The vast majority of devices installed today are Single Ended, therefore, for the purpose of the following discussion, it is assumed that you will have a Single Ended bus and devices.
There is no substitute for quality cabling and termination! Excessive electrical interference is invisible to see, however, the result is always 1 the device is not recognized by the adapter card, 2 it completely fails to work, 3 seems to work but has excessive errors or, 4 works for awhile but then has a fatal error.
The device may seem to backup OK but then will fail when attempting to read the media. These are all classic SCSI problems. In some instances, the cause for electrical interference on a SCSI bus already exists but is only manifested when either a new device is added or when an application is installed which drives the bus to higher performance levels.
Either condition can cause a marginal system with a problem to fail where previously the level of corruption did not cause any noticeable failures. A SCSI peripheral will retry sending a block of data a number of times before failing, with a hardware error, therefore, note that a system with a marginal problem can also cause the system to run slowly but without any hard errors until something changes on the system like adding a new device.
Cabling can be too long, too short, be of poor quality or simply defective. As a general rule, keep cables as short as possible but not under 6 inches. Be wary of inexpensive cables, which are often delivered with a new device. A quality cable is always thick because it has every data wire wrapped with a separate ground wire. There are two types of termination, passive and active. An external terminator is a cap that fits on the last device in the chain.
The internal chain is also terminated on the last device, however, termination may be provided by either a jumper setting on the last device typically a disk drive , a set of removable resistors either tape or disk , or by an external terminator block which is specially designed to terminate the last device on an internal SCSI bus.
Only SCSI 1 systems can use passive terminators! Passive terminators do not regulate voltage, which can end up being either too high or too low. Either condition will cause corruption that gets progressively worse as the length of the chain gets longer. Even if you have a SCSI 1 system, use an active terminator. All current model disk drives provide active termination when this option is used.
Never mix passive and active terminators on the same bus! Because of this, the signals hit a "wall of impedance" and are echoed reflected back down the SCSI bus. Reflected signals will cause line noise problems and possible data errors. To keep this from happening, terminators are placed at both physical ends of the SCSI bus.
The terminator's job is to closely match the impedance of the SCSI bus cable to prevent the signals from reflecting back down the cable. Termination is required at both physical ends of the SCSI bus. Many SCSI controllers automatically enable termination at the controller itself when it is at one physical end of the bus. However, it may be necessary for you to enable or disable termination at the controller.
This means that the host adapter device is always at the end of the SCSI bus and should always be terminated. Termination is usually enabled by default on host adapters. In fact, you may not need to be concerned about host adapter termination at all, since many Adaptec host adapter boards can automatically adjust their termination for any configuration of devices.
However,you need to disable termination on your host adapter if the host adapter board doesn't automatically adjust its termination and you connect both internal and external SCSI devices to it.
See your host adapter documentation to learn how to change the termination setting. Termination must be disabled if a device is in the middle of the bus.
There are several different ways to change termination on SCSI devices: Physically remove terminating resistors from their sockets, or insert them in the sockets for internal SCSI devices Change a switch setting on the device's switch block Remove or install a terminator plug for external SCSI devices Read the device documentation to find out how to change termination on your SCSI devices.
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