RS485 has strong anti-interference capability and low cost, making it widely used in various fields such as industrial smart instruments and communication devices.
The RS485 circuit can be divided into non-isolated and isolated types. Among them, the isolated circuit adds isolation performance based on the non-isolated circuit, giving the circuit stronger anti-interference and system stability. Below, we will introduce several isolation solutions for RS485.
1. When Is RS485 Isolation Needed?
When external nodes connected to the RS485 communication interface are high voltage, it can easily damage the backend circuit and may even cause electric shock to the user.
When the distance between RS485 communication nodes is too far, the reference ground of each node is connected to the local ground. If there is a significant voltage difference between the two grounds, the ground potential will superimpose on the signal line as common-mode voltage, which may exceed the common-mode voltage range that the port can withstand, affecting normal communication and potentially damaging the backend circuit.
When distant RS485 communication nodes are connected via cables (such as RS485 shielded cables), the ground wire will form a ground loop with the earth. This loop can couple external common-mode noise and generate ground loop currents, which may cause the entire circuit system to fail.
2. Isolation Solutions for RS485
However, regardless of the solution, to achieve isolated RS485 communication, an isolated power supply is necessary. Typically, a small integrated power module like this can be used:
There are several methods for RS485 isolation:
● Optical Isolation
Optical isolators are the most commonly used isolation devices and are relatively low in cost. The following image shows three optical isolators implementing RS485 isolated communication. It is important to note that the rate of the optical isolator must meet the baud rate requirements, and high-speed optical isolators are generally chosen for RS485 isolation.
● Dedicated Digital Isolation Chips
Standard optical isolators have limited communication rates, and even high-speed optical isolators cannot compare with dedicated digital isolation chips, such as ADuM1301, a three-channel digital isolator, which can achieve maximum communication rates of several tens of Mbps (generally RS485 communication does not require such high rates). Moreover, compared to optical isolators, they occupy less PCB area but are relatively more expensive.
The circuit diagram is as follows:
● Integrated RS485 Isolation Chips
The above two solutions are achieved by combining isolation chips with RS485 chips. Another type of chip is the integrated RS485 isolation chip, which integrates the isolation chip and RS485 level conversion chip together, such as ADI’s ADM2483, TI’s ISO1410, and domestic options like Naxin Micro’s NSi83085 and Chiplink’s SIT3485ISO. This method occupies the smallest PCB area, has moderate communication rates, can meet most applications, and is reasonably priced.
The circuit diagram is as follows:
● Isolation Chips with Power Supply
The above three solutions use separate isolated power modules. Additionally, there is a solution that implements both power isolation and signal isolation within the chip, such as ISOW7841.
This chip integrates isolation power functionality, providing external 3.3V or 5V power, with output currents reaching tens to 100mA, capable of powering general RS485 level conversion chips.
This solution is similar to the one described in 2.2, but does not require a separate isolated power supply, which can further reduce PCB area, but the cost is also higher.
3. Summary
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|