Purchasing Hints, Troubleshooting Tips

With the various SCSI standards and devices, how do you choose what equipment to buy? First off, I recommend purchasing devices that support an external terminator; this will let you add another SCSI device to the chain without the hassle of taking apart a hard drive to remove a terminator from an internal component. This will also make troubleshooting easier, because you can add and remove devices from the chain simply by moving the external terminator and changing a few cables. (The SCSI-3 specification states that devices may not be internally terminated.)

Often a failure can occur if a power supply is inadequate. The terminator needs power and will not get enough if it is coming from a marginal bus or power supply.

One of the biggest problems is when a bus becomes too long. As the number of connected devices increases, the total length of your SCSI bus increases. Cheap cables, terminators, and connectors mean you will have to have a shorter bus for devices to work. When you determine the bus length, don't forget to allow for the impedance that devices add. You can check with your vendor on how much impedance to allow. If they don't know, find another vendor.

Even with perfect cables, the maximum cable length for single-ended SCSI is only 6 meters. And because neither cables nor connectors in the real world are perfect, you want to stay below the maximum cable length. Thus, when ordering devices, you should request short SCSI cables. Many vendors routinely ship 6-foot-long cables, which limit you to a chain of only three devices. Another way of keeping the bus short is to use cabinets that can house multiple disks; this simplifies the use of shorter cables. If you buy a packaged system of SCSI disks, ask the vendor how much cable length to allocate for the internal cablin g. If you are close to the 6-meter-maximum length, adding another SCSI device may require that you buy and add another SCSI controller. Or, you might have to replace two small disks with a single, larger-capacity drive. Note that if you are using fast SCSI devices, you need to be well below the 6-meter-length limit. Ask your vendor what bus length it recommends.

The biggest problems with SCSI devices generally involve connectors. You might move a working hard drive from one office to another, only to find that it no longer works. I recommend keeping an extra supply of SCSI cables that can be swapped around to help troubleshoot these problems when they occur. After eliminating the cables, the system configuration, and the terminator as potential offenders, you may find that the problem is in the enclosure itself. Sometimes the connection between the internal SCSI ribbon cable and the external connectors is marginal; moving the box causes the cable to shift and perhaps short out. Many drive failures get diagnosed as a problem with the hard drive when the fault really lies with the connector inside the enclosure.

Unless you have the equipment and skills to diagnose and fix this problem yourself, you will have to return the whole enclosure or replace the suspect drive (and hope that it is still under warranty or a maintenance contract). Many times, the root of the problem is sloppy work by the drive's distributor or OEM. By keeping track of failures, you may discover that hardware from some vendors suffers from a usually high failure rate, while similar equipment from other vendors does not.

Also, be wary of warranties. All five-year warranties are not the same. Some vendors require you to return the faulty device to them first. Other vendors will ship a replacement on notification before they receive the returned unit. This kind of service is usually worth paying a little extra for, especially when it can get your system back up and running in one or two days.

Finally, whenever you buy a SCSI adapter, make sure your vendor provides good support for the particular software you use. Alternatively, consider buying the set of drivers offered by Corel (Ottawa, Ontario, Canada). Paying extra for any given adapter may be well worth it if the vendor provides good support.