Choosing a SCSI Host Adapter

 

When choosing a SCSI host adapter for your storage system, consider the following:

 

Calculating Total SCSI Cable Length and 
the Number of SCSI Devices

 

Single-ended SCSI adapters support a maximum electrical cable length of 6 meters (approximately 19' 6").  Each SCSI device requires an additional 0.6 meters (2') of electrical cable length in addition to the cable length required to connect all devices.  Be sure to consider this when calculating total cable length. 

 

The SCSI addressing specification allows you to address several daisy-chained SCSI devices but you should limit the amount to a maximum of 4 to 5 devices per bus when using a single-ended SCSI adapter. It may be possible to operate more devices on one single-ended SCSI bus, however this conservative method will help ensure superior reliability.

If your configuration requires more devices or more cable length, you have three choices:

 

      Choose a differential SCSI host adapter and differential SCSI interfaces for all devices on this bus (best choice for large libraries with six storage drives or more).

      Use multiple single-ended SCSI host adapters and distribute the load (best choice for multiple libraries or drives or combinations of these two).

      Add a SCSI repeater to the circuit.

Differential SCSI adapters
support a maximum electrical cable length of 25 meters (approximately 81' 6") but MUST be connected to differential SCSI devices ONLY! Connecting a single-ended SCSI device to a differential SCSI host adapter can result in the destruction of the single-ended device's SCSI interface (and in some cases smoke!). Additionally, differential SCSI requires differential SCSI cables, terminators and devices. Single-ended SCSI devices cannot be mixed or used upon a differential SCSI bus.

Some SCSI library manufacturers include differential SCSI bus capabilities within their devices, and have single-ended SCSI drives residing within the library. These drives are connected to a SCSI differential to single-ended converter. If your device has this option, and you have five or more SCSI drives, choose a differential SCSI host adapter. If the differential SCSI bus is not standard equipment within your library, and you have five or more SCSI drives, order all devices with differential SCSI interfaces.

I/O Bus Performance

 

Some people feel that there is no reason to spend money on a fast SCSI host adapter when it is connected to traditionally slower SCSI devices (like storage disk drives).

 

Although storage disk drive speeds are increasing, they are still slower when compared to a multi-head, multi-platter hard drive. Given this, some users believe that a slower device will not benefit from a faster I/O bus performance, so why spend the money?

Things to Consider:

 

      ISA bus adapter cards are typically clocked at approximately 7.15 MHz which is very slow by today's standards

      ISA bus adapter cards support 16 bit I/O

      ISA bus adapter cards typically use dated SCSI chip sets

      EISA bus adapter cards are typically clocked at 10 MHz

      EISA bus adapter cards support 32 bit I/O

      VESA/VLB adapter cards are typically clocked at approximately 33 MHz

      VESA/VLB adapter cards support 32 bit I/O

      PCI bus adapter cards are typically clocked at 33 MHz

      PCI bus adapter cards support 32 bit I/O

      PCI bus adapter cards use advanced SCSI chip sets

Summary

 

A PCI bus SCSI host adapter will provide substantially higher transfer rates and overall superior performance over an ISA bus adapter. Even though the storage drive may not be able to read or write anywhere near the 10 MB/second performance provided by an ISA adapter, the drive will still operate substantially faster when connected to the faster PCI bus card. This is especially true when the SCSI adapter is used with a multitasking, multithreaded operating system like the InveStore.

 

View current list of supported SCSI adapters at:

 

http://www.pegasus-afs.com/SCSIadapters.html