SCSI terminators are used on SCSI chains to ensure that the SCSI data packets are transmitted and received without any errors. It is extremely important that the chain is terminated properly using SCSI terminators, since without them different problems can occur that can be very difficult to narrow down. If a SCSI chain is not properly terminated or you have too many terminators (read: more than two) the transfer speed can vary, it might just not work at all, or you can even cause physical damage to the system in some cases.

A SCSI chain works very similar to a radio aerial (in fact, it is a radio aerial, but it keeps the signal inside the cable instead of transmitting radio waves into the ether). When transmitting radio signals, you have to shield the transmitter from transmitted signals bouncing back down the aerial after they have been transmitted. The same applies to SCSI chains. On a SCSI chain you only want to hear signals that have actually been transmitted by a device on the chain, not echos from previous signals which cause interference. To remove the echos, you terminate the SCSI chain at both ends using terminators. A terminator simply soaks up any signals hitting it, so that nothing will be transmitted back.

Most SCSI host adapters tend to auto-terminate their side of the chain if it thinks that it is connected at one end of the chain, but this varies, so check the documentation for the card.

There are four main types of terminators, depending on the type of SCSI used:

  • Passive terminators are just a collection of resistors, that will dampen all the signals as it receives them. Passive terminators should only be used on Differential (aka HVD) SCSI really, or SCSI slower/older than Fast SCSI.

  • Active terminators actually controls the signals on the wire, canceling them out. Active terminators should be used on all Single-Ended SCSI from Fast SCSI and faster.

  • LVD terminators are a kind of active terminators. When connecting a Single-Ended device on an LVD chain, the entire chain will go into Multimode Single-Ended (MSE) which is much slower than LVD (or it might shut itself down, depending on how good the SCSI host adapter is), so avoid that if possible. When mixing devices, make sure that the terminators are also capable of MSE, or use regular active terminators.

  • FPT (Forced Perfect Terminator) terminators are for single-ended SCSI. They are the best terminators from a signal canceling point of view, but they also require more power than the SCSI specification allows, and because of that they are not approved for all platforms. If there are a high number of devices on the chain however, FPT's are supposed to be the best thing.

Terminators generally come in two forms. The most common one is just a regular terminator, using one SCSI connector, and will not let anything else connect there, in effect wasting one connector. But they can also come as pass-through terminators, so that it is still possible to connect a SCSI device on it as well, so no connector needs to be wasted on the chain. Pass-through terminators can usually be used without anything connected to them, just as if it was a regular terminator. Depending on the type of cable used in the system, one or the other might be desirable, since it is preferable that devices are connected at even distances across the cable. Some cables have the last two connectors very close to each other, making it more suitable to have a regular terminator at the end instead of a pass-through, since any devices connected there would be very close to each other.