Introduction of several RS485 isolated communication schemes

RS485 Have strong anti-interference ability 、 Low cost features , It is widely used in industrial intelligent instruments , Communication equipment and other fields .

RS485 Circuits can be divided into non isolated and isolated . Isolated circuit is to increase isolation performance on the basis of non isolated circuit , The circuit has stronger anti-interference and system stability . Today, we will introduce some isolation RS485 The plan .

1. What situation needs 485 Isolation       

When 485 When the external node of the communication interface is connected to the high voltage , It is easy to damage the back-end circuit , It may even cause electric shock at the end of use ;

When 485 When the communication node is too far away , The reference ground of each node is connected to the local earth , When there is a large pressure difference between the earth at both ends , The ground potential will be superimposed on the signal line in the form of common mode voltage , Thus, it is possible to exceed the common mode voltage range that the port can withstand , Affect normal communication , It will even damage the back-end circuit

When the distance is far 485 When the ground plane between communication nodes is connected by cables （ Such as 485 Shielded cable ）, The earth wire will form a ground loop with the earth , This loop will couple external common mode noise , And generate ground loop current , It may lead to the failure of the whole circuit system .  

2. Isolation RS485 programme

First of all, no matter which scheme , To achieve isolation 485 Communications , There must be an isolated power supply . Generally, small integrated power modules like this can be used .

and 485 There are many ways to isolate ：

2.1 Optocoupler isolation

Optocoupler is the most commonly used isolation device , And the cost is low . The following figure for 3 Realized by an optocoupler 485 Isolated communication , It should be noted that , The speed of the optocoupler should meet the requirements of baud rate , Usually choose high-speed optocoupler 485 Isolation .

2.2 Special digital isolation chip

The communication rate of ordinary optocoupler is limited , Even the high-speed optocoupler cannot compete with the dedicated digital isolation chip , Such as ADuM1301, Three Channel Digital Isolator , The maximum communication rate can reach dozens Mbps（ commonly 485 Communication can't use such a high rate ）. And relative to the optocoupler ,PCB Smaller area , But the cost is also relatively high . The circuit diagram is as follows ：

2.3 Integrated 485 Isolation chip

Both of the above two schemes are isolated chips +485 Chip implementation , Another kind of chip is integrated 485 Isolation chip , About to isolate the chip and 485 Level conversion chips are integrated , Such as ADI Of ADM2483、TI Of ISO1410、 Domestic ones also include nano core micro NSi83085、 Xinlite's SIT3485ISO. This way occupies PCB Minimum area , The communication rate is moderate , It can meet most applications , The price is also moderate . The circuit diagram is as follows ：

2.4 Isolation chip with power supply

The above three schemes all adopt separate isolated power modules , Another solution is to realize power isolation and signal isolation in the chip , Such as ISOW7841. The chip integrates the function of isolating power , Can be provided externally 3.3V or 5V Power Supply , The output current can reach tens to 100mA, Can give general 485 Level conversion chip power supply . The plan is related to 2.2 The introduction is similar to , Just no longer need a separate isolated power supply , It can be further reduced PCB area , But the cost is also high .

3. summary

programme	Communication rate	Occupy PCB area	cost
Optocoupler isolation	low	Big	low
Special digital isolation chip	high	more	high
Integrate 485 Isolation chip	Moderate	smaller	Moderate
Isolation chip with power supply	high	Small	high

in general , Integrated 485 The scheme of isolating chips can meet most applications , Best value for money .

Recommended reading ：

A kind of RS485 Automatic transceiver circuit

STM32 Serial port hardware flow control (RS232/RS485)

The embedded GUI Inventory - How many do you know ？

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