How Does RS-485 Transmit Differential Signals?

• Sending device
• Receiving device
• Twisted pair

What you will learn

• Describe a differential signal sent over two wires.
• Explain why a twisted cable helps against noise.
• Say how many devices share one RS-485 line.

Before you start

It helps to know what UART and a resistor are.

What is a differential signal?

Each RS-485 signal is transmitted over a Sig+ line and a Sig- line. The RS-485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line.

Under the standard common-mode range, an RS-485 receiver accepts input common-mode voltage from −7 V to +12 V. This is the voltage common to both bus lines relative to the receiver reference; confirm the exact transceiver's operating range before connecting.

• Transmitter
• Sig+ line
• Sig- line
• Comparing receiver

Why does a twisted cable resist noise?

One of the main problems with RS-232 is the lack of immunity to noise on the signal lines. Noise is easily picked up and limits both the maximum distance and the communication speed.

The best results are achieved when the Sig+ and Sig- lines are twisted. In the reference example of magnetic-field pickup, twisting reduces the induced noise current compared with straight wires; the benefit depends on the environment and geometry.

• Noise picked up on both wires
• The difference stays clean
• Twisted cable

Differential signals and twisting allow RS-485 to communicate over much longer distances than RS-232. In this comparison of interfaces, RS-485 can support higher bit rates than RS-232; actual performance depends on the link design. If high noise immunity is needed, a combination of twisting and shielding is often used.

How do many devices share one line?

Of RS-232, RS-422, RS-423 and RS-485, RS-485 can connect multiple transmitters and receivers in one network. For a multipoint network sharing one signal pair, use a main line with short drops rather than a star.

On long lines or at high speeds, suitable termination at both ends can help reduce reflections; whether it is needed depends on the transmission line and transceiver characteristics.

• Shared line
• Device on the line
• Termination at both ends

Real-world example

With an adult, think of a row of sensors along a workshop. One twin cable runs past all of them, each sensor joins by a short drop, and the two ends of the cable have termination resistors.

Common misunderstandings

RS-485 measures voltage against ground. The receiver compares the voltage difference between both lines, instead of the absolute level on one line; the common-mode voltage must still remain within the receiver’s allowed range.

A star layout is better because the wires are shorter. For a multipoint RS-485 network sharing one signal pair, a main line with short drops is generally a better fit than a star.

Frequently asked questions

What is a differential signal?

Each signal goes over two lines, Sig+ and Sig-, and the receiver compares the voltage difference between them.

Why does RS-485 reach farther than RS-232?

Differential signals and twisting allow RS-485 to communicate over much longer distances than RS-232.

What does a twisted cable do?

Twisting helps reduce induced noise; the benefit depends on the noise source and geometry.

What are termination resistors for?

They help reduce signal reflections at both ends when transmission-line conditions call for them.

Summary

• RS-485 sends each signal over two wires, Sig+ and Sig-, and the receiver compares the voltage difference between them.
• Twisting helps reduce induced noise depending on the conditions.
• A multipoint network shares one main line with drops; suitable termination helps reduce reflections when needed.