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What are termination resistors and why use them?

Issue

What are termination (end of line) resistors and why use them?

A LAN of any type has a node or two somewhere in the middle having problems but the rest seem stable

Environment

  • I/NET Site
  • Controller LAN
  • 7808 Repeater

Cause

Termination resistors (also called clamping or end-of line resistors) are to be installed between lan+ and lan -, not to ground. Their purpose is to prevent the characteristic impedance of the wire from increasing to infinity at the end of the cable.

Resolution

Since most wire's characteristic impedance is between 80 and 150 ohms, use 100 to 120 ohms on an unbiased controller or host lan. This will "clamp" reflections from the end of the wire which are seen as noise by the controllers.

If a 7808 repeater is used, it requires bias. Using 100 ohm termination resistors would kill that lan's communication; therefore, use 620 ohms instead.

As cheap as resistors are and as easy as this installation is, they can only help. Typically they help more on a longer LAN, so anything longer than 1000 feet (about 300 meters) should definitely have terminations.

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