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RS485 data links may be up to 500 meters, so require some type of surge suppression. The diagrams below show two methods for protecting RS-485 links [RS422].
The diagram below shows an external implementation of Fault Protection for a RS485 circuit. The RS485 interface IC is from TI; SN75ALS176. The external implementation requires more parts on the circuit board. The surge protection circuit uses two back-to-back zener diodes. These device types are also called Transient Voltage Suppressors
The diagram below shows an internal implementation of Fault Protection for a RS485 circuit. The SN75LBC184 IC from TI supplies the protection inside the IC reducing the parts required on the PWB.
RT is a Termination Resistor used to terminate the signal reflections.
Both of the IC's may be purchased from Texas Instruments. The graphs are from a TI application note.
Key words: Overvoltage, fault, RS485 bus, interface, EMI, EIA485, EIA-485, Voltage Surge, Chip Protection
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