RS-232, RS-485, and RS-422 are common serial communication standards, each with unique characteristics and applicable scenarios. Below is a detailed analysis of these three communication standards:
1. RS-232
1. Origin and Purpose:
● Established by the Electronic Industries Alliance (EIA) in 1962.
● Aimed at solving low-speed, short-distance communication issues between early computers and peripheral devices.
2. Core Features:
● Electrical Characteristics: Uses asymmetric (single-ended) signal transmission, transmitting data through different voltage levels (typically ±12V), where logic 1 is -3V to -15V, and logic 0 is +3V to +15V, providing some level of anti-interference capability.
● Transmission Distance: Maximum transmission distance is about 15 meters (can reach 50 meters in practical applications, but requires a lower baud rate).
● Communication Method: Full-duplex communication (point-to-point).
● Baud Rate: Common range is 300bps to 115.2kbps (can reach up to 1Mbps, but limited by distance).
● Interface: DB9 connector (9 pins) and DB25 connector (25 pins) are common interfaces.
3. Typical Applications:
● Debugging interfaces in industrial control.
● Connecting Programmable Logic Controllers (PLCs) to host computers.
● CNC machines transmitting G-code instructions via RS-232.
● Medical devices such as electrocardiograms (ECG) and blood glucose meters outputting test data via RS-232.
4. Advantages and Disadvantages:
● Advantages: Simple hardware and software implementation, easy to promote and apply.
● Disadvantages: Short transmission distance, low speed, poor anti-interference capability, and can only communicate between two devices (point-to-point), unable to support multi-device communication.
2. RS-485
1. Origin and Purpose:
● Established by the Electronic Industries Alliance (EIA), specifically designed for multi-node, long-distance, and high anti-interference industrial communication scenarios.
2. Core Features:
● Electrical Characteristics: Uses differential signal transmission, where logic 1 is A+ voltage > B- voltage (typical difference ≥ +1.5V), and logic 0 is A+ voltage < B- voltage (typical difference ≤ -1.5V), providing strong anti-interference capability.
● Transmission Distance: Theoretical maximum is 1200 meters (at a rate ≤ 100kbps), with speed inversely proportional to distance.
● Communication Method: Half-duplex/full-duplex (requires switching).
● Node Count: A single bus supports 32 to 256 nodes (depending on the transceiver’s driving capability).
● Baud Rate: Supports from 300bps to 10Mbps (commonly seen in practical applications at 9600bps to 115.2kbps).
3. Typical Applications:
● Fieldbus in industrial automation.
● Device communication in smart buildings and power systems.
● Data collection and control between PLCs and sensors under the Modbus RTU protocol.
4. Advantages and Disadvantages:
● Advantages: Supports multi-node communication, long-distance transmission, and strong anti-interference capability.
● Disadvantages: Complex protocol management, risk of conflict in multi-master communication, and defaults to half-duplex communication.
3. RS-422
1. Origin and Purpose:
● Established by the Electronic Industries Alliance (EIA), specifically designed for long-distance, high anti-interference, and full-duplex communication scenarios.
2. Core Features:
● Electrical Characteristics: Uses differential signal transmission, where logic 1 is A+ voltage > B- voltage (typical difference ≥ +2V), and logic 0 is A+ voltage < B- voltage (typical difference ≤ -2V), providing strong anti-interference capability.
● Transmission Distance: Theoretical maximum is 1200 meters (at a rate ≤ 100kbps), with speed inversely proportional to distance in practical applications.
● Communication Method: Full-duplex communication (independent sending and receiving differential pairs).
● Driving Capability: A single driver can drive 10 receivers (multi-point reception, but only supports point-to-point transmission).
3. Typical Applications:
● High-precision temperature/pressure sensors transmitting data in noisy industrial environments.
● Transmission of uncompressed video signals between professional cameras and broadcasting stations.
● Aircraft avionics systems transmitting critical data via RS-422.
4. Advantages and Disadvantages:
● Advantages: Strong anti-interference capability of differential signals, supports full-duplex communication, and long-distance transmission.
● Disadvantages: Does not support multiple sending devices competing for the bus, only suitable for point-to-point or multi-branch receiving scenarios.
4. Summary Comparison
| Feature | RS-232 | RS-485 | RS-422 |
| Signal Type | Single-ended voltage signal | Differential signal | Differential signal |
| Communication Method | Full-duplex (point-to-point) | Half-duplex/Full-duplex (requires switching) | Full-duplex (independent send/receive channels) |
| Maximum Node Count | 1 (point-to-point) | 32~256 | 1 send, 10 receive (multi-branch reception) |
| Transmission Distance | ≤15 meters | ≤1200 meters (at 100kbps) | ≤1200 meters (at 100kbps) |
| Anti-interference Capability | Poor | Strong | Strong |
In summary, RS-232, RS-485, and RS-422 each have unique characteristics and applicable scenarios. When choosing, one should consider specific application requirements, transmission distance, node count, and anti-interference capability.