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What is the Difference Between Shield and Common on a RS485 Communications Port.

Issue
User maybe confused about when to use the shield or common on a RS485 port

Product link
PowerLogic and ION Power Meters

Environment
Serial Communications

Cause
Some RS485 communication ports have both a common and shield pin. Please see the picture below for reference

Resolution
The shield terminal is a landing terminal not internally connected. It is meant as a landing for connecting incoming and outgoing shield.

The Common terminal is electrically connected to the internal floating RS485 circuit that drives + & -.
A recommended example of a daisy chain connection to a meter without a "common “is shown below. To be sure please check the install guide for the specific meter you are installing.  The "best" connection scheme depends on the internal circuits of the other device and the type of wire being used. Most devices without a labelled common use the terminal labeled "shield" also as a common (when it is connected internally to the circuit driving + and -).





 
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