Tips to Minimize the Effects of Differential Noise in RS485 Communication

Altium Designer
|  Created: November 9, 2018  |  Updated: December 15, 2020

Differential noise can corrupt RS485 data packets.

I’ve never been a morning person and pride myself on simply getting dressed and making it to work on time. Although I typically beat the traffic and appear at the office just in time, there have been occasions when I’ve arrived wearing mismatched socks. In these situations, I walk straight to my desk and hope that my embarrassment stays unnoticed.

In electronics, you might deal with a completely different type of embarrassment if you fail to keep differential noise out of RS485 communication. Corrupted data packets plague the receiving controller and, if unchecked by the underlying firmware, can cause the system to behave unpredictably.

For any number of devices, the network you are working with for a communication protocol can have varying applications contributing to characteristic impedance calculations like ground loops, a transceiver, resistor, converter, or transmitter, and varying other network topology or layout choices. Ensure your design works as intended by following smart design steps.

Understanding RS485

RS485 is a form of serial communication that uses differential signaling for data transmission over a long distance. A maximum of 32 transceivers can be connected on an RS485 bus. A typical RS485 bus consists of a pair of wires and operates in a half-duplex mode.

This means that at any particular time, a single transceiver can either transmit or receive data packets but not both. Due to the differential mode signaling employed by RS485, it can transmit data up to 4000 feet at up to 90kbps. Theoretically, it has relatively high immunity to mode noise.

Differential Noise Versus Mode Noise

Electrical noise is a significant concern in electronics. In RS485 communications, there are two types of noises: differential and mode. The RS485 signal is transmitted over a pair of wires. Differential noise occurs when electrical noise is coupled over one of the wires; mode noise, on the other hand, occurs when the same amplitude of interference is coupled to both wires.

As the RS485 receiver decodes the signal by comparing the differential voltage between the wires, differential noise has little effect on the transmission. However, differential noise can effectively corrupt the data transmitted over the RS485 bus. A spike on one of the wires can easily toggle the logic of the decoded value.

How to Minimize the Effects of Differential Noise in RS485 Communication

The performance of RS485 transceivers is as good as the susceptibility to the noise along the bus. While it is generally immune from mode noise, the right design efforts are needed to minimize differential noise.

Use Twisted Pair Cable

Using a twisted pair cable dramatically reduces electrical interference. With RS485 mainly being used for long-distance communications, the likelihood of electrical noise coupling over the cable is pretty high. It is important that the cable used for RS485 is a twisted pair cable. Each of the differential signals of the RS485 transceiver needs to be connected to the respective wires of the twisted pair.

Data rate and cable length affecting input impedance

Differential signaling with a twisted pair cable can impact applications

As current passes through a wire, an electromagnetic field forms around the circumference of the wire. A twisted pair utilizes the magnetic field that forms between the twists to cancel out the noise induced in one of the wires. This effect of noise cancellation due to the electromagnetic field does not occur in parallel pair wires.

Fail-Safe Biasing

The RS485 architecture involves multiple transceivers connected to a single bus. When no transceivers are transmitting, the bus is released and placed in an idle state. This floating state of the bus can sometimes introduce differential noise to the receiver circuit and results in random signals being decoded.

A feature that mitigates the effect of the idle RS485 bus is fail-safe biasing. This involves placing a series of resistors to form a voltage divider on the RS485 bus. This ensures that a minimum voltage occurs across the bus that places it in a known state. This effectively prevents the bus from fluctuating randomly.

PCB Routing

Hardware designers have an important role to play in preventing differential noise from coupling into the RS485 bus on the PCB. For instance, other high-frequency signals need to be kept away from RS485 transceivers and the traces. Besides that, the differential signal pair needs to be routed in parallel and positioned close to each other.

Whether baud rate or personal data in your devices, have strong routing

Tight routing will help to control the noise of your board

This ensures that electrical noises couple to both of the traces equally, resulting in mode noise instead of differential noise. If you’re using Altium Designer®, you can easily route RS485 traces in parallel with the differential pair routing tool.

Altium is PCB design software looking out for designers with signal integrity issues and layout challenges within their devices. If you’re looking for design software with a user-friendly design environment, there truly is nothing quite like Altium Designer’s intuitive and easily integrated unified design environment for a circuit board.

Need more tips for reducing differential noise in RS485? Talk to an expert at Altium.

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PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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