Why is 485 isolation needed?

RS485 is a commonly used data communication protocol, widely used in industrial control systems, building automation, security systems and other fields.

What is a 485 isolation circuit?

In some applications, in order to prevent electrical noise interference or electrical shock, the RS485 interface needs to be isolated. RS485/RS232 IO circuits can be divided into non-isolated and isolated types. Isolated circuits add isolation performance on the basis of non-isolated circuits, which can make the circuit have stronger anti-interference and system stability.

The following situations require 485 isolation:

1. When the RS485 external interface is connected to high voltage, it is easy to damage the back-end circuit and even cause electric shock at the user end.

2. When the RS485 communication nodes are too far away, the reference ground of each node is connected to the local earth. When there is a large voltage difference between the two ends of the earth, the ground potential will be superimposed in the form of common mode voltage. On the signal line, the common-mode voltage range that the port can withstand may be exceeded, affecting normal communication and even damaging the back-end circuit.

3. When the ground planes between distant 485 communication nodes are connected using cables, the ground wire will form a ground loop with the earth, and at the same time couple external common mode noise, resulting in a ground loop current, which may cause the entire circuit to malfunction. System failure.

What methods can achieve 485 isolation?

To achieve isolation on the 485 circuit, isolation devices need to be used, such as optocoupler isolators, magnetic isolators, isolation chips, etc. Optocoupler isolators are the most commonly used isolation devices and are relatively low cost. The speed of the optocoupler must meet the baud rate requirements, and high-speed optocouplers are generally selected for 485 isolation. Compared with optocouplers, isolation chips have 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 supply.

1. Optocoupler isolation

Optocoupler isolation communication speed is low, so high-speed optocoupler is generally chosen for 485 isolation. Optocoupler isolation occupies a large PCB area, but the cost is low.

Optocoupler isolation solution reference circuit diagram

2. Integrated 485 isolation chip

Integrated 485 isolation chip, the isolation chip and the 485 level conversion chip are integrated together, such as TI's ISO1410, Industrial Router/Gateway ADI's ADM2483, Nanocore's NSi83085, etc. This solution occupies the smallest PCB area, has moderate communication speed, and is affordable, and can meet most applications.

Integrated 485 isolation chip solution reference circuit diagram

3. Dedicated digital isolation chip

The dedicated digital isolation chip has a fast communication speed and occupies a smaller PCB area than the optocoupler, but the cost is relatively high.

Dedicated digital isolation chip solution reference circuit diagram

4. Isolation chip with power supply

The first three isolation solutions all use separate isolated power modules, while the isolation chip with power supply integrates the isolation power function to achieve power isolation and signal isolation in the chip. The chip can provide 3.3V or 5V power supply to the outside world, and the output current can reach tens to 100mA, which can power the general 485 level conversion chip. The isolation chip with power supply is similar to the dedicated digital isolation chip solution, except that a separate isolation power supply is no longer required, and the PCB area occupied is further reduced, but the cost is higher.