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






It is well known that the overall purpose of a serial interface is to provide a single path for data transmission wirelessly or through a cable, most common serial communication protocols, including RS232, RS485, RS422, which are widely used in Industrial Control and Measurement equipment, Automation controllers, Distributed data acquisition, Medical monitoring equipment, Power System Automation and Industrial Internet of Things.  


From this article, we will learn:

1 .What is RS232

2. What is RS422

3. What is RS485

4. What's the difference between RS232/422/485

5. How to choose RS232/422/RS485 Converters

6. Products of RS232/RS44/485 converters

7. How to connect RS232/422/485 Converters



What is RS232

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.


EIA  RS-232 Specification Summary

Parameter

Conditions

Min

Max

Units

Driver Output  Voltage, Open Circuit

25

V

Driver Output Voltage, Loaded

3kΩ < RL < 7kΩ

±5

±15

V

Driver Output Resistance, Power Off

-2V < V < 2V

300

Slew Rate

4

30

V/µs

Maximum Load Capacitance

2500

pF

Receiver Input Resistance

3

7

Receiver Input Threshold

Output = Mark (Logic 1)

-3

V

Output = Space (Logic 0)

3

V


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.

What is RS422

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.


EIA RS-422 Specification Summary

Parameter

Conditions

Min

Max

Units

Driver Output  Voltage, Open Circuit

±10

V

Driver Output  Voltage, Loaded

RL = 100Ω

±2

V

Driver Output  Resistance

A to B

100

Ω

Driver Output  Short-Circuit Current

Per output to common

150

mA

Driver Output  Rise Time

RL = 100Ω

10

% of bit width

Driver Common-Mode  Voltage

RL = 100Ω

 ±3

V

Receiver  Sensitivity

VCM < ±7V

±200

mV

Receiver  Common-Mode Voltage Range

-7

7

V

Receiver Input  Resistance

4

Differential  Receiver Voltage

Operational

±10

V

Withstand

±12

V


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.

What is RS485

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.


EIA RS-485 Specification Summary

Parameter

Conditions

Min

Max

Units

Driver Output  Voltage, Open Circuit

±1.5

±6

V

Driver Output  Voltage, Loaded

RL = 100Ω

±1.5

±5

V

Driver Output Short-Circuit  Current

Per output  to common

±250

mA

Driver Output  Rise Time

RL = 100Ω , CL=  50pF

30

% of bit width

Driver  Common-Mode Voltage

RL =  54Ω

 ±3

V

Receiver  Sensitivity

-7V < VCM < 12V

±200

mV

Receiver  Common-Mode Voltage Range

-7

12

V

Receiver Input  Resistance

12


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.

Difference Between RS23 vs RS485

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


Pinout

Characteristics


Characteristics of RS232, RS422, and  RS485

RS232

RS422

RS485

Signaling

Unbalanced

Balanced

Balanced

Max number of drivers

1

1

32

Max number  of receivers

1

10

32

Duplex

full  duplex

full  duplex

half  duplex/full duplex

Network  topology

point-to-point

Multi-drop

Multi-point

Max distance (acc. standard)

15m

1200m

1200m

Max speed  at 12 m

20 kbs

10 Mbs

10Mbs

Max speed at 1200 m

(1 kbs)

100 kbs

100 kbs

Max slew  rate

30 V/μs

n/a

n/a

Receiver input resistance

3~7 kΩ

>=4kΩ

>=12kΩ

Driver  load impedance

3~7 kΩ

100 Ω

54 Ω

Receiver input sensitivity

±3 V

±200 mV

±200 mV

Receiver  input range

±15 V

±10 V

–7~12 V

Max driver output voltage

±25 V

±6 V

–7~12 V

Min driver  output voltage

±5 V

±2.0 V

±1.5 V


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).


Products of RS232/RS422/485 converters

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.


RS232/RS485/422 Converters


Photo Model Name Description
UP-485-C2 Industrial RS-232 to RS-485 Converter, 300bps~115.2Kbps, Port-powered
UP-485-C3 Industrial RS-232 to RS-485/RS422 Converter, 300bps~115.2Kbps, Port-powered
UP-485-C4 Industrial RS-232 to RS-485/422 converter, 300bps~38.4Kbps, 2500V Opto-isolation, Port-Powered
UP-485-C8 Industrial RS-232 to RS-485/422 Converter, 2500V isolation, DC5V/DC9-30V


USB to RS232/485/422 Converters


Photo Model Name Description
UP-USB-1 USB to RS232 interface converter
UP-USB-2 USB to RS485/422 interface converter
UP-USB-4 Industrial USB to RS232/RS485/ RS422 interface converter


RS232/485 Isolator/Repeater/Hub


Photo Model Name Description
UP-232-P RS232 Three Wire Signal Isolated Protector, 300~38.4Kbps, 2500V Opto-isolation
UP-232-H4 Industrial 1-port RS-232 to 4 ports RS-232 Isolation HUB, 2500V Opto-isolation
UP-485-H4 Industrial RS-232/RS-485 to 4 ports RS-485 Isolation HUB, 2500V Opto-isolation
MW1040-I-R Industrial RS-485/RS-422 Isolator/Repeater/Converter, 3000V Opto-isolation
UP-485-R Industrial RS-485/RS-422 Isolator/Repeater/Converter, 2500V Opto-isolation

How to choose RS232/422/RS485 Serial Converters

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:

  • 1. Do you need Industrial or commercial type Converter?

    1. Do you need Industrial or commercial type Converter?

    Industrial serial converter is designed for harsh environment, price is higher than commercial type.

  • 2. Do you need Isolated or non-isolated Converter?

    2. Do you need Isolated or non-isolated Converter?

    Isolated converter is support 600W surge protection, 15KV electrostatic protection.

  • 3. Do you need externally powered converter or port-powered?

    3. Do you need externally powered converter or port-powered?

    You can choose a port-powered converter or DC5V external power supply converter.

  • 4. Which type of connector do you need?

    4. Which type of connector do you need?

    There are several connectors for selecting, DB9 or DB25 connectors, terminal block, RJ11 connector.

  • 5. Do you need serial converter with LED indicators?

    5. Do you need serial converter with LED indicators?

    Serial converters with LED indicators can be very helpful when troubleshooting.

How to connect RS232/422/485 Converters

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


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