What is 485?

Smart meters were developed with the maturity of microcontroller technology in the early 80s, and the world meter market was basically monopolised by smart meters, which was attributed to the need for enterprise informatization, and one of the necessary conditions for enterprises in the selection of meters was to have a networked communication interface. Initially it was the data analogue signal output simple process quantity, later the instrument interface was RS232 interface, this interface can realize the point to point communication way, but this way can not realize the networking function, then appeared the RS485 solved this problem.

Chinese name: RS-485

Foreign name: RS-485

Time: 1983

Related: Microcontroller technology

Reason: The need for enterprise informatization

Belonging to: Communication interface

I. Definition

RS-485 is also known as TIA-485-A,ANSI/TIA/EIA-485 or TIA/EIA-485.

RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems, as defined by the Telecommunications Industry Association and the Electronics Industry Alliance. Digital communication networks using this standard are able to transmit signals efficiently over long distances and in electrically noisy environments. RS-485 makes it possible to connect local networks and to configure multi-branch communication links.

RS485 has two types of wiring, two-wire and four-wire. The four-wire system can only achieve point-to-point communication and is rarely used nowadays, but mostly the two-wire wiring is used, which is a bus topology and can be hooked up to 32 nodes on the same bus.

In RS485 communication networks, a master-slave communication method is generally used, i.e. a master with multiple slaves. In many cases, the RS-485 communication link is simply connected to the "A" and "B" ends of each interface with a pair of twisted-pair cables, ignoring the connection of the signal ground. Reason 1 is common mode interference: The RS-485 interface uses differential signal transmission and does not need to detect the signal relative to a reference point, the system only needs to detect the potential difference between the two lines. The network can work normally; when the common mode voltage in the network line exceeds this range it will affect the stability and reliability of communication, and even damage the interface; the second reason is the problem of EMI: the common mode part of the output signal of the transmitter driver needs a return path, such as the absence of a low resistance return channel (signal ground), it will return to the source in the form of radiation, the whole bus will be like a huge antenna radiating outwards electromagnetic waves.

II. Cables

At low speeds, short distances and without interference ordinary twisted pair cables can be used, conversely, at high speeds and long line transmissions, impedance matched (typically 120Ω) special RS485 cables (STP-120Ω (for RS485&CAN) pair 18AWG) must be used, while in environments with harsh interference armoured twisted shielded cables (ASTP-120Ω (for RS485&CAN) pair 18AWG) should also be used.

RS-485 communication cable construction diagram

When using the RS485 interface, for a given transmission line, the maximum cable length allowed for data signal transmission from the RS485 interface to the load is inversely proportional to the baud rate of the signal transmission, this length figure is mainly influenced by factors such as signal distortion and noise. Theoretically, the maximum transmission distance of RS485 can be up to 1200 m at communication rates of 100 Kbps and below, but in practice the transmission distance varies depending on the transmission characteristics of the chip and cable. The signal can be amplified during transmission by adding relays, up to a maximum of eight relays, which means that in theory the maximum transmission distance of RS485 can be up to 10.8 km. If long distance transmission is required, optical fibre can be used as the transmission medium, with a photoelectric converter at each end of the transceiver. The transmission distance of multimode fibre is 5 to 10 km, while single-mode fibre can be used for a transmission distance of up to 50 km.

We have grouped industrial networks into three categories: RS485 networks, HART networks and fieldbus networks.

HART network: HART is an excessive bus standard proposed by Emerson, which mainly superimposes digital signals on top of 4~20 mA current signals, and the physical layer adopts BELL202 frequency shift keying technology to achieve some of the functions of intelligent meters, but this protocol is not an open standard in the real sense.

FieldBus fieldbus network: Fieldbus is one of the hot technologies in the field of automation today and is known as the computer local area network in the field of automation. Its appearance marks the beginning of another new era in automation control technology. Fieldbus is a digital, serial, multi-station communication network connecting the instruments at the control site with the control devices in the control room. Its key mark is its ability to support bi-directional, multi-node, bus-based, fully digital communication. Fieldbus technology has become a hot spot in the international development of automation and instrumentation, and its emergence has revolutionised the structure of traditional control systems, enabling automatic control systems to move further in the direction of "intelligence, digitalisation, informatisation, networking and decentralisation", forming a new type of fully distributed control system with network communication. --Fieldbus Control System (FCS). ProfiBus, CANbus, CC-Link and other standards exist in parallel, and all have their own living space. When will they be unified, it is far from certain. The variety of instruments supporting fieldbus is still relatively small, and the choice is small, the price is high, and the usage is also small.

RS485 network: RS485/MODBUS is a popular way of laying networks, simple and convenient to implement, and there are many instruments supporting RS485.

III. Laying the net

The network topology generally uses a bus-type structure with matched terminals. When building the network, the following points should be noted.

(1) A twisted pair cable is used as the bus to connect the nodes in series. The length of the lead wire from the bus to each node should be as short as possible so that the reflected signal in the lead wire has the least impact on the bus signal. Some network connections may still work normally at short distances and low rates despite incorrect connections, but the adverse effects will become more and more serious as the communication distance is extended or the communication rate is increased, mainly because the signal is superimposed on the original signal after being reflected at the end of each branch, which will cause the signal quality to deteriorate.

(2) Attention should be paid to the continuity of the characteristic impedance of the bus, and the reflection of the signal will occur at the point of impedance discontinuity. Such discontinuities are likely to occur in the following cases: different cables are used for different sections of the bus, or too many transceivers are installed close together on a section of the bus, or too long branch lines are led out to the bus. In short, a single, continuous signal path should be provided as a bus.

(3) Pay attention to the issue of terminal load resistance. The whole network works well without adding terminal load resistance when there are few devices at short distances, but the performance will decrease as the distance increases. In theory, when sampling at the midpoint of each received data signal, matching can be disregarded as long as the reflected signal is attenuated to a low enough level at the start of sampling. However, this is difficult to master in practice, and an article by MAXIM USA mentions an empirical principle that can be used to determine at what data rate and cable length matching is required: when the signal transition time (rise or fall time) is more than three times the time required for the electrical signal to travel in one direction along the bus it can be left unmatched.

The RS-485 should be connected to a terminal resistor at the beginning and end of the bus cable. The termination resistor is 120 Ω in RS-485 networks, which is equivalent to the characteristic impedance of the cable, as most twisted pair cables have a characteristic impedance of approximately 100 to 120 Ω. This matching method is simple and effective, but has the disadvantage that the matching resistor consumes more power and is less suitable for systems with strict power consumption limits. Another more power-efficient matching method is RC matching. Using a capacitor C to isolate the DC component can save most of the power. However, the value of the capacitor C is a difficult issue and requires a compromise between power consumption and matching quality. There is also a diode matching method, which does not achieve true "matching", but it uses the clamping effect of the diode to quickly weaken the reflected signal and improve the signal quality, with significant energy savings.

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