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 the third revision of the standard. This interface consists of 25 signal lines, divided into a primary channel and an auxiliary one. In practice, most applications only use the main channel, which typically requires just three lines: transmit, receive, and ground. For basic full-duplex communication, these three lines are usually sufficient. 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 capacitive load, which limits the maximum communication distance. For example, with a cable that has 150 pF per meter, the maximum distance is around 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. As a result, it is generally used for short-range communications, typically 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 makes it highly resistant to electromagnetic interference. With high sensitivity, RS-485 transceivers can detect signals as low as 200 mV, allowing reliable communication over long distances. It operates in half-duplex mode, meaning only one device can transmit at a time, requiring an enable signal to control the transmission circuit. RS-485 is ideal for multi-point networks, supporting up to 32 drivers and 32 receivers on the same bus. RS-422 is similar to RS-485 in that it also uses differential signaling, eliminating the need for a digital ground. Unlike RS-232, which is single-ended, RS-422 enables full-duplex communication using two pairs of twisted wires, allowing simultaneous sending and receiving without interference. However, RS-485 is limited to half-duplex operation and only requires a single pair of twisted wires. Both standards can support communication over 1,200 meters at 19.2 kbps, making them suitable for industrial applications. Historically, communication between PCs and smart devices relied heavily on RS-232, RS-485, and Ethernet, depending on the device’s interface requirements. However, these standards primarily define the physical and data link layers, leaving higher-level communication protocols to be implemented manually. Most custom programs lacked compatibility with ISO/OSI standards, resulting in limited versatility and applicability to specific devices. In larger systems with more than two devices, RS-485 is often preferred, where a master device coordinates communication with multiple slave devices. Fieldbus technologies, on the other hand, are built upon the ISO/OSI model and offer a complete software framework for managing bus control, collision detection, and link maintenance. This makes them more scalable and reliable for complex networked environments.

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