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Serial communication is a commonly used communication method in PLC networks. Choosing the correct interface type and protocol standard is crucial for ensuring communication reliability.
The RS-232 interface complies with the serial data communication interface standard established by the Electronic Industries Alliance (EIA), originally numbered EIA-RS-232 (commonly referred to as 232, RS232). It is widely used for connecting peripheral devices to computer serial interfaces. RS-232 is one of the mainstream serial communication interfaces today. Below, we will introduce the basics of RS232, RS422, and RS485 serial communication and protocols.
1. RS232
RS232 is a standard serial physical interface, with 232 being the identifier. Each RS232 interface has two physical connectors (plugs), available in 9-pin and 25-pin configurations.
The 9-pin plug is more commonly used, with pin functions primarily as follows: pin 2 represents RXD, pin 3 represents TXD, and pin 5 represents GND.
RS232 operates in half-duplex communication mode. Due to interference and wire resistance, the communication distance is limited; it can reach tens of meters at low speeds, but in practical applications, it is generally within 15 meters.
It is important to clearly define the pin functions of the serial port; please refer to the diagram below:
2. RS485
The maximum communication distance of RS485 is 1200 meters, with a maximum transmission rate of 10M/S.
The transmission rate is inversely proportional to the transmission distance; the maximum transmission distance can only be achieved at a transmission rate of 100 kb/s. If longer distances are required, RS485 repeaters must be added. RS485 supports up to 32 nodes.
In low-speed, short-distance, and non-interference scenarios, ordinary twisted pairs can be used. Conversely, in high-speed and long-distance transmissions, it is necessary to use RS485 specific cables with impedance matching (generally 120Ω) (STP-120Ω (for RS485 & CAN) one pair 18 AWG), and in environments with severe interference, armored twisted shielded cables (ASTP-120Ω (for RS485 & CAN) one pair 18 AWG) should be used.
The RS485 interface not only facilitates data transmission between two points but also conveniently supports interconnection among multiple stations.
3. RS422 and RS485 Interfaces
The standard electrical interface circuit uses balanced differential driver and receiver circuits to receive and send signals in different ways, thereby reducing interference. The difference between RS422 and RS485 is that the former is full-duplex (receiving and sending can occur simultaneously), while the latter is half-duplex (receiving and sending must be separated).
Interface converters can facilitate the mutual conversion between RS232 and RS485. Generally, computers only have RS232 interfaces. To read RS485 data, an RS232 to RS485 converter is required.
Above is a detailed introduction to RS232, RS422, and RS485. I hope it is helpful to you, and thank you for reading.
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