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Definition: SCSI signaling

Following are the three types of SCSI signaling, which determine the total length of a SCSI chain. See SCSI.

Single Ended (SE)
Single-ended SCSI allows devices to be attached to a total cable length up three or six meters, depending on type (see below). Single-ended signaling uses data and ground lines.

High Voltage Differential (HVD)
Also called just "differential SCSI" because it was available before there was a second differential option, HVD signaling supports cable lengths up to 25 meters. In order to increase distance, differential signaling uses data low and data high lines and costs more than single-ended (see differential signaling).

Low Voltage Differential (LVD)
Ultra2 SCSI introduced LVD signaling, which supports cable lengths up to 12 meters. LVD requires less power and is less costly than HVD, because the transceivers are built into the controller chips.

SCSI Versions
The different SCSI types provide backward and forward compatibility. If a new SCSI host adapter is used with an older SCSI drive, the drive will run at its maximum speed. If an older SCSI host adapter is used with a newer drive, the drive will run at the host adapter's maximum speed. The following data are courtesy of the SCSI Trade Association (STA), San Francisco (www.scsita.org).


            Rate  Bus Dimensions
 SCSI        MB  Width  Length(m)  #  Max
 Type       Sec  bits SE LVD HVD Pins Dev

 SCSI-1        5   8   6  -  25   25   8
 Fast SCSI    10   8   3  -  25   50   8
 Fast Wide    20  16   3  -  25   68  16

 Ultra        20   8  1.5 -  25   50   8
 Ultra        20   8   3  -   -   50   4

 Ultra Wide   40  16   -  -  25   68  16
 Ultra Wide   40  16  1.5 -   -   68   8
 Ultra Wide   40  16   3  -   -   68   4

 Ultra2       40   8   - 12  25   50   8
 Ultra2 Wide  80  16   - 12  25   68  16

 (Ultra160)  160  16   - 12   -   68  16

 (Ultra320)  320  16   - 12   -   68  16

 Serial Attached SCSI (SAS) Speed
  (for comparison)

 (2003)       1    3.0 Gbps (300 MB/sec)
 (2009)       1    6.0 Gbps (600 MB/sec)
 (2013)       1   12.0 Gbps (  1 GB/sec)
 See serial attached SCSI.

The Advanced SCSI Programming Interface (ASPI) and Common Access Method (CAM) provide common software interfaces between drivers and SCSI host adapters. ASPI was developed by Adaptec, and CAM is an ANSI standard. Most SCSI products are ASPI or CAM compliant. Prior to ASPI and CAM, hooking up two SCSI devices often meant plugging in two host adapters, negating SCSI's advantage of connecting multiple peripherals.

IDs, LUNs and Termination
External SCSI devices have two ports, one for the incoming cable and another for the outgoing cable to the next device. An internal SCSI device has a single port that attaches to a ribbon cable with multiple connectors. Each device must be set to a unique ID number, which is normally done by flipping rotary switches on external devices or by setting jumpers on internal ones. The ID determines the device priority, which starts at 7 and goes to 0 and then from 15 to 8. The host adapter defaults to 7, the highest priority.

SCAM Sets IDs Automatically
A subset of Plug and Play, called "SCSI Configured Automatically" (SCAM), allows IDs to be set by software rather than manually. Both the host adapter and peripheral must support this.

Logical Unit Numbers (LUNs)
Each SCSI device can be further broken up into logical units, identified by logical unit numbers (LUNs) 0 to 7 for 8-bit devices and 0 to 15 for 16-bit devices. Although most SCSI drives contain only one disk inside and are addressed as LUN 0, optical disc libraries and RAID arrays contain multiple drives, each of which can be addressed independently via LUN numbers. See LUN.

The device at the end of a SCSI chain must be terminated by setting a switch or plugging a resistor module into the open port. Usually, host adapters default to terminated. If both internal and external devices are used, the host adapter termination must be removed, and termination must be applied to the ends of both chains.