What is RS485
RS-485, also known as TIA-485(-A) or EIA-485, is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems.
As explained by Wikipedia:
RS-485 is electrical signaling which is balanced, and multipoint systems are supported. The standard is jointly published by the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multidrop bus. These characteristics make RS-485 useful in industrial control systems and similar applications.
RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. It is generally accepted that RS-485 can be used with data rates up to 10 Mbit/s[a] or, at lower speeds, distances up to 1,200 m (4,000 ft).[2] As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. [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. RS485 specifies the data transmission rate, cable length, and the relationship between branch line length and configuration.
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 like 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 series 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.
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RS485 employs a wiring approach that enables the seamless connection of numerous devices in a series using a single wiring setup.
｜Schematic diagram of connecting multiple devices in series with RS485
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.
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- 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, is responsible for transmitting forward data, while the other line, called the B line, handles the transmission of reverse data. This utilization of a differential signal transmission mode effectively mitigates interference and enhances overall communication reliability. [2]
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- Topology
Topology can be classified into various categories, including:
｜Schematic diagram of various topologies
Among various topologies, the daisy chain topology (also known as Daisy) is often considered the optimal choice for RS-485 due to its minimal impact on signal integrity. In a daisy chain topology, each device is connected in a linear fashion, forming a ring-like structure. This configuration facilitates efficient data transfer between devices while minimizing the risk of signal distortion.
However, it is important to note that as the length of the daisy chain increases, the transmission of the signal along the communication line may suffer from distortion, resulting in a reduced maximum data transmission rate. It is crucial to consider the length and characteristics of the communication line when implementing a daisy chain topology to ensure reliable data transmission. [3]
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- Wiring suggestions for RS485
- 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 the terminating resistance on both ends of network, master and farest device. It is generally recommended to use 120Ω terminating resistor. For the actual value, please refer to the wire specifications.
- If the signal is unstable, please apply biasing circuit at one point on the line.
- The 8-core cable shipped with device, if used in RS485 communication, please trim the length, try to use a short wire to reduce noise interference. Connect the shielding net to the main communication wire shielding and then ground it.
- Where there is a lot of signal interference, the software may need to do multiple inquiries to get a valid response. [4]
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Case study
Application example of LORRIC paddle wheel flowmeter in the Centers Chemical Dispensing Control (CDC) of Fastprint Circuit Tech Co.,LTD
｜Application scenario of LORRIC paddle wheel flowmeters connected to PLA with RS485 in Fastprint Circuit Tech CDC.
The central dosing and dispensing system of Fastprint Circuit Tech Co.,LTD incorporates a central chemical removal tank along with dispensing systems installed at each process endpoint. Valve boxes are utilized to regulate the supply of chemical agents.
To measure the flow rate of the medication accurately, each group of valve boxes is equipped with LORRIC paddle wheel flowmeters. These flowmeters relay real-time flow information to the central control system via RS485 communication. Whenever the specified amount of medication is reached, the central control system receives the feedback and halts the supply accordingly.
In this setup, RS485 plays a pivotal role as a bridge, enabling seamless flow information communication between the central control system and each valve box.
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Citations
1. ⌃ RS-485 - Wikipedia (2023-06-17)
2. ⌃ Duplex (telecommunications)-Wikipedia (2023-06-17)
3. ⌃ RS485 vs Ethernet: Which One is Most Used in Industry? (2023-06-17)
4. ⌃ LORRIC paddle wheel flowmeter Mechanical and electrical installation-Wiring suggestions (2023-06-17)