Adaptec
 
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Differential SCSI

SCSI devices use Single-Ended (SE), Low Voltage Differential (LVD or Ultra2), or High Voltage Differential (HVD) electrical interfaces between the device and host adapter. A single-ended interface uses one wire for each data or command/status line, with a corresponding ground wire for that signal. Voltage is carried only on the signal line.

A differential interface uses signal transmitters and receivers to drive the signal over longer distances and for noise rejection. The signal is split into positive and negative components and transmitted over cable pairs. Voltage is carried on both wires. The signal is re-combined at the receiving device. If a single-ended device such as a CD-ROM is connected to a differential host adapter (e.g., AHA-2944UW), or a differential device (e.g., a hard drive with a D suffix or differential tape drive) is connected to a single-ended host adapter such as the AHA-2940UW, irreparable damage can be done to either or both devices. Adapters are available from third-party suppliers which adapt differential devices to single-ended uses. These third party adapter configurations are not supported by Adaptec.

Low Voltage Differential (LVD) devices are also known as ULTRA2 and should be used only on multi mode adapters which support ULTRA2 and/or SE devices.(The AHA-2930U2, AHA-2940U2W, and AHA-3950U2 support both LVD and SE devices).


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