Fault Protection

Transient Suppression and Fault Protection on data lines.
Two examples are provided below, either external protection that requires additional components or protection within an IC [like a transceiver chip].

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

Circuit Protection for RS485 Devices

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.

RS485 line Protection

Design note; Always place the fault protection near the source so no additional damage occurs. For example if fault protect is being used to protect lines coming onto a Printed Wiring Board [PWB], place the protection near the edge of the board as the signals enter the PWB. Otherwise damage may occur to traces on the card between the entry point and the protection. While damage would be limited if the protection is nearer the point the signals come onto the PWB.

Of course the situation is different when signals are on the same PWB or if the protection is contain within an IC.
In the case that the fault protect is contained within a transceiver, the guidelines hold true. as normal design practices dictate that the transceiver be located near the board connector any way [normally within a few inches], as the signal enters the board.

Both of the IC's may be purchased from Texas Instruments. The graphs are from a TI application note.

PC motherboard

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Modified 3/17/12
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