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SCSI extenders are stand-alone modules or cards used to extend the distance at which peripheral devices may be placed from the host computer. Standard SCSI cabling is restricted to 5 meters in single ended mode and 25 meters in differential.  This design creates significant network limitations, especially in relation to sharing peripherals.  The use of SCSI extenders allows the network to be configured so that large peripherals including tape libraries and RAID arrays may be shared and linked to the network even if they are as far away as 33,000 feet (depending upon the power and style of the SCSI extenders in use, generally in connection via a fiber optic cable) from the server or workstation.  While SCSI boosters and repeaters may be used to regenerate the bus and provide the same type of connections, they must be used in multiples to extend long distances, and this may cause skew problems.  SCSI extenders may use standard SCSI cables for parallel SCSI, coaxial cable for serial SCSI transmission, or fiber optic cable for high-speed, large-distance serial transmission.  Note: when using SCSI-3 they are referred to as SCSI expanders instead of SCSI extenders.

 

Small Computer Systems Interface (SCSI) is an intelligent I/O parallel peripheral bus with a standard, device-independent protocol that allows many peripheral devices to be connected to the SCSI port. A single SCSI bus can drive up to eight devices or units: the host adapter or controller, and seven other devices. Each device is assigned a different SCSI ID, ranging from 0 to 7.

How SCSI Extenders Function

SCSI extenders function by converting SCSI parallel signals into serial data, which is then transmitted to another extender that converts the information back to SCSI signals. The two extenders communicate in full duplex mode and provide transparent communications for the SCSI bus.  They may be used in conjunction with any application where converting single-ended devices to LVD, or where longer cables on a single-ended SCSI interface are required. For example: RAID Systems and remote arrays, DLT & DAT Tape Drives, Jazz Drives (removable media), streaming tape drives, optical drives, laser printers, Hard disk drives, optical CD-ROM and CDR drives, optical juke boxes, etc.

 

SCSI extenders are either single ended or differential.  With single ended devices, each signal is carried on only one wire while the other wire is grounded.  In differential SCSI the signal transmitted is the voltage difference between the two lines of the cable.  Differential SCSI extenders are dived into two types, HVD (where 5 volts represent the SCSI signals), and LVD (which uses 3.3 volts to represent the SCSI signals).