What is RS485 and its difference between RS232

You probably have heard of the RS232 which is a serial communication method between computers and devices. But have you heard of the RS485 communication? What are their differences?

Today, through this guide, we will cover just that:

  • What is RS485
  • How does the RS485 works?
  • RS485 Advantages
  • Examples of RS485 Devices
  • RS485 vs RS232 – What are their differences?

Without further ado, let us jump right into what is RS485:

What is RS485

  • The RS485 also known as TIA-485 (-A) or EIA-485 is a serial communication method for computer and devices similar to the RS232.
  • Not only is RS485 a single device-to-device interface, but it is also a communication bus that is used to form simple networks of multiple devices.
  • RS485 allows multiple devices of up to 32 to communicate at half duplex on a single pair of wire plus a ground wire at distances up to 1200 meters.
    • If you wish to extend the length of the network and number of nodes, you can easily do that by using repeaters.
  • RS485 is used commonly as a protocol for POS, industrial, telecom and especially in noisy environments due to its wide common-mode range which enables data transmission over long cable lengths in noisy environments. The RS485 is also common in computers, PLCs, microcontrollers and intelligent sensors in scientific and technical applications.

How does the RS485 work? (Wiring Arrangements)

  • In RS485 data is transmitted differentially through 2 wires twisted together which is often referred to as “Twisted Pair Cable”. The twisted property is what gives the RS485 high noise immunity and long-distance capabilities.
    • As you can see in the photo above, the direction of noise current that is generated by magnetic fields from the environment is flowing opposite from the current in other parts of the cable which allows the noise current to be lower than an ordinary straight cable.
  • In an RS485 network, it can be configured in 2 common ways: either in a 2 wire (half-duplex system) or 4 wire configuration (full-duplex system)

2 Wire Half Duplex System

  • In an RS485 2 wire half-duplex system, data can only pass in one direction at a time. (You can only transmit data or receive data on one time) In this setup, the host TX and RX signals will share a single pair of wires which means you will save money on installation costs with the ability for the nodes to talk amongst themselves too.
  • In this setup, the transmitter and receiver are connected to each of the nodes on one twisted pair.
  • However, in this setup, it is limited to half-duplex and requires attention to turn-around delay.

4 Wire Full Duplex System

Ref: Windmill
  • With this RS485 4 wire full duplex system, data can pass simultaneously both to and from the nodes. (You can simultaneously receive and transmit data) In this system, 2 wires will be used for transmission and the other 2 will be used for receiving.
  • In this setup, one master port with the transmitter is connected to each of the nodes receiving data on one twisted pair.
  • However, in this full-duplex setup, they are limited to master and slave communication where nodes cannot communicate with each other.

Do note that an RS485 network should be in one line with multiple drops but not as a star. Even though total cable length may be shorter in a star, this can cause large interference, poor communication, and adequate termination is not possible anymore.

RS485 Advantages

  • RS485 compared to the other interfaces is probably the only one that is able to internetwork multiple transmitters and receivers in the same network.
  • With the RS485, you can also connect many devices to the network.
    • With the default RS485 receivers with an input resistance of 12 kΩ, you are able to connect 32 devices.
    • With high-resistance RS485 inputs, you can connect up to 256 devices.
    • With RS485 repeaters, you can further increase the number of devices over long distances as well!
  • In addition, with its arrangement (two wires are required for each signal), signals can be transmitted faster over great distances.

Examples of RS485 Devices

Now that you have a basic idea on what RS485 is and how it works, let us look at a few RS485 Devices and uses:

Industrial USB to RS485 Converter

  • This is an industrial USB to RS485 converter with the original FT232RL inside. It features embedded protection circuits such as lightning-proof, resettable fuse, ESD protection, and TVS diode, etc. It’s pretty small in size.
  • The USB TO RS485 is very easy to use, allows automatic transceiving. Due to its fast communication, stability, reliability, and safety, it is an ideal choice for industrial control equipment and/or applications with high communication requirements.

Grove – RS485

  • The Grove RS-485 allows your Arduino connect with RS-485 device easily, you just need to plug and play.  
  • About Grove:
    • Grove is a modular, standardized connector prototyping system. Grove takes a building block approach to assemble electronics. Compared to the jumper or solder based system, it is easier to connect, experiment and build which simplifies the learning system.
    • To know about our Grove System, check out our wiki page here!

USB TO RS232 / RS485 / TTL Industrial Isolated Converter

  • This is an industrial USB to RS232/485/TTL isolated converter with the original FT232RL inside.
  • It features embedded protection circuits such as power isolation, ADI magnetical isolation, and TVS diode, etc.
  • In addition, it comes with an aluminum alloy enclosure which makes it solid and durable.
  • The USB TO RS232/485/TTL is very easy to use, fully automatic transceiving without delay. Due to its fast communication, stability, reliability, and safety, it is an ideal choice for industrial control equipment and/or applications with high communication requirement.

RS485 vs RS232 – What are their differences?

Let us compare their characteristics:

RS232RS485
Line ConfigurationSingle-EndedDifferential
Max Number of Drivers132
Max Number of Nodes132
Wiring Arrangementhalf duplex
full duplex
half duplex
full duplex
Network Point to PointMultipoint
Max Communication Distance15m1200m
Max Communication Speed20 KBit/s 10MBit/s
Typical Logic Levels±5 to ±15 V±1.5 to 6 V
Max Slew Rate 30 V
Receiver Input Resistance3 to 7 kΩ12 kΩ
Receiver Sensitivity ±3 V±200mV

So, what is different?

Number of Drivers and Nodes

  • The first thing you may notice will be the difference between its number of drivers and nodes. With the RS485, one device is able to control 32 other devices. But with the RS232, there is only 1 driver and 1 receiver.
    • This is as RS232 uses point to point communication while RS485 uses a multipoint network.

Distance

  • Secondly, it is the difference in communication distance between the RS485 and RS232. RS485 max communication distance is much higher than RS232 at 1200 compared to RS232 with only 15m.
    • This is as RS232 has a lack of immunity for noise on the signal lines. Noise is easily picked up on the RS232 which limits its maximum communication distance and communication speed.
    • If you are looking to operate equipment at a far distance from a transmitter, RS485 will be ur pick.

Speed

  • Lastly, similarly to communication distance, RS485 communication is also much higher at 10MBit/s (decreases as distance increases) compared to 20KBit/s with RS232.
    • For RS485, lines are twisted to add immunity to noise and allow magnetic fields to pass and not disrupt communication which allows RS485 to communicate as such high speeds.
    • As RS485 line configuration is differential as well, it allows for higher bit rates compared to a non-differential connection which RS232 has.
    • The slew rate of RS232 is also limited at 30V compared to RS485 with an indefinite slew rate which has been done to avoid reflection of signals. With a maximum slew rate, it limits the communication speed on the line for RS232. (To avoid reflections on longer cables for the RS485, it is necessary to use appropriate termination resistors.)

Summary

In summary, if you are looking to

  • Connect DTE (Data Terminal Equipment) directly without the need of modems
  • Connect more than 1 DTE in your network
  • Communicate over long distances
  • Communicate at fast communication speeds

RS485 will be your pick as they perform well in these 4 factors.

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