1. What is a 485 isolation circuit?
RS-485 is a commonly used data communication protocol, widely used in industrial control systems, building automation, security systems and other fields. In some applications, the RS-485 interface needs to be isolated in order to prevent electrical noise interference or electrical shock. The following situations require 485 isolation:
1. When the RS485 external interface is connected to a high voltage, it is easy to damage the back-end circuit, and even cause an electric shock at the end of use.
2. When the communication nodes of RS485 are too far away, the reference ground of each node is connected to the local ground. When there is a large voltage difference between the two ends of the ground, the ground potential will be superimposed on the signal in the form of common mode voltage. On the line, it may exceed the common-mode voltage range that the port can withstand, which will affect normal communication and even damage the back-end circuit.
3. When the ground plane between 485 communication nodes with a long distance is connected by cables, the ground wire will form a ground loop with the ground, couple external common mode noise, and generate ground loop current, which may cause the entire circuit system to fail .
Sample image (for reference only)
2. What methods can achieve 485 isolation?
If you want to achieve isolation on the 485 circuit, you need to use isolation devices, such as optocoupler isolators, magnetic isolators, and isolation chips. Optocouplers are the most commonly used isolation devices and are relatively low cost. The speed of the optocoupler must meet the requirements of the baud rate. Generally, a high-speed optocoupler is selected for 485 isolation. Compared with the optocoupler, the isolation chip has a smaller PCB area, but the cost is also higher. Commonly used isolation chip solutions generally include dedicated digital isolation chips, integrated 485 isolation chips, and isolation chips with power supplies.
The specific implementation steps are as follows:
1. Use an isolator: First, you need to choose a suitable isolator. Common RS-485 isolators include ADI's ADM2483, TI's ISO3082, etc. These isolators typically integrate an RS-485 transceiver and an isolated power supply that can provide isolation voltages up to 2500V.
2. Power supply design: The isolator needs two independent power supplies, one for the circuit before isolation (such as MCU), and one for the circuit after isolation (such as RS-485 bus). Isolated power can be achieved using an isolated DC-DC converter.
3. Interface design: RS-485 interface usually requires a 120-ohm terminal resistor to suppress signal reflection. In addition, TVS diodes and ferrite beads can be added for electrostatic discharge protection and high frequency noise filtering.
4. Wiring design: In order to reduce electromagnetic interference, the A and B lines of RS-485 should run together as much as possible, and keep away from high-speed or high-current signal lines as much as possible.
5. Software design: In the software, it is necessary to correctly set the working mode of the RS-485 transceiver (such as half-duplex or full-duplex), and set the data rate, data bits, stop bits and parity bits as required.
What we need to pay attention to is that the specific design may need to be adjusted according to actual application requirements and environmental conditions. At the same time, it is also necessary to pay attention to the selection and quality of the isolation device to ensure the stability and reliability of the isolation circuit.
3. The role of 485 isolation circuit
Improve the reliability and stability of communication: The isolation circuit can prevent electrical noise and electrical shock from interfering with the system, thereby improving the reliability and stability of the system.
Protect equipment safety: Prevent damage to equipment caused by high voltage shocks, thereby protecting equipment safety.
Improve signal quality: Isolation circuits can reduce interference during signal transmission, thereby improving signal quality.
Extended system distance: The RS-485 interface can support long-distance data transmission, and the isolation circuit can further extend this distance.
Improve system flexibility: Make the system more flexible in design and layout, and devices can be placed in different locations according to needs.
Good compatibility: RS-485 isolation circuit scheme can be compatible with various equipment and systems, which is convenient for system integration and expansion.