RS-485, also known as the TIA/EIA-485 standard, is a serial communication standard that uses differential signaling and allows for multi-drop (multi-point) communication. It is commonly used in industrial automation, building automation, and data acquisition systems.

History and Standardization

The RS-485 standard was defined in 1983 by the Electronic Industries Alliance (EIA). It was developed as an extension of the RS-422 protocol, offering a structure that allows multiple devices to share the same data line. Today, the standard is officially known as TIA/EIA-485-A.

Technical Specifications

Physical Layer

• Signaling Type: Differential (balanced)
• Cable: Typically twisted pair
• Connection Topology: Bus structure
• Maximum Number of Devices: 32 drivers and 32 receivers (standard); more with advanced drivers
• Cable Length: Up to 1200 meters (depending on data rate)

Electrical Characteristics

• Voltage Levels: Typically ±5V
• Data Rate: Up to 10 Mbps (varies with cable length)
• Transmission Type:
• Half-duplex: Bidirectional over a single pair, but not simultaneously
• Full-duplex: Bidirectional and simultaneous over two cable pairs

Application Areas

• Industrial Automation: PLCs, motor drivers, sensors
• Building Automation: HVAC systems, smart meters, fire alarm systems
• Data Acquisition Systems: SCADA, MODBUS RTU communication
• Transportation and Energy Sectors: Railway systems, transformer stations

Relation to Communication Protocols

RS-485 defines only the physical layer. The protocols that operate on it vary depending on application needs. The most common protocols include:

• MODBUS RTU
• Profibus DP
• BACnet MS/TP
• DMX512 (for lighting control systems)

Design Considerations

• Termination Resistors: To prevent reflections, 120-ohm resistors should be used at both ends of the line.
• Biasing Resistors: Prevent undefined line states when idle.
• Grounding and Shielding: Protect against EMI (Electromagnetic Interference).
• Star Topology: Should be avoided; use bus topology instead.

Advantages and Disadvantages

Advantages

• Suitable for long-distance communication
• Noise-resistant
• Supports multiple devices on the same line
• Well-suited for low-cost applications

Disadvantages

• Requires careful design in complex topologies
• Distance limitations at high data rates
• No built-in standard application protocol; must be implemented separately

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