There are five main differences between RS232 and RS485

Many communication protocols are often used in embedded systems, such as RS232 protocol, RS485 protocol, lightweight TCP/IP protocol, LoRa and Zigbee, etc. This article shares with you the differences between RS232 and RS485, which are the most commonly used in embedded development (also often used in single-chip microcomputer development)

introduce

RS232 is a standard for serial communication developed in 1970 by the Electronic Industries Association (EIA) of the United States, the Bell System, modem manufacturers and computer terminal manufacturers. Its full name is "Technical Standard for Serial Binary Data Exchange Interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE)". The standard specifies the signal content of each pin of the connector and also specifies the levels of various signals. RS232 is an asynchronous transmission standard interface. Usually, the RS-232 interface appears in the form of 9 pins (DB-9) or 25 pins (DB-25).

RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems. The standard is defined by the Telecommunications Industry Association and the Electronic Industries Alliance. Digital communication networks using this standard can effectively transmit signals over long distances and in environments with high electronic noise. RS-485 makes it possible to configure inexpensive local networks and multi-branch communication links. RS485 has two types of wiring: two-wire and four-wire. The four-wire system can only achieve point-to-point communication and is rarely used. The two-wire wiring method is now more commonly used. This wiring method is a bus topology structure, and up to 32 nodes can be connected on the same bus.

• the difference
• Connection
• Level signal
• Transmission distance
• Maximum baud rate for data transmission
• Networking (multi-machine communication)

The specific contents are as follows:

• Connection method: The most commonly used connection method for RS232 is three wires: a sending line, a receiving line and a ground line; RS485 generally uses two-wire transmission: two transmission lines A and B.
• Level signal: RS232 logic 1 is -3V~-15V, logic 0 is +3~+15V; RS485 -2V~-6V represents logic 0, +2V~+6V represents logic 1, and the voltage is the voltage of AB.
• Transmission distance: RS232 transmission distance is limited, the theoretical transmission distance standard value is 15 meters, and the maximum transmission distance in actual projects does not exceed 30 meters; RS485 transmission distance is longer than RS232 transmission distance, the theoretical transmission distance standard value is 1200 meters, and it can reach 2500 meters in actual projects.

Maximum baud rate of transmitted data: RS232 has a lower transmission rate, with the maximum baud rate being 19200bps; although the transmission speed is slow, it is still practical in many engineering applications; RS485 has a higher transmission rate, which can reach 10Mbps; the transmission speed is also much faster, and is often used in engineering control signal transmission.

Networking (multi-machine communication):

• RS232 cannot realize multi-machine communication because RS232 is a full-duplex communication mode. The general connection mode is TTL level. When the line is idle, the TTL level of sending and receiving is high level. The RS232 level obtained after the level conversion chip is always low. Now assume that A is the host, B and C are slaves, then there is no problem when A wants to send a signal to B, because A's TXD line is connected to B and C's RXD line. At this time, B and C's RXD lines are normally in the waiting for reception (idle) state, but if B receives a signal and wants to reply to A, B's TXD and C's TXD lines are connected to A's RXD line. B is in the state of sending data (B wants to control the change between the high and low levels of its TXD line), but C is in the idle state (C wants to ensure that its TXD line is low level unchanged), and there is a communication conflict between B and C's TXD lines. Therefore, RS232 can realize point-to-point communication, but this method cannot realize the function of networking;
• RS485 can realize multi-machine communication because it is a half-duplex communication mode, that is, receiving and sending are realized in time-sharing. When the bus is idle, it is necessary to ensure that the state is logic 1, that is, the voltage of AB meets the level value of logic 1. Assuming that 1 is the host, 2 and 3 are slaves, the data line connection method is that A of 1, 2, and 3 are connected together, and B of 1, 2, and 3 are also connected together. There is no problem with the RS232 connection method.