RS-485 bus communication fault details processing

The RS-485 (RS232 to RS485) bus interface is an electrical specification for multipoint, differential data transmission and has now become one of the most widely used standard communication interfaces in the industry. The RS-485 standard only specifies the electrical characteristics of the interface and does not involve connectors, cables, or protocols. Therefore, users can build their own high-level communications protocols on this basis.

RS-485 bus communication mode is widely used in instrumentation, intelligent sensor collection and distribution control, building control, monitoring and alarming, etc. due to its simple structure, low cost, proper communication distance and data transmission rate. However, the RS485 bus has some disadvantages such as self-adaptive and self-protection functions. If you do not pay attention to the processing of some details, communication failures or system failures often occur. Therefore, it is very important to improve the operational reliability of the RS-485 bus.

The network topology generally adopts a terminal-matched bus structure and does not support ring or star networks.

Bus nodes are networked in a daisy-chain or bus topology. That is, each node is connected to the main cable by a very short line head. The interface bus is usually designed for half-duplex transmission, that is to say it uses only one pair of signal lines, the driving data and the receiving data can only appear on the signal line at different times.

As for the RS485 cable (RS485 to RS232), common twisted-pair cable can be used in general applications, and coaxial cable with shielding layer can be used in a relatively high environment.

Opto-isolated circuits In some industrial control areas, due to the complexity of the on-site situation, there is a high common-mode voltage across the nodes. Although the RS-485 interface uses a differential transmission method and has a certain resistance to common-mode interference, when the common-mode voltage exceeds the limit receiving voltage of the RS-485 receiver, that is, greater than +12V or less than -7V, the receiver It can no longer work normally. In serious cases, it even burns chips and equipment.
The solution to this problem is to isolate the system power supply from the RS-485 transceiver's power supply via DC-DC; to isolate the signal by optocoupler and completely eliminate the effects of common-mode voltage.

Signal reflection (connected to the termination resistor for matching)
During communication, there are two signals that cause signal reflection: impedance discontinuities and impedance mismatches. The impedance is not continuous, and the signal suddenly encounters little or no cable impedance at the end of the transmission line. The signal will cause reflections at this point. The principle of this kind of signal reflection is similar to the reflection caused by light entering from one medium to another medium. To eliminate this kind of reflection, you must connect a termination resistor of the same size as the characteristic impedance of the cable at the end of the cable so that the impedance of the cable is continuous. Since the transmission of the signal on the cable is bidirectional, a termination resistor of the same size can be connected across the communication cable.

Bus isolation (Low-resistance and cross-connect diodes between interface line and bus)
The RS-485 bus is a parallel-connected two-wire interface. If there is a single chip failure, the bus may be "pulled to death." Therefore, a 4-10 Ω PTC resistor is connected in series between its two-wire port VA, VB and the bus. A 5V TVS diode is also connected across the ground at the same time to eliminate line surge interference. In addition, the chip should be reasonably selected. For example, in order to prevent strong electromagnetic shock to external equipment, it is recommended to use anti-lightning chips.

Ground Problem (1) Common Mode Interference Problem The RS-485 interface uses a differential mode to transmit signals. It does not need to detect the signal with respect to a certain reference point. The system only needs to detect the potential difference between the two lines. However, it should be noted that the transceiver can only work if the common-mode voltage does not exceed a certain range (7 to +12 V). When the common-mode voltage exceeds this range, it will affect the reliability of communication until the interface is damaged.

(2) Electromagnetic dry radiation (EMI) problems The common mode part of the driver output signal requires a return path. If there is not a low-resistance return path (signal ground), it will return to the source in the form of radiation. The entire bus will be A large antenna radiates electromagnetic waves outward. Therefore, despite the differential transmission, a low impedance signal ground is still needed for the RS 485 network.

If the above problem does not solve the RS485 bus communication failure, contact the manufacturer Shenzhen Yutai Technology http://