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Introduction of several RS485 isolated communication schemes
2022-07-06 04:38:00 【Tianwaifeixian CUG】
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 .
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