RS485 interface 3.3V and 5V

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#1
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Hi there.

 

I have some products that use the SN65HVD3082ED (RS485 5V tranceiver). Now I am bulding a new product that uses the SN75HVD12DR (RS485 transeiver 3.3V). 

 

I am wondering if there is any way to buid a compatible communication between 5V and 3V3 RS485 transeivers.

 

Regards.

 

 

Michael.

User of:
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RS485 to RS485 works from 3V3 to 5V devices. The RS485 spec says the minimum differential is 200mV

 

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Kartman wrote:

...the minimum differential is 200mV

 

That's the important bit. RS485 works by looking at the difference between the two lines not their absolute values.

 

I have products out in the field working very reliably with a mix of 3v3 and 5v transceivers.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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Brian Fairchild wrote:
I have products out in the field working very reliably with a mix of 3v3 and 5v transceivers.

As do I.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I'm a software guy. So, please be gentle. Isn't there an issue that 5V from a 5V device might "spill" over the transceiver and into a 3.3 micro (that is not 5V tolerant)?

Last Edited: Wed. May 25, 2016 - 08:13 PM
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ezharkov wrote:

Isn't there an issue that 5V from a 5V device might "spill" over the transceiver and into a 3.3 micro.

 

No. RS485 receivers will withstand a common-mode voltage in the range of -7v to +12v. That is to say, the incoming voltages can be offset with respect to the receivers 0v pin by voltages within that range.

 

So, in normal operation the A line swings either side of 2.5v (for a 5v supply device) or 1.65v (for a 3.3v chip), and the B line does the same but in the opposite direction to A. However, that 2.5v or 1.65v can be shifted by up to 7v in the negative direction or 12v in the positive and it'll still be seen as a valid signal.

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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The reason I asked was because I had seen this issue myself. It is possible that there was some design issue. Maybe that depends on transceiver? I believe that in my particular case it was an ST485. Maybe it was because there were no termination resistors? Whatever that was, I saw >4V on the side of the 3.3V micro (which was an xmega and everything still seemed to work just fine).

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The ST485 is a 5V part. Was it powered from 5V or 3V?

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Sorry, not ST485. ST1480ABDR. Powered from 3.3V. ST485 was on the other side, on the 5V board.

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I'd suggest there were problems elsewhere. Maybe it was bus bias resistors injecting 5V levels into the 3V supply?

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One more simple question

 

My network comes up to 150 meters Daisy-Chain Connection.

 

What I have to do in order to protect the drivers from any short circuit between A and B line ?

 

 

Michael.

User of:
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Altium Designer

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icarus1 wrote:

What I have to do in order to protect the drivers from any short circuit between A and B line ?

 

What does the data sheet say about short-circuit protection?

#1 Hardware Problem? https://www.avrfreaks.net/forum/...

#2 Hardware Problem? Read AVR042.

#3 All grounds are not created equal

#4 Have you proved your chip is running at xxMHz?

#5 "If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

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It says

 

"Bus-Pin Short-Circuit Protection From –7 V to 12V"

 

http://www.ti.com/lit/ds/symlink...

 

I can;t see anything more

 

 

Michael.

User of:
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Altium Designer

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They should survive a bus short, but this situation is probably the least of your problem - at 150m, lightning might be an issue. I normally use tranzorbs to protect the chip.
Note to he chips can draw 100's of mA in a short circuit condition, so make sure your power supply can cope.

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ezharkov wrote:
The reason I asked was because I had seen this issue myself.

Back when I was your age, many [more than today] apps had RS232.  Weren't you worried then about the +/- 10V blowing out your 5V micros and etc.?

 

Hmmm--indeed, we pull A/B to the rail with bias resistors.  While fairly stiff there is a lot of current limiting in say 1kohm.  But only the transceiver should see the level on the field side, not on the micro side.

 

I picked a 3V transceiver at DigiKey more or less at random to see the A/B specs/ratings...

 

Another:

 

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

Last Edited: Fri. May 27, 2016 - 08:03 PM
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I saw 4V on the micro side. Don't remember the details. But I think you can simulate that a condition by setting both transceivers to output at the same time (well, at least with the ST transceivers that I mentioned above.)

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Kartman wrote:
Note to he chips can draw 100's of mA in a short circuit condition, so make sure your power supply can cope.
Wasn't aware of the RS-485 current limit mentioned in one transceiver's datasheet

http://www.intersil.com/en/products/interface/serial-interface/rs-485-rs-422-fault-protected/ISL32452E.html (datasheet, Built-In Driver Overload Protection, file page 14)

and that transceiver has staged fold-backs to greatly reduce the current.

Another manufacturer's datasheet mentions fold-back but does not state the current(s).

 

"Dare to be naïve." - Buckminster Fuller