increased the electrical characteristics of the cable becomes more important. Cabling outside specification may cause data skewing (where some data bits are delayed due to variations in electrical impedance on some of the lines) and signal reflections in the cable. Some Ultra160 cabling may work at Ultra320 speeds, some may not. Note also that the SCSI controller communicated directly with each device at startup and can determine transfer data rates and supported functions for each device individually. So if an Ultra320 SCSI controller is connected to an Ultra320 drive and an Ultra160 drive then it will transfer data at 320Mb/sec to the Ultra320 drive and 160Mb/sec to the Ultra160. Adding an Ultra2 Wide (40Mb/sec) drive will however cause the LVD bus to switch to Ultra2 speeds and everything will then have an 40MB/sec maximum transfer rate.
Connectors are the same here as are used with SCSI-3 above.

Fiber Channel SCSI is another SCSI-3 enhancement to be released. It allows for a 1Gb/sec and recently 2Gb/sec transfer rates using either up to 30meters of copper cabling or up to 10km or fiber optic cabling. This provides the backbone for a SAN (Storage Area Network), with Servers, hard drives and other storage devices located on a separate fiber network and all data storage is in a single physical pools. Each of the servers is provided with an allocation from the SAN data storage pool. This provides the advantage of easily reallocating storage capacity between servers, and allowing for a "server less" backup with the tape system directly backing up the storage system. Fiber Switches and Hubs are available to control the fiber network and result in a network layout similar to a twisted pair Ethernet layout although all devices actually run in a ring type topology.


As a signal is sent along the cable, when it reaches the end, it may be reflected back along the cable causing interference with the next signal. For this reason each end of the SCSI cable must be terminated. The terminators basically try to ensure that signal is not reflected back along the cable. There are several types of termination: Passive, Active, and Forced Perfect.
SCSI Controllers have terminators built into them, so you would only have to worry about the other end of the cable, but each end of a SCSI cable MUST be terminated and there MUST NOT be any terminated devices in between.

This is correct:
  Device 1   Device 2   Device 3
This is not:
  Device 1   Device 2
  Device 3

As well it is the PHYSICAL END of the cable that must be terminated, not the last device. So you should put devices on each end of the cable and terminate them. (Or just put a Terminator on the last connector of the cable).

The arrangement shown below is not acceptable.
  Device 1   Device 2
Try one of these arrangements:
  Device 1   Device 2

  Device 1   Device 2   Terminator

Types of Terminators
This type uses an array of resistors to try to prevent signals being reflected back down the cable. This type of terminator is usually used for lower speed SCSI-2 devices such as tape units and CD-ROM drives.

This type used a voltage regulator to try to hold the end of the cable at a constant voltage, and absorb any signals reaching the end of the line. This is more effective than the Passive Terminator, and is the usual type used on a SCSI bus.

Forced Perfect
This type is used mainly when there are SCSI bus errors occurring, and other terminator types have not resolved the problem. The terminator attempts to force the signal line to a specific voltage, and uses a voltage regulator configuration that acts faster than an Active Terminator can. It also requires more power than an Active terminator, and this may cause problems in the supply of termination power.

Low Voltage Differential Terminator: If you plan to run your bus in LVD mode, you will need an LVD or LVD/SE terminator or a Twist 'n Flat cable that is terminated with one of these types of termination. If you are content to run in SE mode, then an Active terminator will do. KEEP IN MIND THAT ACTIVE TERMINATORS ARE FOR SE MODE, YOU NEED AN LVD OR LVD/SE TERMINATOR TO RUN IN LVD MODE. People commonly think you need active termination for LVD, which is incorrect.

stands for "Multimode Low Voltage Differential and Single Ended". Most LVD devices support LVD/SE. The term multimode is very tricky though, because it makes you think that devices can run in LVD and SE mode at the same time. This is not the case, a LVD/SE drive must run in LVD or SE mode.

SCSI Types / Symbols
SE SE SCSI (also called Single-Ended SCSI) - Most SCSI devices use SE SCSI signaling. 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 or HVD Differential SCSI (also called HVD or High Voltage Differential SCSI) - Differential SCSI is completely incompatible with SE SCSI above because it uses "differential" signaling rather than "single-ended" signaling. 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, however our ACC7229 can be used to connect a HVD device to a LVD bus or the other way around. Note: Never mix Differential SCSI with SE or LVD SCSI without the ACC7229 converter.
LVD 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.
LVD/SE LVD/SE equipment is equipment that can work either LVD or SE; however, the two can't be mixed on the same bus. The entire bus must be one or the other.
In the tables below, the connector on the left is typically found on devices while the connector on the right is typically found on cables. Drawings are in the same scale.
SCSI-1, Used with the older Apple Macs, Zip drives, and many scanners. Old Future Domain controllers have a DB25 with a different design then other devices with the DB25.
DB25 Female
DB25 Male
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Click here to see SCSI Cables with DB25
Click here to see SCSI Adapters with DB25
Click here to see SCSI Terminators with DB25

SCSI-1, SCSI-2, SCSI "narrow", Typically, almost all internal 8 bit SCSI controllers & drives use this connector. The "Male" is sometimes called the "Socket" since it is found on the device (controller, motherboard, or drive). The "Female" is sometimes called the "Plug" since it is found on cables.
ID50 Male
ID50 Female
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SCSI-1, SCSI-2, Appears mostly on peripherals, the peripheral most often has a Female connector and the cable has the Male.
CN50 Female
CN50 Male
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Apple PowerBook, Not suitable for connecting multiple SCSI devices.
HDi30 Male
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SCSI-1, Usually found on old Sun Sparc Stations. This is less common but has been used on DEC, HP and Sun SCSI devices.
DB50 Female
DB50 Male
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SCSI-2, SCSI "narrow". Most 8-bit "Fast SCSI" controllers use this connector. Often referred to as the SCSI-2 connector. From a distance this and the DM68 look almost the same. The DM50 is 1-3/8" wide where the DM68 pin is 1-7/8" wide.
DM50 Female
DM50 Male
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SCSI-3, Ultra Wide, LVD, HVD. The mating area of these is the same for both external and internal, but the internal connectors tend to be designed for internal ribbon cables. This is a wider version of the SCSI-2, the 68 pin is 1-7/8" wide while the DM50 is only 1-3/8" wide.
DM68 Female
DM68 Male
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Click here to see External SCSI Cables w/DM68 or Click here for Internal Cables w/DM68
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VHDCI .8mm, HPCN68, sometimes called SCSI-5, very popular with RAID cards. (VHDCI=Very High Density Centronics Interface). This connector is quite a bit smaller and densely packed. Also called an Ultra2 SCSI-P.
CU68 Male
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SCA, Found on internal SCA drives. Also called HP Centronics 80, Micro Centronics 80, or HD Centronics 80. 68 pins are used for SCSI (LVD or SE) and then the extra pins are used to provide power to the drives. The pins are numbered backwards from what we're used to.
CH80 Female
CH80 Male
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Click here to see Cables with CH80
Click here to see Adapters with CH80
Most SCA drives have built in terminators

Other High Density Centronics
These connectors are not seen too often, the CH60 was found on IBM RS6000 equipment. There is also some IBM equipment with the CH68 connector. These pictures show only the connector that's found on the cable.
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