The difference between RS232 and RS422, RS485

RS-232-C is a serial communication standard developed by the Electronic Industry Association (EIA). The term "RS" stands for "Recommended Standard," while "232" is the designation number, and "C" indicates it's a revised version of the original. This standard defines 25 signal lines, including a main channel and an auxiliary one. However, in most practical applications, only a few of these lines are used for basic full-duplex communication—typically the transmit line, receive line, and ground. The RS-232-C standard supports data rates ranging from 50 to 19,200 baud per second. It allows a driver to handle up to 2,500 pF of capacitance, which limits the communication distance. For instance, with a cable that has 150 pF per meter, the maximum reliable distance is about 15 meters. If the cable’s capacitance is reduced, the distance can be extended. Another limitation is that RS-232 uses single-ended signaling, making it susceptible to common-mode noise and ground loops. This is why it is typically used for short-range communications, usually under 20 meters. In contrast, RS-485 is widely used for longer-distance communication, ranging from tens of meters to several kilometers. It employs balanced transmission and differential signaling, which significantly improves its ability to reject common-mode interference. With high receiver sensitivity, it can detect signals as low as 200 mV, allowing reliable communication over long distances. RS-485 operates in half-duplex mode, meaning only one device can transmit at a time, requiring an enable signal to control the transmitter. It supports multi-point connections, reducing the number of required signal lines and enabling networked systems with up to 32 transmitters and 32 receivers. RS-422 is similar to RS-485 in that it also uses differential signaling, eliminating the need for a digital ground. Unlike RS-485, however, RS-422 supports full-duplex operation using two pairs of twisted wires, allowing simultaneous transmission and reception without interference. While RS-485 can only operate in half-duplex and requires a single pair of wires, RS-422 offers greater flexibility for bidirectional communication. Both RS-422 and RS-485 can transmit data over 1,200 meters at 19.2 kbps. They allow multiple devices to be connected on the same bus, making them ideal for industrial and automation applications. In the past, communication between PCs and smart devices often relied on RS-232, RS-485, or Ethernet, depending on the device's interface requirements. However, RS-232 and RS-485 only define the physical and data link layers. To achieve two-way data exchange, custom software was needed, which often lacked versatility and compatibility. In contrast, fieldbus technologies are built on the ISO/OSI model, offering complete software support for tasks like bus management, collision detection, and link maintenance. These systems enable more robust and scalable communication networks, especially when multiple devices are involved. In an RS-485 network, a master device controls the communication, while other devices act as slaves. This structured approach simplifies system design and improves reliability in industrial environments.

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