As a signal is sent along the cable, when it reaches the end, it may be reflected back along the cable causing interference with the next signal. For this reason each end of the SCSI cable must be terminated. The terminators basically try to ensure that signal is not reflected back along the cable. There are several types of termination: Passive, Active, and Forced Perfect.
SCSI Controllers have terminators built into them, so you would only have to worry about the other end of the cable, but each end of a SCSI cable MUST be terminated and there MUST NOT be any terminated devices in between.
This is correct:
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| Controller Terminated |
Device 1 | Device 2 | Device 3 Terminated |
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This is not:
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| Controller Terminated |
Device 1 | Device 2 Terminated |
Device 3 | ||||||
As well it is the PHYSICAL END of the cable that must be terminated, not the last device. So you should put devices on each end of the cable and terminate them. (Or just put a Terminator on the last connector of the cable).
The arrangement shown below is not acceptable.
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| Controller Terminated |
Device 1 | Device 2 Terminated |
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Try one of these arrangements:
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| Controller Terminated |
Device 1 | Device 2 Terminated |
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| Controller Terminated |
Device 1 | Device 2 | Terminator | ||||||
Types of Terminators
Passive
This type uses an array of resistors to try to prevent signals being reflected back down the cable. This type of terminator is usually used for lower speed SCSI-2 devices such as tape units and CD-ROM drives.
Active
This type used a voltage regulator to try to hold the end of the cable at a constant voltage, and absorb any signals reaching the end of the line. This is more effective than the Passive Terminator, and is the usual type used on a SCSI bus.
Forced Perfect
This type is used mainly when there are SCSI bus errors occurring, and other terminator types have not resolved the problem. The terminator attempts to force the signal line to a specific voltage, and uses a voltage regulator configuration that acts faster than an Active Terminator can. It also requires more power than an Active terminator, and this may cause problems in the supply of termination power.
LVD
Low Voltage Differential Terminator: If you plan to run your bus in LVD mode, you will need an LVD or LVD/SE terminator or a Twist ‘n Flat cable that is terminated with one of these types of termination. If you are content to run in SE mode, then an Active terminator will do. KEEP IN MIND THAT ACTIVE TERMINATORS ARE FOR SE MODE, YOU NEED AN LVD OR LVD/SE TERMINATOR TO RUN IN LVD MODE. People commonly think you need active termination for LVD, which is incorrect.
LVD/SE
stands for “Multimode Low Voltage Differential and Single Ended”. Most LVD devices support LVD/SE. The term multimode is very tricky though, because it makes you think that devices can run in LVD and SE mode at the same time. This is not the case, a LVD/SE drive must run in LVD or SE mode.