High Voltage Differential (HVD) Tutorial

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The ins and outs of buying HVD equipment.

High Voltage Differential is different from the regular SCSI you encounter in most computer stores today.

1) Signaling: High Voltage Differential uses "Differential" signaling as opposed to "Single Ended" signaling. This makes HVD and SE completely incompatible with each other. HVD to SE converters commonly cost $300.00+. New HVD controllers are $150.00+. (Your adapter AHA-2940UW will NOT work with HVD equipment. You would need the AHA-2944UW.)

2) Pinout: When you go from one type of connector to the same style, all SCSI uses the same pinout: "straight through". This means that any SCSI cable will work (the pinout will match) as long as the connectors are the same on both ends. (ex. 68-68 50-50 but not 68-50 or HD50-Centronics50). When you go from 68-50 (which is the most common adaptation) you must use adapters wired for HVD. All adapters in stores are SE unless they state otherwise.

3) Impedance: Most "Ultra" rated cable has the proper impedance for SE or HVD, even though the ratings are different. I will illustrate:

Cable A:

Differential Impedance: 135 ohms
Single-Ended Impedance: 90 ohms

Cable A has two different ratings. This is because impedance changes based on which type of signaling/testing you use. Cable A has the typical value of a Fast-20/Ultra Madison cable. We use Fast-20(40MB/Sec.) Madison cable for all of our SE external cables unless otherwise stated (which means our cables work for HVD as well. Refer to point #2 for more details).

4) Why HVD???: HVD allows a maximum bus length of 25 meters. It is commonly used for long runs in noisy areas.

5) Why not HVD???: HVD was great for it's purpose in the past, but a new technology has arrived that makes HVD obsolete. LVD aka Low Voltage Differential is out, allowing 12 meters of maximum bus length. LVD supports Ultra2 Wide, allowing a bus bandwidth of 80 to 160 MB/Sec.

High Voltage Differential Information

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