Small Computer System Interface (SCSI)


SCSI Terminating
SCSI Symbols
Find SCSI Cables by Connector

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
DB25 Female
DB25 Male
CN50
CN50 Female
CN50 Male
DB50
DB50 Female
DB50 Male
ID50
ID50 Male
ID50 Female

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
CN50 Female
CN50 Male
HDi30M
HDi30 Male
DM50
DM50 Female
DM50 Male
DM68
DM68 Female
DM68 Male
ID50
ID50 Male
ID50 Female

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
DM68 Female
DM68 Male
CU68
CU68 Male

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.