Difference Between RS-232/RS-422/RS-485

The RS-232 interface (TIA / EIA-232) is related to the serial data transmission interface standard between the transmitter or terminal (Data Terminal Equipment, DTE) and the receiver or communication equipment (Data Communications Equipment, DCE), was established by the Electronic Industries Alliance (EIA) in 1960.

RS232 is widely used in the connection of peripherals of the computer’s serial interface. Connect the cable to mechanical, electrical, signal, and transfer processes, such as Servo Controllers, CNC Machines tools and PLC machines. but most computers currently do not have RS232 COM ports, because USB to Serial Converter is more convenient to connect devices such as serial printers, scanners and GPS devices.

Since the communication data is based on the voltage on the wires relative to the ground level, the ground connection must be made between the RS-232 communication device. the following is RS232 wiring diagram and Specification Summary.

D-subminiature connector includes 9-pin DB9 and 25-pin DB25, DB9 including Male and Female type, the following figure shows the pinout.

RS232 Feature

• The information is digitally transmitted through logical 0 and 1.

• The logical "1" (MARK) corresponds to the voltage in the range from -3 to -15 V.

• The logical "0" (SPACE) corresponds to the voltage in the range from +3 to +15 V.

• The speed of RS-232 depends on the distance between the devices, the speed is 9600 bps at a distance of 15 meters.

• RS-232 runs in full-duplex mode, allowing data transmission and reception simultaneously.

• Normally, Only TxD, RxD and GND ground lines are used, the remaining lines are used to control the data flow.

RS232 Disadvantages

• Easy to damage the chip ----The signal level of the interface is high, valid signals are either in the range of +3 to +15 volts or −3 to −15 volts, easy to damage the chip of the interface circuit.

• Limited Distance - --The Cable length is limited to 15meters.

• Susceptible to Noise ---RS232 is single-ended, which means that the transmitting and receiving lines are referenced to a common ground.

• Transmission rate is low---The data transmission rate specified in the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second. In asynchronous transmission, the maximum bit rate is 20Kbps

• Not support multi-point ---RS-232 only support point-to-point, doesn’t support multi-point communication.

RS-422, also known as TIA/EIA-422, is a technical standard that specifies electrical characteristics of a digital signaling circuit originated by the Electronic Industries Alliance.

The RS-422 line is 4 wires for data transmission and one common GND ground wire, allows you to send and receive data on separate lines (full duplex) simultaneously, each signal uses two wires to pass the data. The differential voltage on the A and B wires represent the digital value. If B>A, the value is 1. If A>B, the value is 0.

It has a maximum transmission distance of 4000 feet (about 1219 meters) and a maximum transmission rate of 10 Mb/s.

Contrast to RS232, RS422 adopts a combination of balanced driver and a differential receiver, which has strong anti-common mode interference capability enables systems to operate over long distance, much better than RS232.

RS422 Feature

• The RS-422 operates in full-duplex mode, allows you to send and receive message simultaneously, but uses a differential signal

• The data transfer speed in RS-422 depends on the distance.

       • Speed: 10Mbits/sec, Distance: 40 Feet (12 meters);

       • Speed: 1Mbits/sec, Distance: 400 Feet (122 meters);

       • Speed: 100kbits/sec, Distance: 4000 Feet (1219 meters)

• In the RS-422 network, there can only be 1 driver and up to 10 receiving devices.

• The RS-422 line is 4 wires for data transmission (2 twisted wires for transmission and 2 twisted wires for receiving) and one common GND ground wire.

• The logical difference between A and B is greater than +0.2 V.

• Logical 1 corresponds to the difference between A and B less than -0.2 V.

• The RS-422 standard does not define a specific type of connector, usually it can be a terminal block or a DB9 connector or Ethernet.

• RS-422 pinout depends on the manufacturer of the device.

• Twisting wires (twisted pair) allows you to get rid of interference. twisted-pair cable can be either #22 or #24 AWG solid wire

• Terminating load resistors are required to ensure a matched line condition, which prevents reflections that introduce data errors.

The most widely used serial interface is the RS485 or EIA-485 protocol. the RS-485 standard specifies differential signaling on two lines to generate effective common-mode noise cancellation. It has a great advantage over RS232 interfaces by using a multi-point topology, multiple receivers and transmitters can be connected.

RS485 can operate in half-duplex mode (with 2 contacts) or full-duplex (with 4 contacts), the transmission distance can reach up to 4000 feet (about 1219 meters) and the maximum transmission rate is 10 Mb/s. but if the distance is too long, you should put an Industrial RS-485 repeater/Isolator in the middle or at the beginning of the bus, the repeater with photoelectric Isolation can be extended to 3000 meters.

RS485 is used commonly as a protocol for POS, metering instruments, data acquisition and control and especially in noisy environments, which make data transmission on a higher speed and longer distance, the RS485 is also common use for computers, PLCs, microcontrollers and intelligent sensors in scientific and technical applications.

RS485 Feature

• In the RS-485 network, it allows up to 32 devices.

• The data transfer speed in RS-485 depends on the distance.

       • Speed: 10Mbits/sec, Distance: 40 Feet (12 meters);

       • Speed: 1Mbits/sec, Distance: 400 Feet (122 meters);

       • Speed: 100kbits/sec, Distance: 4000 Feet (1219 meters)

• The logic “1” is represented by the voltage difference between the two lines +2V~+6V, and the logic “0” is represented by the voltage difference between the two lines -6V~-2V.

• The RS-485 standard does not define a specific type of connector, it can be a terminal block or a DB9 connector or Ethernet.

• RS-485 pinout depends on the manufacturer of the device.

• Twisting wires (twisted pair) allows you to eliminate interference. twisted-pair cable can be either #22 or #24 AWG solid wire.

• Terminating load resistors are required to ensure a matched line condition, which prevents reflections that generate data errors.

• The RS-485 interface can be used in simplex of half-duplex modes with a single-pair cable.

• Full-duplex or simultaneous transmit and receive operations can be implemented with a two-pair cable.

• A common configuration is bus network with multiple drops or connections.

• RS-485 HUB adopts star bus topology to realize star-type centralized network connection of RS-485 devices, by splitting 1-port RS232/RS485 into 4-port RS485 and enhancing the load capacity of bus signal.
  

There are many differences between RS232 vs RS485, mainly in Pinout, Number of Drivers, Nodes, Distance, Speed, and Application,  as shown below:

Pinout

Characteristics

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. RS232 uses point to point communication while RS485 uses a multi-point network.

Distance

The maximum communication distance of RS485 is 1200m while RS232 only supports 15m, RS232 has a lack of immunity for noise on the signal lines. RS232 is susceptible to Noise which limits its maximum communication distance and speed.

Speed

RS485 communication is also much higher at 10MBit/s (decreases as distance increases) compared to 20KBit/s with RS232.

Application

RS-232 only allows one-to-one communication (single station capability), The RS-485 interface allows up to 128 transceivers on the bus (with multi-station capability).

There are several types of products related to USB, RS232, RS42, RS485 interface, including RS-232 to RS-422/RS-485 Converters, USB to RS232/RS485/RS422 Converter, RS232/RS-485 Isolator/Repeater/HUB. which are widely used in Industrial Control ,Automation control, Distributed data acquisition, and Power System Automation.  

The RS232 to RS485 converter comes in many models, sizes and shapes, so how should we choose ideal product for our application, here are some advice for you:

The following video shows how to connect RS232/RS422/RS485 converters.