terminator

Definition of terminator in The Network Encyclopedia.

What is Terminator?

Terminator is a device connected to one end of a bus or cable that absorbs signals. Terminators prevent signal reflection, which can produce interference that causes signal loss. Most communication systems such as networks and computer buses require some form of termination at the ends of the data path, although this is often provided internally by the devices at the ends of the data path.

How It Works

In a bus-based system, a single wire or series of wire segments connects network components in a chain formation. If the ends of the cable are not terminated, a signal placed on the wire by one component will bounce back and forth between the ends of the cable, hogging the cable and preventing other components from signaling. Terminators eliminate this signal bounce by absorbing the signal after each component has seen it once, allowing other components to place their signals on the cable.

Terminators can be passive (simple resistors) or active (more complex electronics) depending on the type of bus being terminated. By supplying a load equal to the impedance of the cable, the terminator prevents reflections or standing waves from developing on the cable. Passive terminators use resistors to provide this impedance matching, while active terminators generally use voltage regulators.

Terminator types include the following:

  • Coaxial cabling terminators:
    Passive terminators that come in various sizes and use BNC threading to terminate

     

    • RG-58 thinnet cabling for 10Base2 Ethernet networks with termination resistance of 50 ohms
    • RG-59 cable television terminators with resistance of 75 ohms
    • RG-62 ARCNET cabling terminators with resistance of 93 ohms

  • Small Computer System Interface (SCSI) terminators:
    The ends of a SCSI cable must always be terminated in a chain of SCSI devices. The internal termination is usually supplied by the SCSI adapter card, and the external termination is supplied by the last device in the chain. SCSI terminators can be passive, active, differential, or forced-perfect. Forced-perfect terminators compensate for the differences in impedance along the length of a SCSI bus. Diagnostic terminators analyze and display the condition of the data paths within a SCSI bus and are useful for high-availability uses such as clustering.

     

  • Free connectors:
    Connectors on the hubs at both ends of a series of stackable hubs. These terminators are specific to the type of hub sold by a vendor.

     

TIP

You can test the termination of a long 10Base2 network without having to hunt for the ends of the cable. Simply use an ohmmeter and test the resistance between the central conductor and the shield of any BNC T-connectors (after removing the cable from the network card it is attached to). If the reading is around 25 ohms, the cable is properly terminated; if the reading is around 50 ohms, one of the terminators is loose or missing. If the cable appears to be properly terminated but network problems persist, remove one of the terminators and use the ohmmeter to test the connection to the T-connector that you just exposed. If the result is less than 50 ohms, you probably have a short in the cable; if it is more than 56 ohms, there is probably a loose T-connector somewhere on the network.

Graphic T-7. A terminator can be used to test thinnet cabling.

See also: