RS485, RS422 and RS232

What is serial communication?

A communication method that uses one or two transmission lines to send and receive data, one bit at a time. Since it can be connected with fewer signal lines, it has the advantage of suppressing the cost of wires and transfer devices. RS-232, RS-422 and RS-485 are three common serial communication standards, each with different characteristics and application scenarios. The following is a detailed comparison of these three standards:

RS-232

RS232 is a 9-pin (DB-9) or 25-pin (DB-25) serial communication port on older computers.

Features

1. Communication mode: Single-ended
2. Transmission distance: Usually within 15 meters (50 feet), depending on the baud rate and cable quality
3. Baud rate: From 300 bps to 115200 bps, or even higher
4. Voltage: The signal voltage is usually between ±3V and ±25V, and the voltage range of logic 0 and logic 1 is wide
5. Signal line: mainly transmit data (TXD), receive data (RXD), ground (GND), etc.

Application

Usually used for short-distance communication between computers and peripheral devices (such as modems, printers)

Advantages

1. Simple and easy to implement
2. Widely compatible, supported by many devices

Disadvantages

1. Limited communication distance
2. Simplex communication, unable to perform two-way data transmission at the same time (although half-duplex can be achieved through control lines)
   

   RS-422

RS-232C has the disadvantages of short transmission distance and slow transmission speed. RS-422A is a specification formed by improving these disadvantages. It is also called "EIA-422A". Although the purpose and timing of each signal line are specified, the connector is not specified. Many products that support EIA-422A mainly use D-SUB 25-pin or D-SUB 9-pin.

Features

1. Communication mode: Differential
2. Transmission distance: usually up to 1200 meters (4000 feet)
3. Baud rate: from 300 bps to 10 Mbps
4. Voltage: The signal is transmitted via differential voltage, usually ±2V to ±6V
5. Signal: usually includes data transmission (TX+, TX-), data reception (RX+, RX-)

Application

Suitable for long distance and high data transmission rate occasions, such as industrial control systems and data acquisition systems

Advantages

1. Support longer transmission distance
2. Higher anti-interference ability due to the use of differential signals
3. Support half-duplex and full-duplex communication

Disadvantages

1. More complex to implement than RS-232
2. Requires differential receiver and transmitter

RS-485

This is a specification that was formed to improve the shortcoming of RS-422A, which has a small number of connections. It is also called "EIA-485". RS-485 is a specification that is upwardly compatible with RS-422A. Although the purpose and timing of each signal line are specified, the connector is not specified. Many products that support EIA-422A mainly use D-SUB 25-pin or D-SUB 9-pin.

Features

1. Communication mode: Differential
2. Transmission distance: typically up to 1200 meters (4000 feet)
3. Baud rate: from 300 bps to 10 Mbps
4. Voltage: Signals are transmitted via differential voltage, typically ±1.5V to ±5V
5. Signal lines: mainly data lines (A, B) and ground lines (GND), supporting multi-point bus structure

Application

Widely used in industrial automation, building management and multi-point communication systems

Advantages

1. Supports multi-point communication, up to 32 devices can be connected (more devices are also possible through appropriate drivers and receivers)
2. High anti-interference ability due to the use of differential signals
3. Capability of long distance and high data transmission rate

Disadvantages

1. More complex to implement than RS-232
2. Requires correct terminal matching and network design

Summary and comparison

1. RS-232: Suitable for short-distance, simple point-to-point communication, easy to implement, but not suitable for long-distance and multi-point communication.
2. RS-422: Supports long-distance communication and higher speed, but is mainly used for point-to-point or multi-point (up to 10 devices) communication.
3. RS-485: Most suitable for long-distance, multi-point (up to 32 devices) communication, with high anti-interference and flexible network topology, but the implementation complexity is high.
4. The maximum communication distance of RS232 port is 15 meters, while the maximum communication distance of RS422/485 is 1200 meters.

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