What is the use of the RS485 bus, and how does the RS485 communicate?

Bus time-sharing multi-machine communication.

RS485 bus common sense.

1. Basic characteristics of RS485 bus.

According to the RS485 industrial bus standard, the RS485 industrial bus is a half-duplex communication bus with a characteristic impedance of 120 and a maximum load capacity of 32 payloads (including the master control device and the plant setup).

2. RS485 bus transmission distance.

When using twisted pair as a communication cable, the theoretical value of the maximum transmission distance is as follows:

Baud rate Maximum distance.

2400bps 1800m

4800bps 1200m

9600bps 800m

When using thinner communication cables, or using this product in an environment with strong electromagnetic interference, or when there are more devices connected to the bus, the maximum transmission distance is shortened accordingly; Conversely, the maximum distance is extended.

3. Connection method and terminal resistance.

1) The RS485 industrial bus standard requires that the equipment be daisy-chained, and the two ends must be connected with 120 terminal resistors (as shown in Figure 1), and the simplified connection can be connected in Figure 2, but the distance between the "D" section can not exceed 7 meters.

Figure 1. Figure II.

2) The connection method of the ball machine terminal 120 matching resistor.

The dome terminal 120 matching resistor can be connected by the DIP switch DIP on the dome chassis, as shown in Figure 3. When the dome machine leaves the factory, the 120 matching resistor is not connected by default, and the 120 matching resistor can be connected to the line by dialing the 10th position of the DIP switch to on. On the other hand, if you do not connect the 120 matching resistor, you can set the 10th bit to off.

Figure III. 4. Problems in practical application.

In actual construction and use, users often adopt the star-shaped connection mode, and the terminal resistor must be connected to the two devices with the farthest road distance (as shown in Figure 4, 1 and 15 equipment), but because the connection mode does not meet the use requirements of the RS485 industry standard, when the distance between the equipment lines is far, it is easy to produce problems such as signal reflection and anti-interference ability, resulting in the reliability of the control signal decreasing. In this case, the phenomenon is that the dome machine is completely out of control, or it cannot be stopped by itself.

Figure IV. In this case, it is advisable to add an RS485 splitter. This product can effectively convert the star connection to the connection specified in the RS485 industry standard, thereby avoiding problems and improving communication reliability, as shown in Figure 5. Figure 5.

Half-duplex data transfer allows data to be transmitted in both directions, but at a certain point, data is only allowed to be transmitted in one direction, which is actually a simplex communication that switches directions; Only one party can receive or send information at the same time, and two-way communication can be achieved. Example: Walkie-talkie.

Full-duplex data communication allows data to be transmitted in both directions at the same timeTherefore, full-duplex communication is a combination of two simplex communication methods, which requires both the sending device and the receiving device to have independent receiving and sending capabilities, and can receive and send information at the same time to achieve two-way communication. Example: Communication.

Standard. The RS-485 standard is a half-duplex communication protocol, and RS-485 is suitable for the transmitter and receiver to share a pair of lines for communication, and also for multiple points to share a pair of lines for bus networking.

RS-485 interface chips have been widely used in many fields such as industrial control, instruments, meters, multi-** networks, mechatronics products, etc. There are also more and more types of chips that can be used for RS-485 interfaces. How to find the most suitable chip among a wide variety of interface chips is a problem in front of every user.

The RS-485 interface has different requirements for chips and usage methods in different use occasions. What factors should users consider in the selection of chips and the design of circuits, due to the inherent characteristics of some chips, some faults in communication even need to be adjusted accordingly in the software. The RS-485 interface can be connected to both half-duplex and full-duplex communication modes.

The RS485 bus communicates with two conductors, the other of which is a normal conductor and is a necessary connection for the suppression of interference by the ground G [long-distance cable shield]. There were a total of three wires that could not be connected during the test.

Since RS-485 was developed on the basis of RS-422, many of the electrical specifications of RS-485 are similar to RS-422. For example, the balanced transmission method is used, and the terminal resistor is connected to the transmission line.

The RS-485 can be used in both two-wire and four-wire systems, and the two-wire system can achieve true multi-point two-way communication. As with the RS-422, with a four-wire connection, only point-to-many communication can be achieved, that is, there can only be one master and the rest are slaves, but compared with the RS-422, there are improvements. Whether it is a four-wire or two-wire connection, more than 32 devices can be connected on the bus.

RS485 generally uses twisted pair cables.

With the maturity of single-chip microcomputer technology in the early 80s, the world instrument market is basically monopolized by smart meters, which is due to the needs of enterprise informatization, and one of the necessary conditions for enterprises in instrument selection is to have a network communication interface. At first, it was a simple process quantity of data analog signal output, and then the instrument interface was RS232 interface, which could realize point-to-point communication, but this way could not realize the networking function, and then RS485 solved this problem.

The RS485 bus 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. The RS-485 bus network topology generally adopts a terminal-matched bus structure. That is, a bus is used to connect the nodes in series, and ring or star networks are not supported.

Digital communication networks using this standard can effectively transmit signals over long distances and in environments with high electronic noise. RS-485 makes it possible to configure inexpensive local networks as well as multi-leg communication links.

The common communication faults of the 485 bus structure are as follows:

1. Communication is not available and there is no response.

3. The system prompts that it is disturbed during communication, or the communication indicator light flashes constantly when there is no communication.

4. Sometimes it can be communicated, sometimes it can't, some instructions can be communicated, and some instructions can't.

Here are a few suggestions and advice to reduce communication failures.

1. Carry out construction in strict accordance with the construction specifications of the 485 bus, and put an end to any fluke mentality.

2. Adopt scientific and reserved solutions for 485 bus projects with long lines and large loads.

3. If the communication distance is too long, such as more than 1200 meters, it is recommended to use a repeater to solve the problem.

4. If the number of loads is too much, such as more than 30 units on a bus, it is recommended to use a splitter to solve the problem.

5. On-site debugging with complete debugging equipment. On-site debugging must carry a few converters that can connect long distances and multiple loads, a commonly used computer notebook, a multimeter for test path breaks, and a few 120 ohm termination resistors.

Electronic fence system bus system 485 communication wiring diagram.

In the construction of the 485 bus system, the construction must be strictly in accordance with the construction specifications, and the following points should be paid special attention to:

and 485 data cables must be twisted together with each other.

2. The wiring must be multi-strand shielded twisted pair. The multi-strand is for standby, the shielding is for debugging in special circumstances, and the twisted pair is because the 485 communication adopts the principle of differential mode communication, and the anti-interference of the twisted pair is better. It would be a mistake not to use twisted pairs.

The bus must use a hand-in-hand bus structure, and resolutely avoid star connections and bifurcated connections.

4. The alternating current and chassis supplied by the equipment must be truly grounded and well grounded. There are many places where there are triangular sockets on the surface, but in fact, there is no grounding at all, and good grounding can prevent the equipment from being struck by lightning and surge. When static electricity accumulates, it can cooperate with the lightning protection design of the equipment to better release energy and protect the 485 bus equipment and related chips from damage.

5. In order to avoid the interference of strong electricity, the 485 bus should be avoided from walking together with strong electricity.