RS-485 interface is a serial communication standard widely used in industrial automation and communication systems, which is favored for its excellent anti-interference ability, long-distance transmission characteristics, and multi-point communication capability. This paper discusses the technical details and application advantages of the RS-485 interface through systematic analysis, combined with circuit design principles and application examples.
Overview of RS-485 interface
RS-485 uses differential signaling, i.e., data is transmitted through two signal lines (usually labeled A and B). Comparing the potential difference between these two lines instead of the absolute voltage value effectively improves the anti-interference ability of the signal and enables stable and reliable data transmission in industrial environments. Compared with the traditional RS-232 interface, RS-485 not only supports communication distances of up to 1200 meters, but also allows a single bus to connect up to 32 devices and expand to 128 or 256 nodes by using a chip with high input impedance.
RS-485 Interface Circuit Design
Core Components and Circuit Essentials
1. Transceiver Chip The core of the RS-485 interface is a transceiver chip, such as the MAX485 or SP3485, which converts TTL level signals to differential signals and restores the received differential signals to TTL levels.
Transmitter (TX): Responsible for converting serial data from the microcontroller to differential signal output.
Receiver (RX): Reduces the differential signal to serial data that can be read by the microcontroller.
2. Termination Matching Resistor Adding 120Ω termination resistors at both ends of the bus can effectively eliminate signal reflection and ensure signal integrity.
3. Bias Resistor The bias resistor is used to hold the differential line at a defined level when the bus is idle, preventing noise from triggering false triggers.
4. DE and RE Pin Control In half-duplex communication, the drive enable (DE) and receive enable (RE) pins are often connected together to switch the transmit and receive modes by a single control signal.
Take SP3485 for example, its working principle is as follows:
Data input/output: The DI pin of the chip is connected to the serial data output of the microcontroller, and the RO pin is connected to the serial data input of the microcontroller.
Differential signaling: Differential signaling of data through the A and B pins.
Mode switching: The chip is in receive mode when RE pin is low, and in transmit mode when DE pin is high.
In addition, in order to improve the reliability of the system, it is recommended to add lightning protection components such as TVS diodes and other EMC designs such as common mode inductors to the circuit.
Communication mode and topology of RS-485 interface
1. Communication mode
Half-duplex mode: supports only unidirectional communication at the same time, switching the state by controlling the DE and RE pins.
Full-duplex mode: two pairs of differential lines are required to support simultaneous transmission and reception of data.
Full-duplex mode can be used when you need to be able to send and receive data at the same time. In half-duplex mode, you can only send or receive data at any given time.
2. Topology
Bus topology: RS-485 adopts daisy chain connection, avoiding star connection to reduce signal reflection and interference.
Device Expansion: By using signal repeaters, the number of devices can be expanded to tens of thousands of units and a communication range of several kilometers can be realized.
RS485 Interface for RTU: Wiring Methods and Precautions
ZHC493C is a network IO product supporting 2-way dry (wet) node detection, 1-way relay (COM, NO) output, 1-way analog (current 4~20mA) detection, 1-way serial port transmission, compatible with Modbus RTU/TCP protocol. With “remote control” as the core function, highly easy to use, users can easily and quickly integrated into their own systems to realize remote and local control based on LTE, RS485.
RS485 wiring method
In the following, we will take the ZHC493C of IOTRouter as an example to explain how the RS485 interface of the RTU is connected to the device. ZHC493C contains 1 channel 485 interface, which can connect multiple 485 devices in parallel. The interface supports Modbus RTU/TCP protocol, and the configuration software can read data and control commands directly through this interface.
RS-485 interface design considerations
1. Match the termination resistor with the line The termination resistor needs to be matched with the line impedance (usually 120Ω) to reduce reflection and signal attenuation.
2. Wiring Requirements Use twisted pair wiring to reduce in-phase interference and ensure signal quality.
3. EMC Protection Add components such as TVS diodes to improve the system’s ability to resist electromagnetic interference.
4. Bus stability Reasonable planning of bus length and number of nodes to avoid signal quality degradation due to overload.
