DIFFERENCE BETWEEN RS485, RS422, RS232

RS-485 INDUSTRIAL STANDARD OF RELIABILITY

Universal digital interface board converts RS-485 signal to RS-422 & RS-232. In addition to power and ground wires, RS-485 uses a single twisted pair (2 wires twisted together and labeled A & B) for transmitting and receiving communication. RS-485 has one master which either transmits of receives data, but never at the same time since all outbound and inbound communication is performed over the twisted A & B wires together.

Since RS-485 uses pairs of twisted wires with differential signal (see explanation below), it is highly immune to external interference and is very widely used in industrial environments. It supports very long wire lengths of 1200 meters/yards and up to 35 Mbps data rates (not at the same time).  It has one master device which controls the direction of communication and up to 255 slave devices or sensors or nodes.

RS-422 INDUSTRIAL STANDARD OF RELIABILITY & SPEED

RS-422 differs from RS-485 in that it uses two twisted pairs for communication. One twisted pair (2 wires twisted together and labeled RD+, RD-) for inbound communication (inbound to the single master device) & second twisted pair (2 wires twisted together and labeled TX+, TX-) for outbound communication. Since it uses separate wire pairs for sending and receiving messages, it is able to send and receive signals simultaneously. It supports data throughput rates up to 10 Mbps.

RS-422 also uses pairs of twisted wires with differential signal (see explanation below) and is highly immune to external interference just like RS-485. It has one master device which controls the direction of communication, up to 10 slave devices or sensors and supports 1200 meters/yards wire lengths.

RS-232 EASY & UNIVERSAL COMMUNICATION INTERFACE

RS-232 is simple and low cost and differs from RS-485 only in the fact that it uses hardware switching to physically control the direction of communication and does not require twisted pairs of wires because it does not use a differential signal. Apart from the standard power and ground lines, RS-232 uses one wire (labeled TX) to transmit and one wire (labeled RD) to receive data. It is therefore much more susceptible to external interference and not widely used in industrial applications requiring anything but short wire lengths.  Its maximum data rates are well below 1 Mbps and highly dependent on wire length. It also has one master device which controls the direction of communication and up to 16 slave devices with wire lengths of up to 20m.


WHY ARE WIRES IN TWISTED PAIRS IMMUNE TO INTERFERENCE?

 Twisted pair FTP cable.

Twisted pair FTP cable.

Only differential signals require twisted pairs to maintain high interference immunity and low EMI emissions. Wires in twisted pairs without transmitting differential signals are not immune to interference. High data rates and close proximity to interference sources require tighter twist of each wire pair.

Tighter twist has the following benefits:

  • Allows a wire to pass very close to the source of interference. 
  • Allows higher frequency communication and high data rates to be immune to interference.

Why?

For comparison, imagine two parallel wires passing by a source of interference (lightning strike or industrial electric motor).  One wire will most likely pass closer to the interference source and thus will be exposed to stronger radiation (since its closer). This will generate higher voltage levels in one wire than in the other wire, and this difference in levels is noise that will interfere with communication signals passing through the wires. 

HOW DOES TWISTING HELP?

If you take the same wires and twist them together, you will see that in each full 360° twist, first one wire and then the other (alternating) will pass closer to the interference. If you add up the total length of each wire that is closer to the interference, they will be equal.  Thus in each wire the interference source will generate the same voltage levels, since on the average, they are equal distant from the interference source. Therefore, there will not be any differential noise to interfere with communication.

WHY IS DIFFERENTIAL SIGNAL NECESSARY?

Twisting wires into pairs eliminates differential interference (or severely limits it), but equal voltage levels are generated in both wires by the interference source, and this is called common mode interference...it is common to both wires.

If one were to send signals down the wires independently or together (not differentially), this common mode interference would result in communication errors. To overcome this, signal down one wire is sent with a positive voltage and over the other wire with an equal and opposite negative voltage.  This way, common mode interference shifts  the signal in each wire in opposite direction and the difference between the signal voltage levels between the twisted wires remains the same, thus preserving signal strength.  

Example:  Signal in wire A is +5V and in wire B it's -5V.  Common mode interference generated in each wire is +2V. This will shift the signal in wire A to +7V and in wire B to -3V. The difference between +5V and -5V = 10V and even with common mode interference added, the difference is still 10V (+7V and -3V).


For more detailed information on the differences between different hardware communication interfaces see the following link:


RS-485 SENSORS

ANEMOMETER, WIND VANE, TEMPERATURE, HUMIDITY, PRESSURE SENSORS

BARANI sensors use RS-485 with MODBUS (user selectable RTU or ASCII) to transmit measured data to any data logger or PLC computer. PLC is an acronym for Programmable Logic Controller which is the name generally used for customized industrial computer hardware. As you can see from the comparison of different serial communication protocols, RS-485 definitely has significant advantages for sensors in terms of simplicity of implementation, signal robustness, cable lengths and ease of use.

For applications in weather stations that use legacy data loggers or data loggers with analog inputs for wind speed, wind direction, temperature and humidity, we provide simple interface boards that offer the following benefit over analog measurement.

  • Highest level triple layer lightning protection for your analog data logger inputs. RS485 input from BARANI sensor features strong lightning protection on the interface board. This ensures that your logger sees only protected analog signal from the anemometer, wind vane or temperature and humidity sensor.