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SCSI, IDE, and FireWire specs at a glance

James McPherson has a quick summary of the specs for the seven generations of SCSI, plus IDE and FireWire for comparison.


With seven distinct generations of SCSI, figuring out how one version differs from another can be daunting. Recently, I wrote a Daily Drill Down giving an overview and history of the seven generations of SCSI technology (see A SCSI primer: Seven generations of high-performance hardware). For those of you who may want a quick summary, I’ve listed the specs without all the explanations in this Daily Feature. I’ve also included IDE and FireWire information for comparison. And, for those of you who are unfamiliar with the terminology, you’ll find a short SCSI glossary here as well.

SCSI-1
  • Maximum bus speed: 5 MBps
  • Bus width: 8 bits
  • Bus length:
    6 meters, Single Ended
    12 meters, using Low Voltage Differential (LVD)
    (Did not originally support LVD. If all the devices on a chain use LVD mode, use a 12m length. Otherwise use the Single Ended bus length.)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 8

SCSI-2 (Fast SCSI)
  • Maximum bus speed: 10 MBps
  • Bus width: 8 bits
  • Bus length:
    3 meters, Single Ended
    12 meters, using Low Voltage Differential (LVD)
    (Did not originally support LVD. If all the devices on a chain use LVD mode, use a 12m length. Otherwise use the Single Ended bus length.)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 8

Fast Wide SCSI (Wide SCSI 2)
  • Maximum bus speed: 20 MBps
  • Bus width: 16 bits
  • Bus length:
    3 meters, Single Ended
    12 meters, using Low Voltage Differential (LVD)
    (Did not originally support LVD. If all the devices on a chain use LVD mode, use a 12m length. Otherwise use the Single Ended bus length.)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 16

Ultra SCSI (SCSI 3)
  • Maximum bus speed: 20 MBps
  • Bus width: 8 bits
  • Bus length:
    1.5 meters, Single Ended
    3 meters, Single Ended if using up to four devices
    12 meters, using Low Voltage Differential (LVD)
    (Did not originally support LVD. If all the devices on a chain use LVD mode, use a 12m length. Otherwise use the Single Ended bus length.)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 8

Wide Ultra SCSI (Wide SCSI 3)
  • Maximum bus speed: 40 MBps
  • Bus width: 16 bits
  • Bus length:
    1.5 meters, Single Ended, using up to eight devices
    3 meters, Single Ended, using up to four devices
    12 meters, using Low Voltage Differential (LVD)
    (Did not originally support LVD. If all the devices on a chain use LVD mode, use a 12m length. Otherwise use the Single Ended bus length.)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 16

Ultra 2 SCSI
  • Maximum bus speed: 40 MBps
  • Bus width: 8 bits
  • Bus length:
    12 meters, using Low Voltage Differential (LVD)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 8

Wide Ultra 2 SCSI
  • Maximum bus speed: 80 MBps
  • Bus width: 16 bits
  • Bus length:
    12 meters, using Low Voltage Differential (LVD)
    25 meters, using High Voltage Differential (HVD)
  • Maximum number of supported devices: 16

Ultra 3 SCSI (Ultra 160)
  • Maximum bus speed: 160 MBps
  • Bus width: 16 bits
  • Bus length: 12 meters, using Low Voltage Differential (LVD)
  • Maximum number of supported devices: 16

Ultra 320
  • Maximum bus speed: 320 MBps
  • Bus width: 16 bits
  • Bus length: 12 meters, using Low Voltage Differential (LVD)
  • Maximum number of supported devices: 16

EIDE (PIO/DMA mode 4)
  • Maximum bus speed: 16.66 MBps
  • Bus length: 0.5 meter, Single Ended
  • Maximum number of supported devices: 2

ATA/33 (UDMA 33)
  • Maximum bus speed: 33 MBps
  • Bus length: 0.5 meter, Single Ended
  • Maximum number of supported devices: 2

ATA/66 (UDMA 66)
  • Maximum bus speed: 66 MBps
  • Bus length: 0.5 meter, Single Ended
  • Maximum number of supported devices: 2

FireWire (IEEE 1394)
  • Maximum bus speed: 50 MBps
  • Bus length: 1.6 meters, Single Ended
  • Maximum number of supported devices: 63

Fibre Channel (Gigabit)
  • Maximum bus speed: 50 MBps
  • Bus length: 1.6 meters, Single Ended
  • Maximum number of supported devices: 63

Glossary
  • Asynchronous: Data is transferred faster in one direction than in another.
  • Device ID: SCSI devices are assigned an ID number so the system can recognize their data and commands. IDs can be assigned dynamically by the bus at startup, allowing devices to be added or removed without reconfiguration. Some implementations allow for devices to be added and removed even while the bus is active (a hot swap). ID 0 should be used only by a device that can boot an operating system, such as a hard drive.
  • Differential: Differential systems use varying voltages to increase bandwidth in a single direction, allowing for faster reads or writes. Differential devices are not compatible with nondifferential (Single Ended) components. High Voltage Differential (HVD) variants became available with SCSI-1 and cannot be used on the same chain as a Single Ended device. Low Voltage Differential (LVD) devices appeared with Ultra2 SCSI and will function on the same chain as a Single Ended device but will be limited to Ultra SCSI bus speeds and the Single Ended bus length.
  • Fast Wide SCSI: Third-generation SCSI. Standardized in 1996, Fast Wide SCSI used a 16-bit data bus, doubling the data rate to 20 MBps. This version supported Single Ended and High Voltage Differential devices. An 80-pin single connector attachment (SCA) allowed hot swapping on wide bus devices.
  • HVD: See Differential.
  • LVD: See Differential.
  • Narrow: The 8-bit bus implementation of a particular SCSI version. Narrow is an optional designation, e.g., Narrow SCSI-2 and SCSI-2 are both acceptable.
  • SCSI 1: The original 8-bit SCSI implementation did not have a Wide variant, standardized in the mid-80s.
  • SCSI 2: Second-generation SCSI available in both Narrow and Wide variants, standardized in 1990. Also known as Fast SCSI.
  • SCSI 3: See Fast Wide SCSI.
  • Single Ended: The default format requiring a termination signal to indicate the end of a chain. Most modern devices are self-terminating.
  • Synchronous: Data is sent at the same rate in either direction.
  • Ultra SCSI: Fourth-generation SCSI. Available in both Narrow and Wide variants. Also known as Fast 20.
  • Ultra2 SCSI: Fifth-generation SCSI. Available in both Narrow and Wide variants, standardized in 1999. Only Differential devices are supported: Low Voltage Differential signaling (LVD) was developed for this standard. Also known as Ultra2 Narrow SCSI.
  • Ultra3 SCSI: See Ultra160.
  • Ultra160: Sixth-generation SCSI. Available only with a 16-bit bus, it does not have a Narrow or Wide designation. Only Low Voltage Differential devices are supported. Also known as Ultra3 SCSI.
  • Ultra320: Seventh-generation SCSI. Available only with a 16-bit bus, it does not have a Narrow or Wide designation. Only Low Voltage Differential devices are supported.
  • Wide: 16-bit bus implementation of a particular SCSI version. Must be specified for SCSI-2 through Ultra-2.

James McPherson has served his time in the trenches of technical support, honed his skills as a network administrator, and still managed to complete a B.S. in Engineering. After working for four different companies without changing offices, he is currently a freelance consultant and the bane of computer salesmen everywhere.

The authors and editors have taken care in preparation of the content contained herein but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for any damages. Always have a verified backup before making any changes.

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