Bobby Brown
Post 2023-07-31
What is RS485? Understand RS485 in 5 Minutes


  • RS485 is a hardware architecture that specifies only the electrical characteristics of transmitters and receivers, without endorsing any particular transmission protocol.
  • RS485 transmits binary signals by generating high and low voltages, representing binary 0s and 1s (on and off), enabling efficient long-distance communication in electronically noisy environments.
  • Devices utilizing RS485 can communicate with central control systems using communication protocols such as Modbus and ASCII.


What is RS485?

RS485, also known as TIA-485(-A) or EIA-485, is a standard for the electrical characteristics of drivers and receivers in serial communication systems. As outlined by Wikipedia,

"RS485 operates within the physical layer of the OSI model, offering a two-wire, half-duplex, balanced transmission line standard for multi-point communication. Issued jointly by the Telecommunications Industry Association (TIA) and the Electronic Industries Alliance (EIA), this standard ensures efficient communication over long distances in electronically noisy environments. In a linear bus topology, it supports multiple receivers within a network, using cable voltage differentials to indicate signal transmission, with any voltage differential above 0.2V being valid, and accepting any differential not exceeding 12V or below -7V as correct. Although RS485 does not specify or recommend any data protocols, it defines the electrical characteristics of transmitters and receivers, using twisted pair cables for high-speed data communication (up to 35Mbit/s at 10m, and 100kbit/s at 1200m)."[1]

In essence, RS485 does not define the specific communication protocol, including transmission speed, format, and protocol. Instead, it focuses solely on outlining the electrical characteristics of the transmitters and receivers, known as the physical layer. The RS485 standard enables the transmission of binary signals as high and low voltages, representing the states of 1 and 0 (on and off) within the circuit.
Schematic diagram of binary 0 and 1 signal transmission

In most cases, RS485-based devices can employ communication protocols such as Modbus and ASCII to establish communication with a central control system. To illustrate, let's consider LORRIC's paddle wheel flowmeter, which utilizes Modbus as the underlying protocol for defining various signal transmissions. The flowmeter leverages RS485 for signal transmission and is connected in a serial configuration with the customer's central control system. Through appropriate programming and configuration settings within the central control system, relevant flow measurement data can be read from the flowmeter.

1. RS485: Wiring Solution for Multi-Device Daisy Chain

RS485 provides a means of interconnecting multiple devices in a series using a twisted pair of wires to facilitate data exchange. It offers two primary wiring methods: two-wire half-duplex and four-wire full-duplex. While four-wire full-duplex exists, it is less prevalent in current applications, with the two-wire half-duplex configuration being the predominant wiring method in use today.

RS485 Multi-Device Daisy Chain Wiring Scheme

The picture shows the RS485 wiring scheme, including device interconnection, twisted pair cable structure, and signal transmission.
• Devices (Device 1, Device 2, Device N) are connected in series, forming a daisy chain topology. Each device's A+ and B- ports are connected to the corresponding ports of the next device using twisted pair cables. The wiring method uses A+ and B- lines, representing a two-wire half-duplex configuration, which is the most commonly used wiring method in RS485. 
• The left side of the image shows the structure of the twisted pair cable, including copper wire, shielding, and insulation. This cable structure helps maintain stable data transmission in electrically noisy environments. It should be noted that the connection wires between each device should be kept as short as possible to minimize signal attenuation and interference, improving communication reliability. 
• The entire daisy chain network is ultimately connected to a PLC (Programmable Logic Controller), facilitating data exchange between multiple devices and the central control system. Using the RS485 standard, signals are transmitted through A+ and B- lines, with high and low voltages representing binary 1 and 0 (on and off), ensuring reliable data communication.

2. Two-wire half-duplex

The two-wire half-duplex system enables bidirectional data transmission between two devices, but not simultaneously. For instance, consider devices A and B. During a specific timeframe, data transmission is allowed from A to B, and once completed, data transmission from B to A can take place. Below is a commonly used circuit diagram for RS-485:

Commonly used circuit diagram for RS-485

The provided circuit illustrates the fundamental connections for a two-wire wiring setup. In this method, all nodes within the network share the same pair of communication lines. One line, known as the A line (or Data+), is responsible for transmitting differential signals, while the other line, called the B line (or Data-), handles the complementary differential signals. This utilization of a differential signal transmission mode effectively mitigates interference and enhances overall communication reliability.[2]

1) Topological Structure

Schematic diagram of various topologies

Network topology refers to the arrangement of various nodes (such as computers, servers, switches) in a computer network. Different topology designs suit different network needs and applications, affecting performance, reliability, and scalability. Each topology has its specific advantages and disadvantages. Choosing the right topology is crucial for building an efficient and stable network. Below are introductions to common topologies:

Type Diagram Instruction
Backbone with stubs A network design where a central backbone connects various smaller networks, or stubs. The backbone provides the main communication path, handling high-capacity or critical traffic, while stubs manage local traffic. This design simplifies network management and improves efficiency by centralizing critical pathways and decentralizing local traffic handling.
Backbone with stars/clusters A central backbone connecting multiple star or cluster networks. Each star network has a central node linking to peripheral nodes. This design balances load and improves fault tolerance by segmenting the network into manageable clusters while maintaining robust interconnectivity through the backbone.
Star Topology A central hub connecting directly to multiple peripheral nodes. Each node communicates through the hub, simplifying network management. If one peripheral fails, the rest remain unaffected, enhancing reliability. However, if the hub fails, the entire network is compromised, making the hub a single point of failure.
Ring Topology

Ring topology arranges network nodes in a circular configuration where each node connects to two others, forming a closed loop. Data travels in one or both directions around the ring. This topology offers efficient data transfer but can be disrupted if any single connection breaks, though redundancy protocols can mitigate this risk by providing alternative pathways for data.

Daisy Chain Topology Daisy chain topology connects each node in series, where each node has exactly two connections, one to the preceding and one to the next node. This simple design is cost-effective and easy to implement for small networks. However, a break in any connection can disrupt the entire network, making it less reliable for larger setups.

Among various topological structures, the Daisy Chain topology is considered the optimal choice for RS-485 due to its minimal impact on signal integrity. The characteristic feature of the Daisy Chain topology is the sequential connection of each device along a single line, not forming a loop structure. This configuration facilitates efficient data transmission between devices while reducing the risk of signal distortion. However, it is important to note that as the length of the cable increases, signal distortion can occur during transmission along the communication line, thereby reducing the maximum data rate that can be achieved.[3]

2) Recommendations for RS-485 Wiring

  • It is generally recommended to use a 24AWG twisted pair cable with grounded shielding. The RS485 network must be designed as one line with multiple drops, not as a star or ring topology.
  • When the connection is long, please use terminating resistors at both ends of the network, i.e., at the master and the farthest device. It is generally recommended to use a 120Ω terminating resistor. For the actual value, please refer to the wire specifications.
  • If the signal is unstable, please apply a biasing circuit at one point on the line.
  • If the 8-core cable shipped with the device is used for RS485 communication, please trim the length and try to use a short wire to reduce noise interference. Connect the shielding net to the main communication wire shielding and then ground it.
  • In areas with a lot of signal interference, the software may need to perform multiple inquiries to get a valid response.

3. Case study

Application Example of LORRIC Paddle Wheel Flow Meters in the Central Chemical Dosing and Dispensing System at Hingsen Semiconductor

The application scenario of LORRIC paddle wheel flow meters within the central chemical dosing and dispensing system at Hingsen Semiconductor

In the central chemical dosing and dispensing system of China's Hingsen Semiconductor, the configuration comprises central chemical storage tanks coupled with dispensing systems at each process point, controlled through valve boxes to regulate the supply of chemical reagents. Each valve box is equipped with a LORRIC paddle wheel flow meter to measure the flow rate of reagents. The flow meters relay flow information in real-time to the central control system via RS-485, halting the supply once the specified amount of reagent is dispensed. Within such a system, RS-485 serves a pivotal role as a bridge, establishing communication of flow information between the central control system and each valve box.


  1. ^ Duplex (telecommunications)-Wikipedia
  2. ^ EIA-485 - Wikipedia
  3. ^ RS485 vs Ethernet: Which One is Most Used in Industry? - Robotiq
  4. ^ Mechanical and electrical installation - LORRIC
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