Learn and Implement RS-485

What is RS-485?

RS-485 is an industrial specification that defines the electrical interface and physical layer for point-to-point communication of electrical devices. The RS-485 standard allows for long cabling distances in electrically noisy environments and can support multiple devices on the same bus.

When, Why, and Where to Use RS-485 ?

RS-485 is use in a widely computer automation systems. It is invented in 1998. With the standard allowing for multi-drop (multiple devices on the same bus) and long cabling lengths, it is easy to understand its frequent use in the industrial and automation spaces. RS-485 is also found in theater applications in many devices.

In addition, the noise immunity offered by the RS-485 standard makes the interface very versatile. Engineers are not only using it for long cabling distances, but are implementing it into applications, such as the automotive industry, where it is uncertain what noise could be encountered in the end application. The ability to use RS-485 at high speeds, over long cabling lengths, in electrically noisy environments, and with multiple devices on the same bus, makes it a smart implementation for most applications requiring a serial interface.

How Do I Make RS-485 Connections?

First check the pinout connections and specifications of the devices you want to connect in order to determine whether you need a “2-wire” RS-485 connection or a “4-wire” RS-485 connection. If you can’t obtain this information before selecting a converter or serial card, select a converter or device which can be easily set to work in either mode using switches, jumpers or other options. Most often you will be setting up and connecting a card or converter as a Master to one or more Slave devices. Following are figures for three types of 2-wire Master connections, and one type of 4-wire connection. Usually Termination is not require for baud rates 19.2K or lower at distances up to 4000 ft.(1220 m).

What about biasing?

If an RS485 driver is idle then what? The line is disconnect from the driver in its idle state that is in + and – are floating. What happens if for an instant there appears a differential voltage of more than 0.2 Volts? Well, a differential greater than 0.2 is consider a signal and so it constitutes data noise. Solve this problem by pulling the lines to no voltages when they are idle. To do this connect them to the ground or some other voltage using pull up/pull-down resistors. Nice vendors include this. Nicer vendors provide a choice of biasing resistors selected by switches/jumpers.

To Understand fail safe biasing, you can refer Fail-Safe Biasing document of Texas Instruments


If you want calculate Biasing Resistor Value Online use below link:


There are many ready RS485 Module for converting TTL signal to 485 signal. Below given is one TTL to 485 module which good to start with.


Description of the RS485 interface

The RS485 (EIA485) interface is extremely robust. It is most popular communication protocol use in industry due to its multi-point topology. The RS422 protocol has similarities with RS485 in that both conduct data transmission using differential signals.

There are two types of RS485:

  • RS485 in half-duplex mode with 2 contacts
  • RS485 in full duplex mode with 4 contacts.

Full-duplex mode is used when you need to be able to transmit and receive data at the same time. In half-duplex mode, you can only transmit or receive data at any given time.

The voltage range on the lines varies from -7 V to +12 V.

There is no specific type of connector used to implement the RS485 protocol but in most scenarios, a DB9 connector or terminal block are employed.

Specific RS485 connectors may have different pinouts. You will be able to determine the actual configuration based on the documentation that accompanied the device.

Connect RS485 devices with 2 contacts.

A description of the RS485 technical specifications is provided in the following table.

If you want use RS-485 with Modbus RTU please check link http://embeddedtechsolutions.com/category/protocols/modbus/

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