With the more and more extensive application of RS485 communication mode, the understanding and understanding of RS485 is becoming more and more important, which is the knowledge that every engineer who does communication, intelligent system and intelligent instrument must master.
RS485 connection mode:
RS485 now mostly adopts two-wire connection mode, which is bus topology structure, and can be connected to 32 nodes at most on the same bus.
Cable: ordinary twisted pair can be used in low speed, short distance and non-interference situations. On the contrary, impedance matching (generally 120) must be used in high speed and long line transmission Ω） In addition, the armored twisted pair shielded cable should be used in the environment of severe interference. The single section of cable shielding network needs to be grounded, and the wiring needs to be separated from the high voltage cable.
Understanding of RS485 interface:
485 communication interface is a hardware description of communication interface. It only needs two communication lines, that is, it can transmit data between two or more devices. This data transmission connection is a half duplex communication mode. At a certain time, a device can only send or receive data.
(2) After the hardware communication interface is established, a data protocol needs to be agreed between the instruments for data transmission, so that the receiving end can analyze the received data. This is the concept of “Protocol”.
(3) Communication protocol has a unified standard protocol format, such as “MODBUS” protocol. The standard protocol is comprehensive and contains a lot of content, but it is not easy to understand. Therefore, we can define a kind of protocol, which is simple and practical, that is “custom protocol”.
(4) Including Modbus protocol, custom protocol. Gas turbine is compatible with two protocol formats.
RS485 common connector:
Because RS-485 interface has good noise immunity, long transmission distance and multi station capability, it becomes the preferred serial interface. Because the half duplex network composed of RS485 interface only needs two wires, so the RS485 interface adopts shielded twisted pair transmission. The RS485 interface connector adopts 9-core plug socket of DB-9, the RS485 interface with intelligent terminal adopts DB-9 (hole), and the RS485 interface with keyboard adopts DB-9 (PIN).
Now the 485 bus is prone to failure, and the methods to eliminate these failures are listed as follows:
1. Because the 485 signal uses a pair of unbalanced differential signals, it means that every device in the 485 network must be connected to the ground through a signal circuit to reduce the noise on the data line. Therefore, the data line is best composed of twisted pair, and a shielding layer is added outside as the ground wire to connect the 485 devices in the 485 network, and a single terminal point is reliably grounded.
2. In the industrial field, the field situation is very complex, and there is a high common mode voltage between each node. 485 interface uses differential transmission mode, which has the ability to resist common mode interference, but when the common mode voltage is greater than + 12V or less than – 9V, it exceeds the limit receiving voltage of 485 receiver. The receiver will not work and may even burn the chip and the device together. 485 optical isolation repeater can be used in 485 bus to completely isolate 485 signal and power supply, so as to eliminate the influence of common mode voltage.
3. With the extension of transmission distance, 485 bus will produce echo reflection signal. If the transmission distance of 485 bus is more than 100 meters, it is recommended to match resistance at 120 ohm terminal at the head and end of 485 communication during construction.
4. In 485 bus, the distance between 485 node and trunk should be reduced as far as possible, and it is generally recommended that 485 bus adopt hand-in-hand bus topology. The star structure will produce reflected signal, which will affect the quality of 485 communication.
5. Factors affecting the load capacity of 485 bus: communication distance, wire quality, baud rate, power supply capacity of converter, lightning protection of 485 equipment, selection of 485 chip.
6.485 signal line can be run together with strong current power line. In the actual construction, because the wiring is through the pipeline, the construction party sometimes directly binds the 485 signal line and the power line together for the convenience of the drawing. Because the strong current has a strong electromagnetic signal to interfere with the weak current, it leads to the instability of the 485 signal and the communication instability.
7.485 signal line can use parallel line as wiring, can also use unshielded line as wiring. Because 485 signal is transmitted by differential mode, that is, the voltage difference between 485 + and 485 – is used as signal transmission. If an external interference source interferes with it, when using twisted pair for 485 signal transmission, because of its twisted pair, the interference effect on 485 + and 485 – is the same, the voltage difference is still unchanged, and the interference on 485 signal is minimized. In the same way, if the shielding wire plays a shielding role, the interference effect of external interference sources can be reduced as much as possible.
8.485 bus must be grounded. In many technical documents, it is mentioned that 485 bus must be grounded, but how to ground is not mentioned in detail. Strictly speaking, 485 bus must be grounded reliably at a single point. Single point is that there can only be one point grounding on the whole 485 bus, not multi-point grounding, because it is necessary to keep the voltage on the ground wire (generally shielded wire as ground wire) consistent to prevent common mode interference. If multi-point grounding is just the opposite.