1. RS232/485/422 main differences
These three communication ports are serial ports, which are different in the following aspects:
1.1 Communication distance
The maximum communication distance of RS232 port is 15m, while that of RS422/485 port is 1200m.
1.2 Number of connected equipment
RS232 can only connect one device, while RS485 can connect multiple devices.
1.3 Definition of port
RS232 is a standard interface, which is a D-shaped 9-pin. The signal definition of the interface of the connected equipment is the same.
RS422/RS485 is a non-standard interface, which is generally a 15 pin serial interface (there are also 9-pin interfaces). The pin definitions of each device are different. In addition, there are differences between RS422 and RS485: RS422 is a 4-wire system with full duplex mode; RS485 is two-wire system and half duplex mode.
RS-232, RS-422 and RS-485 are serial data interface standards. RS-232 is the most widely used serial interface in PC and communication. RS-232 is defined as a single terminal standard to increase communication distance in low rate serial communication. RS-232 adopts unbalanced transmission mode, which is called single end communication. RJ45 interface is usually used for data transmission, and the most common application is network card interface.
2. RS232/RS485/RS422 Overview
RS-232 is designed for point-to-point communication, and its driver load is 3~7k Ω. So RS-232 is suitable for communication between local devices.
RS422 is different from RS-485 serial interface standard. RS-422 and RS-485 are different from RS-232. The data signal adopts differential transmission, also known as balanced transmission. It uses a pair of twisted pairs, which is common in early PC communication. The maximum transmission distance of RS-422 is 4000 feet, and the maximum transmission rate is 10 Mb/s.
A complete RS232C interface has 22 wires and uses a standard 25 core plug socket. RS232C adopts logic, logic "1": - 5V~- 15V, logic "0":+5V~+15V
Disadvantages: the data transmission speed is slow, the communication distance is short, and the connector without calibration is easy to generate crosstalk between signals at the interface.
RS-232 is a serial physical interface standard formulated by the Electronic Industry Association (EIA). RS is the abbreviation of "recommended standard" in English, 232 is the identification number, and C is the number of modifications. The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as a transmission line, a receiving line and a ground wire. 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. RS-232-C standard stipulates that the driver is allowed to have a 2500pF capacitive load, and the communication distance will be limited by this capacitance. For example, when a 150pF/m communication cable is used, the maximum communication distance is 15m; If the capacitance per meter of cable decreases, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 belongs to single ended signal transmission, which has problems such as common ground noise and inability to suppress common mode interference. Therefore, it is generally used for communication within 20m.
Basic knowledge of RS232 communication: RS232 communication is also called serial communication. It refers to the way in which the computer communicates with a single device (controller) through RS232 international standard protocol using serial port connection lines.
Communication distance: it is recommended to be within 13 meters at 9600 baud rate.
Baud Rate: 9600 bps is commonly used by default, and 1200, 2400, 4800, 19200, 38400 are also commonly used. The higher the baud rate, the faster the transmission speed, but the shorter the stable transmission distance, the worse the anti-interference ability.
RS-485 serial bus standard is widely used when the communication distance is required to be tens of meters to thousands of meters. RS-485 adopts balanced transmission and differential reception, so it has the ability to suppress common mode interference. In addition, the bus transceiver has high sensitivity and can detect the voltage as low as 200mV, so the transmission signal can be recovered from kilometers away. RS-485 works in half duplex mode, and only one point can be in the transmission state at any time. Therefore, the transmission circuit must be controlled by the enable signal. RS-485 is very convenient for multi-point interconnection and can save many signal lines. The application of RS-485 can be networked to form a distributed system, which allows up to 32 drives and 32 receivers to be connected in parallel.
Basic knowledge of RS485 communication: The basic communication mechanism of RS485 and RS232 is the same. Its advantage is that it makes up for the short communication distance of RS232 and the inability to manage multiple devices simultaneously.
The computer connects several 485 devices (access controller) in turn through RS232 RS485 converter, and uses polling to communicate with the devices on the bus in turn.
The wiring marks are 485+485 -, which respectively correspond to 485+485 - of the link device (controller).
RS422 is a double ended line transmission signal. Through the transmission line driver, the logic level is converted into potential difference to complete the initial information; Through the transmission line receiver, the potential difference is converted into the logic level to realize the information reception of the terminal. The principle of bus, RS485 and RS422 circuits is basically the same. They are all sent and received in a differential way, without the need for a digital ground wire. Differential operation is the fundamental reason for the long transmission distance at the same rate, which is the fundamental difference between RS232 and RS232, because RS232 is a single ended input/output, and duplex operation requires at least three digital ground wire transmission lines and receiving lines (asynchronous transmission), and other control lines can be added to complete synchronization and other functions. RS422 can work in full duplex mode without affecting each other through two pairs of twisted pairs, while RS485 can only work in half duplex mode. Sending and receiving cannot be carried out at the same time, but it only requires one pair of twisted pairs. RS422 and RS485 can transmit 1200 meters at 19 kpbs. A new transceiver line can be used to connect equipment.
The S422 is longer and faster than RS232 in transmitting signals. The maximum transmission rate is 10Mbit/s. At this rate, the allowable cable length is 120m. If a lower transmission rate is used, such as 9000 baud rate, the maximum distance can reach 1200m.
RRS422 uses two signal lines for each channel. If one is in logic "1" state, the other is in logic "0" state. The circuit consists of transmitter, balanced connecting cable, cable terminal load and receiver. It is stipulated that there is only one transmitter and multiple receivers in the circuit, so point-to-point communication is usually adopted.
Basic knowledge of RS422 communication: Its communication principle is similar to that of RS485. The difference is that its bus is two groups of twisted pairs (four wires), marked R+ R- T+ T- respectively. The disadvantage is that the wiring cost is high and it is easy to make mistakes. It is used less now.
RJ45 is a type of various connectors (for example, RJ11 is also a type of connector, but it is used on the telephone); There are two different methods for sorting RJ45 headers and lines. One is orange white, orange, greenwhite, blue, bluish white, green, brownish white, and brown; The other is green white, green, orange white, blue, blue white, orange, brown white, brown; Therefore, there are also two types of lines using RJ45 connectors: straight line and cross line.
4. Application Scenario
In the past, the communication between PC and intelligent devices mostly used RS232, RS485, Ethernet and other methods, mainly depending on the interface specification of the device. However, RS232 and RS485 can only represent the physical media layer and link layer of communication. If you want to achieve two-way access to data, you must write your own communication application program. However, most of these programs cannot conform to the ISO/OSI specification, and can only achieve a single function, which is applicable to a single device type. The program does not have universality. (So Siemens later defined the ModBus protocol. In order to consider factors such as cost, stability, use place, etc., intelligent devices in industrial, building and other scenarios must be different from the way of communication between PCs, so they not only include Internet communication protocols, but also involve some other communication protocols, which need to be understood and learned.)
In the equipment network connected by RS232 or RS485 devices, if the number of devices exceeds 2, RS485 must be used as the communication medium. The communication between devices in the RS485 network can only be achieved through the transfer of "master" devices. This master device is usually a PC, but only one master device is allowed in this equipment network, and the rest are slave devices. Fieldbus technology is based on the ISO/OSI model, with a complete software support system, which can solve the problems of bus control, conflict detection, link maintenance, etc. Fieldbus devices are automatically networked without master/slave devices or multiple masters are allowed. At the same level, the products of different manufacturers can be interchanged, and the equipment has interoperability.
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