Why Does the Bus Need Termination?

Posted on June 10, 2009
Filed Under FAQ |

According to MIL-STD-1553B, the bus must be terminated at both ends of the cable with resistors equal to the cable characteristic impedance.  The purpose of termination is to minimize the effects of signal reflections that can cause waveform distortion.  If termination is not used, the communications signal can be compromised causing disruption or intermittent communications failures.  Furthermore, both ends of the bus, whether it includes one coupler or a series of couplers connected together, must be terminated. Since typically 78 ohm cable is utilized, the terminators should contain 78 ohm resistors.  Some couplers have built-in terminators and are generally used at the ends of the bus in multi-coupler applications. These types of couplers are mainly for vehicle applications as they limit the flexibility of test lab set-ups.

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