RS485 interface electromagnetic compatibility design scheme

成都亿佰特

RS485 interface electromagnetic compatibility design scheme [Copy link]

Overview

The RS485 interface forms a half-duplex network, adopts a combination of balanced drivers and differential receivers, uses shielded twisted pair transmission, and has strong resistance to common-mode interference. Electrical characteristics of RS485: the voltage difference between the two lines is + (2V---6V) for logic "1", and the voltage difference between the two lines is - (2V---6V) for logic "0". The interface communication level is compatible with the TTL level. The most commonly used RS485 bus allows up to 32 transceivers to be connected. The specific connection is related to the chip input impedance. For example: when the input impedance of an ordinary chip is ≥12kΩ, the corresponding number of driving points is 32, and when the input impedance is designed to be 1/2 load (≥24kΩ), the corresponding number of driving points is 64.

EMC issues

The RS485 bus standard is one of the most widely used physical protocols for industrial equipment. It is usually used in building automation, industrial automation, network control, security systems and other scenarios. Therefore, the protection measures for the RS485 interface become particularly important. Generally, if the RS485 bus is built indoors, the electromagnetic environment is relatively mild, and only anti-static protection needs to be added. If the RS485 bus is used outdoors, it may be in danger of lightning strikes, and generally a lightning protection circuit needs to be added.

RS485 is used for communication between devices and computers or other devices. When the number of application devices increases, the communication signal and the routing signal become mixed together. The ordinary RS485 interface design may cause EMC and other problems. Interference problem: RS485 may encounter communication anomalies caused by external interference such as pulse interference and electromagnetic interference. At the same time, if it is used in the outside world, it may also cause lightning strikes and cause permanent damage to RS485. Radiation problem: The RS485 interface will radiate the internal interference of the PCB board to the outside through the power line, causing the radiation test index to exceed the standard.

RS485 protection design

1. Use GDT gas discharge tubes, which are mainly used to discharge industrial mode surges and bypass most lightning currents, and can protect up to 4KV or even higher surge current protection capabilities. When selecting devices, it is necessary to consider that the DC breakdown voltage needs to be greater than the maximum operating voltage allowed by the line, and the current allowed to pass through the discharge tube needs to exceed the maximum current designed to pass.

2. R uses a self-resettable fuse with a resistance of only a few ohms. When the current flowing through the loop exceeds the normal operating current, its internal resistance will increase rapidly, thereby limiting the loop current to a very small range. Effective protection should be the damage of the RS485 interface caused by excessive electrostatic current. The selection needs to consider that the maximum rated operating voltage should be greater than the normal operating voltage of the circuit, and the action circuit needs to be less than the maximum current that the line can withstand.

3. TSS uses transient suppression diode TVS. The actual function of TVS tube is similar to that of GDT. It is used for secondary circuit protection, clamping the voltage within the working voltage range and reducing the residual voltage of surge voltage to the lowest range. The breakdown voltage of device selection TVS should be higher than the signal voltage transmitted on the signal line, but the breakdown voltage should not be too high. Generally, the 6V clamping voltage is indicated, and the actual breakdown voltage is about 10V. At the same time, TVS needs to be selected with a small junction capacitance, generally tens of pF. Too high junction capacitance will cause RS485 transmission abnormality at high data rates.

RS485 circuit protection effectively solves the damage risks such as lightning strike, electrostatic interference, surge overvoltage, etc., thereby achieving the purpose of protecting the entire communication line.

我自己123

good

annysky2012

Would it be better to use an integrated type? In addition, for electromagnetic compatibility considerations, some inductors should be added.