SCSI Cable Length

 

SCSI (Small Computer Systems Interface) provides an industry standard for communication with devices via the SCSI BUS architecture. One of the many standards provided, is the establishment of the maximum SCSI BUS length used to connect the storage device to the Host interface.

 

When planning for the integration of these storage devices, it is important that you determine the correct specification needed for your storage device to communicate reliably. It is critical that you do not exceed the SCSI specifications for your storage device. Doing so can lead to a host of SCSI and ASPI communication problems that may jeopardize the integrity of the data stored to the storage device.

 

The following are the two primary specifications being used with current storage devices:

 

FAST SCSI: 10 MB/S: (Used in a majority of 5.2 GB storage devices and their related libraries)

 

Maximum BUS length supported:

      Single-ended host adapter interface: 3 meters or 9.8 feet

      Differential host adapter interface: 25 meters or 82 feet

 

Low Voltage Differential (LVD)- 

 

Maximum BUS length supported:

      LVD host adapter interface: 12 meters or 39.4 feet

      Single-ended host adapter interface: 3 meters or 9.8 feet

 

Legacy Systems (pre-1998)

 

SCSI-1: 5 MB/S (Used in a majority of 1.3 and 2.6 GB storage devices and their related libraries)

 

Maximum BUS length supported:

      Single-ended host adapter interface: 6 meters or 19.6 feet

      Differential host adapter interface: 25 meters or 82 feet

 

Some storage device manufactures use an electronic component within library configurations that are designed to extend the distance at which peripheral SCSI devices may be placed from the Host adapter. Though these devices are designed to increase the functional length of the single-ended specification, we have seen that there is still internal resistance from the storage drives and controller that adds to this effective BUS length.

 

The calculation of the total SCSI BUS length involves not only the physical cable(s) used to connect the storage device(s) to the Host adapter card, but also includes the storage device itself. You must allow for the "internal" resistance that the storage drive(s) and library controller adds on to this total BUS length.