Serial Communication refers to a communication method that transmits data through a serial port. Data is transmitted bit by bit through data signal lines, ground wires, etc. Common serial ports include RS-232, RS-485 and RS-422. It should be noted that the serial port is only an interface standard. It specifies the electrical standard of the interface, not the communication protocol. It does not specify the interface plug-in cable and the protocol used.
Serial communication has the advantages of long transmission distance, low cost, stability and reliability, and is widely used in low-speed data transmission occasions and fields such as industrial control, automation equipment, instrumentation, and computer peripherals. Many engineers must have come into contact with RS-232, RS-485 and RS-422 in their daily work, so do you know the differences between them?
1. Hardware pin interface definitions are different
RS-232
RS-485
RS-422
2. Working differently
A set of standard serial port protocols was defined in the early days of serial port communication, and DB9 (9 pins) is a typical standard interface. Since serial communication is asynchronous, data can be transmitted using only two wires.
Typical working methods: RS-232: 3-wire full duplex; RS-485: 2-wire half-duplex; RS-422: 4-wire full duplex.
Half Duplex mode uses the same transmission line and can both send and receive data, but cannot send and receive at the same time. Data transmission allows data to be transmitted in both directions, but only one party can send data and the other party can receive data at any time. In half-duplex communication, each end needs to have a transceiver switching electronic switch, which determines which direction the data is transmitted through switching. Because there is a time delay due to switching, the information transmission efficiency is lower.
Full Duplex mode allows data to be transmitted in both directions simultaneously. Therefore, full-duplex communication is a combination of two simplex communication methods, which requires that the sending device and the receiving device have independent receiving and sending capabilities. In full-duplex mode, there is a transmitter and receiver at each end, and the information transmission efficiency is relatively high.
Simplex Communication, one party is fixed as the sender and the other party is fixed as the receiver. Data transmission is unidirectional and can only be transmitted in one direction, using one transmission line.
RS-485 also has two wiring systems and four-wire wiring. When using the four-wire system, it can only achieve point-to-many communication (that is, there can only be one master device and the rest are slave devices). Nowadays, the two-wire system is mostly used. .
3. Communication methods are different
RS-232 is limited to point-to-point communication, but this method cannot realize networking functions. This has led to the emergence of RS-422 and RS-485, which can connect and control multiple devices through a serial port.
RS-232: can only realize point-to-point communication; RS-485 and RS-422: can realize point-to-multiple master-slave communication.
4. Different logical characteristics
We know that the data transmission of serial communication is all 0 and 1. Logic 1 or logic 0 is judged by high and low levels, as follows:
RS-232: Logic “1”: -3V ~ -15 V; Logic “0”: +3V ~ +15 V;
RS-485: Logic “1”: +2V ~ +6 V; Logic “0”: -2V ~ -6 V;
RS-422: Logic “1”: +2V ~ +6 V; Logic “0”: -2V ~ -6 V;
5. Anti-interference, transmission distance and transmission rate are different
Overview of interference immunity, transmission range and transmission rates
RS-232 vs. RS-485
Anti-interference: The RS485 interface uses a combination of balanced driver and differential receiver, which has good anti-noise interference. The RS232 interface uses a signal line and a signal return line to form a common ground transmission form. This type of common ground transmission is prone to common mode interference.
Transmission distance: The standard maximum transmission distance of the RS485 interface is 1200 meters (at 9600bps). The transmission distance of RS232 is limited. The standard maximum transmission distance is 50 meters. In fact, it can only be used at about 15 meters.
Communication capability: The RS-485 interface allows the connection of up to 128 transceivers on the bus. Users can easily establish a device network using a single RS-485 interface. RS-232 only allows one-to-one communication.
Transmission rate: RS-232 transmission rate is low, during asynchronous transmission, the baud rate is 20Kbps. The maximum data transmission rate of RS-485 is 10Mbps.
RS-422 vs. RS-485
RS-422 has 4 signal lines: 2 for sending (T+, T-) and 2 for receiving (R+, R-). Since the reception and transmission of RS-422 are separate, it can receive and transmit at the same time (full duplex).
RS-485 only has 2 data lines: A and B for sending and receiving. Since the receiving and transmitting of RS-485 share two wires, it cannot receive and transmit at the same time (half-duplex).