RS485 is a widely used standard for serial communication, particularly in industrial and commercial environments. Here's a structured overview:

​Basics

• ​Standard: Developed by the Electronic Industries Alliance (EIA), RS485 defines the electrical characteristics of a serial communication system.
• ​Purpose: Designed for robust, long-distance data transmission in noisy environments.

Key Features

1. Differential Signaling:

• Uses two wires (A and B) to transmit data via voltage differences, enhancing noise immunity.
• Voltage Range: Typically ±5V, with a minimum detectable difference of ±0.2V.

2. Multi-Drop Capability:

• Supports up to ​32 devices on a single bus (expandable to 256 with repeaters).
• ​Topology: Bus or daisy-chain configuration.

3. Duplex Modes:

• ​Half-Duplex: Two-wire setup (communication in one direction at a time).
• ​Full-Duplex: Four-wire setup (rarely used; RS422 is more common for full-duplex).

4. Distance and Speed:

• ​Maximum Distance: Up to ​1,200 meters (4,000 feet) at lower speeds.
• ​Data Rates: Ranges from 100 kbps (long distances) to 10 Mbps (short distances).

5. ​Noise Immunity:

• Rejects common-mode noise due to differential signaling, making it ideal for industrial settings.

​Technical Considerations

• ​Cabling: Twisted-pair cables with 120Ω characteristic impedance.
• ​Termination: 120Ω resistors at both ends of the bus to prevent signal reflections.
• ​Biasing: Resistors may be added to stabilize the idle state in half-duplex systems.

​Applications

• Industrial automation (e.g., PLCs, sensors).
• Building automation (HVAC, security systems).
• Protocols like ​Modbus RTU, Profibus, and DMX512.

​Comparison with RS232

• ​RS232: Point-to-point, shorter distances (<15 meters), single-ended signaling, prone to noise.
• ​RS485: Multi-drop, longer distances, differential signaling, noise-resistant.

​Key Notes

• ​Protocol Agnostic: RS485 defines only the physical layer; data protocols (e.g., Modbus) handle addressing and messaging.
• ​Addressing: Devices are identified via software protocols, not hardware.

​Common Misconceptions

• ​Full-Duplex: While possible with four wires, RS485 is typically half-duplex. RS422 is the full-duplex counterpart.
• ​Speed vs. Distance: Trade-offs exist; higher speeds reduce maximum cable length.

RS485 remains a cornerstone of reliable serial communication in demanding environments, balancing distance, speed, and robustness.