RS232, RS485, RS422 detailed introduction

3.1. Serial UART

1. RS232

Full duplex, point-to-point, several meters.

The asynchronous transmission standard interface developed by the Electronic Industries Association (EIA). Usually the RS-232 interface appears in the form of 9 pins (DB-9) or 25 pins (DB-25).

RS232, RS485, RS422 detailed introduction

The data transmission rate specified by the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200, 38400 baud.

RS-232 point-to-point connection, a serial port can only connect to one peripheral.

The signal level value of the interface is relatively high, and it is easy to damage the chip of the interface circuit. Because it is not compatible with TTL level, a level conversion circuit is needed to connect with TTL circuit. The transmission rate is low, in asynchronous transmission, the baud rate is 20Kbps

The transmission distance is limited, the maximum transmission distance standard value is 50 feet, in fact it can only be used at about 15 meters.

2. RS-485

Half-duplex, polling mode, one kilometer

With networking function

RS485 has two-wire and four-wire connections. The four-wire system can only achieve point-to-point communication. It is rarely used now. Nowadays, the two-wire connection method is mostly used. This wiring method is a bus topology. Up to 32 nodes can be connected on the same bus.

RS232, RS485, RS422 detailed introduction

Let's look at the characteristics of RS485 below:

(1) The interface level is low and the chip is not easy to damage. Electrical characteristics of RS485: Logic "1" is represented by the voltage difference between two wires of +(2~6)V; Logic "0" is represented by the voltage difference between two wires of -(2~6)V. The interface signal level is lower than RS232, it is not easy to damage the chip of the interface circuit, and the level is compatible with TTL level, which can be easily connected with TTL circuit.

(2) The transmission rate is high. At 10 meters, the maximum data transmission rate of RS485 can reach 35Mbps, and at 1200m, the transmission speed can reach 100Kbps.

(3) Strong anti-interference ability. The RS485 interface is a combination of a balanced driver and a differential receiver, and the ability to resist common mode interference is enhanced, that is, it has good anti-noise interference.

(4) Long transmission distance and many supporting nodes. The RS485 bus can transmit more than 1200m (rate ≤100Kbps) and generally supports a maximum of 32 nodes. If a special 485 chip is used, it can reach 128 or 256 nodes, and the maximum can support up to 400 nodes.

3. RS-422

Full-duplex, point-to-many, one kilometer

A total of 5 lines. Because the receiver adopts high input impedance and the transmission driver has stronger driving capability than RS232, it is allowed to connect multiple receiving nodes on the same transmission line, up to 10 nodes can be connected. One master device (Master), and the rest are slave devices (Slave). The slave devices cannot communicate, so RS-422 supports point-to-many two-way communication. The maximum transmission distance of RS-422 is 4000 feet (about 1219 meters), and the maximum transmission rate is 10Mb/s.