SCSI Signal Technology

      Single-Ended SCSI This is what normal SCSI is sometimes referred to. Each signal that needs to be carried across the bus has a dedicated wire for it. Cable lengths are limited to 6m for SCSI-1 and SCSI-2 and 3m for all other non-differential SCSI standards.  
      Differential SCSI Differential SCSI involves a pair of dedicated wires for each signal across the bus, versus a single wire. One wire carries the same signal as single-ended SCSI. The other wire carries a logical inversion of the original signal. The receiving device compares the two signals and takes the difference of them, making the bus less susceptible to noise. This allows for cable lengths of up to 25m for high-voltage differential (HVD) SCSI and 12m for low-voltage differential (LVD) SCSI.  
      Mixing Differential and Non-Differential SCSI Devices You cannot mix SE and HVD or LVD and HVD devices on the same bus. You can literally fry the components. However, you can purchase a converter that allows HVD devices to work on an LVD subsystem. Also, both 68-pin and 50-pin connectors on HVD controllers "must" have HVD devices connected.  
        With the design of LVD SCSI, companies are able to design SCSI controllers that allow both non-differential and LVD devices to be on the same bus. Just remember... any LVD device connected to an SE subsystem, or any LVD device that coexists with an SE device, will run at the maximum speed alotted for the SE device. This will also entail shorter cable lengths.  
      HVD Availability HVD SCSI was a short-lived standard. There few peripherals that support it. Adaptec's 2944UW and 3944UW SCSI controllers are a popular option. CSC also produced a Symbios-based HVD controller as well. Quantum's Atlas II UWSCSI drives have an HVD option. The best way to obtain HVD hardware is to visit an auction site (such as E-Bay).  

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