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RS485 Connection: Serial Interface Explained

What is RS485?

RS485 is a serial communication protocol that allows multiple devices to communicate over a single pair of wires. It is a differential signaling system, meaning that it uses two wires, typically labeled as “A” and “B” or “+” and “-“, to transmit data. The differential signaling enables RS485 to achieve high noise immunity and long-distance communication capabilities.

Key Features of RS485

  1. Differential Signaling: RS485 uses differential signaling, where the voltage difference between the two wires determines the logic state. This makes RS485 more resistant to electromagnetic interference and allows for longer cable lengths compared to single-ended communication methods like RS232.

  2. Multi-Drop Network: RS485 supports multi-drop networks, allowing multiple devices (up to 32) to be connected on the same bus. Each device has a unique address, enabling targeted communication within the network.

  3. Half-Duplex Communication: RS485 operates in half-duplex mode, meaning that devices can either transmit or receive data at a given time, but not simultaneously. The direction of communication is controlled by the transmit enable (DE) and receive enable (RE) signals.

  4. High Data Rates: RS485 can support data rates up to 10 Mbps, depending on the cable length and termination. Longer cable lengths generally require lower data rates to ensure signal integrity.

  5. Long Cable Lengths: RS485 can transmit data over long distances, typically up to 1200 meters (4000 feet), making it suitable for large-scale industrial installations and building automation systems.

RS485 Network Topology

RS485 networks are typically arranged in a bus topology, where all devices are connected to a single cable segment. The cable consists of a twisted pair of wires, which helps to cancel out electromagnetic interference.

Termination Resistors

To ensure proper signal integrity and prevent reflections, RS485 networks require termination resistors at both ends of the bus. The value of the termination resistor should match the characteristic impedance of the cable, typically around 120 ohms.

Cable Type Characteristic Impedance
AWG 24 120 ohms
AWG 22 100 ohms
Shielded Cable 100 to 150 ohms

Biasing Resistors

RS485 networks often include biasing resistors to ensure a known state when no device is actively transmitting. Biasing resistors pull the “A” and “B” lines to a defined voltage level, preventing floating inputs and reducing noise sensitivity.

Biasing Type Resistor Value
Pull-Up 680 to 1k ohms
Pull-Down 680 to 1k ohms

RS485 Communication Protocol

RS485 defines the physical layer characteristics for serial communication but does not specify a particular communication protocol. Various protocols can be used on top of RS485, such as Modbus, Profibus, and BACnet.

Data Framing

Data framing in RS485 communication typically includes a start bit, data bits (usually 8), an optional parity bit for error detection, and one or more stop bits. The most common data formats are:

  • 8N1: 8 data bits, no parity, 1 stop bit
  • 8E1: 8 data bits, even parity, 1 stop bit
  • 8O1: 8 data bits, odd parity, 1 stop bit

Baud Rate

The baud rate determines the speed of data transmission in RS485 communication. Common baud rates include 9600, 19200, 38400, 57600, and 115200 bits per second (bps). The maximum baud rate depends on the cable length and the quality of the cable and termination.

Cable Length Maximum Baud Rate
1200 m 100 kbps
600 m 500 kbps
300 m 1 Mbps
150 m 5 Mbps
75 m 10 Mbps

Implementing RS485 Communication

To implement RS485 communication, you need the following components:

  1. RS485 Transceiver: An RS485 transceiver is a device that converts between the UART (Universal Asynchronous Receiver/Transmitter) signals of a microcontroller or PC and the differential signals of the RS485 bus. Common RS485 transceivers include MAX485, SN75176, and ADM2483.

  2. Microcontroller or PC with UART: The microcontroller or PC acts as the master or slave device in the RS485 network. It communicates with the RS485 transceiver using UART signals (TX and RX).

  3. RS485 Cable: Use a twisted pair cable designed for RS485 communication. The cable should have a characteristic impedance of 100 to 150 ohms and be shielded to reduce electromagnetic interference.

  4. Termination and Biasing Resistors: Place termination resistors (120 ohms) at both ends of the RS485 bus. Add biasing resistors (680 to 1k ohms) to pull the “A” and “B” lines to a defined voltage level when no device is transmitting.

Hardware Connection

Connect the RS485 transceiver to the microcontroller or PC’s UART pins as follows:

RS485 Transceiver Pin Microcontroller/PC Pin
DI (Driver Input) TX
RO (Receiver Output) RX
DE (Driver Enable) GPIO
RE (Receiver Enable) GPIO
A RS485 Cable (A)
B RS485 Cable (B)
VCC Power Supply
GND Ground

Software Implementation

  1. Initialize the UART peripheral of the microcontroller or PC with the desired baud rate, data bits, parity, and stop bits.

  2. Set the DE and RE pins of the RS485 transceiver to the appropriate levels for transmit or receive mode:

  3. Transmit Mode: DE = 1, RE = 0
  4. Receive Mode: DE = 0, RE = 1

  5. Implement a protocol layer on top of the UART communication to handle device addressing, data framing, and error checking. This can be done using existing protocols like Modbus or by creating a custom protocol.

  6. When transmitting data, set the DE pin high to enable the driver, send the data via UART, and then set the DE pin low to switch back to receive mode.

  7. When receiving data, set the RE pin high to enable the receiver, read the incoming data from the UART, and process it according to the protocol.

Troubleshooting RS485 Communication

If you encounter issues with RS485 communication, consider the following troubleshooting steps:

  1. Check Wiring: Ensure that the RS485 cable is properly connected, with the “A” and “B” lines paired correctly. Verify that the shield is grounded at one end only to avoid ground loops.

  2. Verify Termination and Biasing: Confirm that the termination resistors are placed at both ends of the bus and have the correct value. Check that the biasing resistors are present and connected correctly.

  3. Inspect Signal Integrity: Use an oscilloscope to examine the RS485 signals for any distortion, noise, or reflections. Ensure that the signal levels are within the RS485 specification (+/-200 mV differential voltage).

  4. Validate Baud Rate and Data Format: Double-check that all devices on the RS485 network are configured with the same baud rate, data bits, parity, and stop bits.

  5. Check Device Addressing: Verify that each device on the network has a unique address and that the addresses are correctly configured in the software.

  6. Analyze Protocol Implementation: Review the protocol implementation on each device to ensure that it adheres to the specified format and error handling procedures.

Frequently Asked Questions (FAQ)

  1. What is the maximum number of devices that can be connected to an RS485 network?
  2. RS485 supports up to 32 devices on a single network without the use of repeaters. With repeaters, the number of devices can be extended to 256 or more.

  3. Can RS485 be used for full-duplex communication?

  4. RS485 is designed for half-duplex communication, where devices can either transmit or receive data at a given time. However, full-duplex communication can be achieved by using two separate RS485 networks, one for transmitting and one for receiving.

  5. Is RS485 compatible with RS232?

  6. RS485 and RS232 are different communication standards and are not directly compatible. However, you can use converter modules or transceivers to interface between RS485 and RS232 devices.

  7. How do I select the appropriate baud rate for my RS485 network?

  8. The baud rate selection depends on the cable length and the desired data throughput. Longer cable lengths require lower baud rates to maintain signal integrity. Refer to the table in the “Baud Rate” section for guidance on maximum baud rates for different cable lengths.

  9. What is the purpose of the shield in an RS485 cable?

  10. The shield in an RS485 cable is used to protect the signal wires from electromagnetic interference (EMI) and reduce noise pickup. It is important to ground the shield at one end of the cable to avoid ground loops and ensure proper shielding effectiveness.


RS485 is a robust and widely used serial communication standard that enables reliable data transmission over long distances in noisy environments. Its differential signaling, multi-drop capability, and half-duplex operation make it well-suited for industrial automation, building automation, and other applications requiring networked communication.

By understanding the key features, network topology, communication protocol, and implementation details of RS485, you can successfully design and troubleshoot RS485-based systems. With proper wiring, termination, biasing, and protocol implementation, RS485 provides a solid foundation for efficient and dependable data communication in various industries.