What is the difference between RS232, RS422 and RS485?

The following is a circuit diagram of [RS232, RS422, RS485 is different]

1, RS-232-C

RS-232-C is a serial physical interface standard developed by the Electronic Industry Association (EIA). RS is an abbreviation of the English "recommendation standard", 232 is the identification number, and C is the number of revisions. The RS-232-C bus standard has 25 signal lines, including one main channel and one auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as one transmission line, one reception line, and one ground line.

The data transmission rate specified by the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second.

According to the RS-232-C standard, the driver is allowed to have a capacitive load of 2500pF, and the communication distance will be limited by this capacitor. For example, when using a 150pF/m communication cable, the maximum communication distance is 15m; if the capacitance per meter is reduced The communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, which has common ground noise and cannot suppress common mode interference. Therefore, it is generally used for communication within 20m.

2, RS-485

RS-485 bus, RS-485 serial bus is widely used when the communication distance is required to be several tens of meters to several kilometers.

RS-485 uses balanced transmit and differential receive, so it has the ability to reject common-mode interference. In addition to the high sensitivity of the bus transceiver, it can detect voltages as low as 200mV, so the transmitted signal can be recovered beyond the kilometer.

RS-485 uses a half-duplex mode of operation, and only one point can be sent at any time. Therefore, the transmitting circuit must be controlled by an enable signal.

RS-485 is very convenient for multi-point interconnection and can save many signal lines. Applications RS-485 can be networked to form a distributed system that allows up to 32 drives and 32 receivers to be connected in parallel.

3, RS-422

RS422 bus, RS485 and RS422 circuit principle are basically the same, are sent and received in differential mode, do not need digital ground.

Differential operation is the fundamental reason for the long transmission distance under the same rate condition. This is the fundamental difference between the two and RS232, because RS232 is a single-ended input and output, and at least digital ground is required for duplex operation. Send line and accept line three lines (asynchronous transmission), you can also add other control lines to complete synchronization and other functions.

RS422 can work and receive without full-duplex operation through two pairs of twisted pairs. RS485 can only work half-duplex, and transmission and reception cannot be performed at the same time, but it only needs one pair of twisted pairs.

RS422 and RS485 can transmit 1200 meters at 19kpbs. A device can be connected to the line on the new transceiver.

(Editor: Circuit Diagram)