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How to acheive RS485 differential signals isolation

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myatham

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Hi,
We have to provide isolation for (Rs485) differential signals. This RS485 interface is used between two different boards with different power supplies. i found few RS232-RS485 transcievers which can provide isolated RS485 communicaiton. But, we are lookin looking for the solution which can provide isolation between RS485-RS485


And also I have to provide a status LED (which can blink during communication) on RS485 interfcae.

Please help me to find out the solution.


Best Regards,
myatham:-?
 

FvM

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Isolated RS485 is commonly used and required by some industrial interface standards, e.g. PROFIBUS. You'll need three fast optocoupler or similar digital isolators and a DC/DC converter or separated transformer winding for an isolated supply.
 

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Please share some more information like how to design it and a reference circuit... Our speed is approx 32.7Kb only...
Isolated RS485 is commonly used and required by some industrial interface standards, e.g. PROFIBUS. You'll need three fast optocoupler or similar digital isolators and a DC/DC converter or separated transformer winding for an isolated supply.
 

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Hi,
The document is explaining about single ended signals isolation. And then they are converting it to differential signals.
But, I am looking for differential signalling isolation. As per my understanding, in the ethernet we use transforemer for isolation. Similary for USB isolation, there are ICs like ADuM4160 (USB isolator). Please suggest me some simple and cost effective techniques to provide isolation for different signals. Thanking you for your quick response!

Use a aDUM device from Analog Devices, circuits will be on data sheet:
Digital Isolators | Interface | Analog Devices
 

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If you want a single chip solution similar to ADuM4160 with built-in DC/DC converter you can refer to ADM2582E or ADM2587E. It has full duplex capabilities but can be used for RS485, review the datasheet.
 
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marce

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Thanks for the memory jog, I have used that part in the past, aslo used another fo that family and a seperate DC DC isolator.
 

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Thanks for your quick support. it looks to be i missed to mention the issue clearly. i am looking for an isolator whose input and out put both should be differential. In the suggested datasheet, input is single ended and the output is double ended. Please see the image for better understanding.
Thanks for the memory jog, I have used that part in the past, aslo used another fo that family and a seperate DC DC isolator.
 

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it looks to be i missed to mention the issue clearly. i am looking for an isolator whose input and out put both should be differential.
I see. I would name this a bus isolator respectively repeater with isolation. If you think about the problem, you'll realize that isolating a multi drop bus requires some intelligence to detect which bus side is sending and control the data direction. You won't find this functionality on a single chip, usually a µP or programmable logic is doing the job. If you are connecting two devices only and get access the transmit enable of one of the peers, it would be very easy.
 

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Hi,
I have thoughts of using transformer as we use in Ethernet. Won't it work?
 

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A transformer can only work for DC-balanced physical layers. You can achieve it by adding 8b/10b or manchester encoding to your protocol. But a standard UART signal is never DC-balanced.
 

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I couldn't understand how it will be connected to encoding schemes? Could you explain me in detail? I think in digitl isolators they use transformers or caps internally to achieve the isolation. Please correct me if iam wrong.
 

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I answered your question "using transformer as we use in Ethernet", not about other possible isolator circuits using transformers. Ethernet is based on DC balanced bit streams, so the twisted pair can be directly connected to transformers.

If you drive an unsymmetrical signal, e.g. from an UART to a transformer, you'll face to problems: 1. the transformer saturates 2. you can't recover the logic level without some a DC restore circuit. You have to encode the data to send it to a transformer. Any digital isolator using transformers does some kind of endcoding, you can review the respective datasheets, e.g. from Analog Devices iCouplers.
 
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