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What is the maximum SCSI cable length I can use with my tape drive, autoloader or library?
(KB # EC12A130)

To ensure stable operation of devices on a SCSI bus, it is required that the total SCSI cable length is less than certain limits. The cable length limit depends on the operating mode of the SCSI bus -- single-ended, LVD, or HVD.

When calculating the total cable length for your SCSI bus, be sure to include the following:

  • All SCSI cables attached to the SCSI controller, both internal to the server and external. In other words, if there are devices attached to your SCSI controller both internally and externally to the server, include cable lengths for both directions. Note: If your SCSI controller has multiple channels, consider each channel as a separate bus.
  • All SCSI cables connected between devices on the bus. In other words, if multiple devices are "daisy-chained" on the bus, add the cable lengths between each device to the total.
  • All SCSI cables internal to the device enclosure. Refer to the SCSI cable and terminator specifications in the Installation and Operation Guide for your product to determine the length of SCSI cabling internal to the device.

Single-ended

Single-ended SCSI uses half of the cable wires for carrying data and control signals and the other half for ground. Because of this configuration, signals on a single-ended bus are more susceptable to corruption due to noise, especially as bus speed is increased. Cable length is therefore more limited than in differential systems.

Maximum cable lengths for a single-ended bus

Single-ended Mode
Bus Speed (MHz)
Maximum Cable Length (m)
Bus Width [a]
Transfer Rate (MB/s)
Maximum Devices per Bus [b]
Single-ended 5 6 Narrow 5 8
Wide 10 16
Fast 10 3 Narrow 10 8
Wide 20 16
Ultra 20 1.5 Narrow 20 8
Wide 40 16

Notes:

[a] Narrow bus uses a 50-pin cable and sends 1 bytes (8 bits) per transfer. Wide bus uses a 68-pin cable and sends 2 bytes (16 bits) per transfer.

[b] The SCSI controller counts as 1 device on the bus.

Differential (HVD)

High Voltage Differential (HVD) SCSI uses a set of two wires for each data or control signal on the bus. Each signal is determined by the difference in voltage between the line pair, resulting in higher noise tolerance. This allows for using longer cables at faster bus speeds. However, HVD is more costly to produce and uses more power than single-ended.

Caution: Do not mix HVD devices on a bus with single-ended or LVD devices. They are not electrically compatible.

Maximum cable lengths for an HVD bus

HVD Mode
Bus Speed (MHz)
Maximum Cable Length (m)
Bus Width [a]
Transfer Rate (MB/s)
Maximum Devices per Bus [b]
HVD 5 25 Narrow 5 8
Wide 10 16
Fast 10 25 Narrow 10 8
Wide 20 16
Ultra 20 25 Narrow 20 8
Wide 40 16

Notes:

[a] Narrow bus uses a 50-pin cable and sends 1 bytes (8 bits) per transfer. Wide bus uses a 68-pin cable and sends 2 bytes (16 bits) per transfer.

[b] The SCSI controller counts as 1 device on the bus.

Low Voltage Differential (LVD)

Like HVD, Low Voltage Differential (LVD) SCSI also uses two wires for each signal. This brings the differential bus advantage of reduced signal corruption due to noise. Another advantage of LVD is that it uses lower voltages than HVD, reducing cost and power consumption.

Most LVD devices are "multimode", automatically detecting when they are attached to a single-ended bus, and operating in single-ended mode. This makes it possible to mix LVD and single-ended devices on the same bus. However, if only one single-ended device is connected to a SCSI bus, all devices on that bus will operate in single-ended mode, and all of the single-ended limitations will apply: shorter cable lengths, fewer devices, and slower bus speeds.

Warning:Because of the high risk of SCSI bus stability issues, Tandberg Data does not support the use of an LVD library connected to a single-ended bus. For optimum performance and stability, only connect LVD devices to an LVD bus.

Maximum cable lengths for an LVD bus

LVD Mode [a]
Bus Speed (MHz)
Maximum Cable Length (m)
Bus Width [b]
Transfer Rate (MB/s)
Maximum Devices per Bus [c]
Ultra-2 40 12 Wide 80 16
Ultra-160
(Ultra-3)
40 12 Wide 160 16
Ultra-320 80 12 Wide 320 16

Notes:

[a] If a single-ended device is attached to the bus, ALL devices operate in single-ended mode. Refer to the table "Maximum cable length for a single-ended bus".

[b] Narrow bus uses a 50-pin cable and sends 1 bytes (8 bits) per transfer. Wide bus uses a 68-pin cable and sends 2 bytes (16 bits) per transfer.

[c] The SCSI controller counts as 1 device on the bus.

Termination

Termination is required at both physical ends of the SCSI bus regardless of the type of SCSI interface. Many SCSI controllers automatically enable termination at the controller itself when it is the physical end of the bus. A terminator at the last device on the SCSI cable is required.

Single-ended: Single-ended terminators are referred to as passive or active. For Fast or Ultra single-ended buses, active termination is recommended for stable operation.

Differential (HVD): A High-Voltage Differential bus requires an HVD terminator. Do not use a single-ended or LVD terminator on an HVD bus.

Low-Voltage Differential (LVD): In order for a SCSI bus to operate in LVD mode, all devices, the SCSI controller, and the terminator must be LVD. Many devices and terminators are "multimode", capable of operating in either LVD or single-ended mode.



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