We have a circuit which master RS 485 module is designed as fail safe mode and (DC GND) connected directly to Earth. It communicates to other RS 485 modules in daisy chain connection with/ with out end node termination & this modules are connected (DC GND) to earth via a RC in parallel network ( 100 R, 0.1 uf) . Now the 485 chip in master module is getting burnt after some days of communication. Can any one help me out what will be the issue. Whether earth is creating issue and if yes why it is.

 

Are your DC commons connected to earth ground at different points/locations, if so this could be the reason.
Strictly speaking RS485 only requires two (differential) signal conductors.
Max.

 

Most likely it's an Electrostatic Discharge problem.

RS-485 lines are very vulnerable to ESD troubles, you need a robust transient suppressing network ahead of the transceiver chip.

 

Are your DC commons connected to earth ground at different points/locations, if so this could be the reason.
Strictly speaking RS485 only requires two (differential) signal conductors.
Max.

Yes. The DC commons are connected to earth in two different places. One is directly connected & one more side with 100 ohm resistor parallel with 0.1 mf cap. Can we know the technical reason for this. Because in lab we cant able to reproduce or see such kind of behaviour, but when in site which is under construction has this problem. Pl can you explain the technical reason? Ya you are correct two wire is enough but the manufacturers recommend to connect DC ground between devices to make a common ground. What is your view on this ?

 

Most likely it's an Electrostatic Discharge problem.

RS-485 lines are very vulnerable to ESD troubles, you need a robust transient suppressing network ahead of the transceiver chip.

But this does not happen in lab or in our office testing. Happens only in site which the construction is n progress. Chip manufacturer claims 15KV for human model & Air discharge. 8KV for contact discharge. So we are confused ?

 

The fact that it works for "some days of communication" tells me that it's an ESD Problem.
The bare chip is rated for "15KV discharge human model" but in the real world there are sources of ESD that are far more energetic than the simple human model.
Just dragging a long cable around on a carpet can induce huge voltages, parts of the system can have much larger capacitance than that of the "human model" - creating far larger discharge currents that destroy even "ESD protected" chips. Bottom line is that a robust RS-485 com link that is expected to survive the real world needs some carefully chosen external ESD protection, the chip is too small to absorb the energy without blowing up.

I use a low capacitance bridge rectifier combined with 2 TVS diodes on all RS-485 lines, if the network is intended for long wire runs, I include a gas discharge tube in the protection circuit as well, for lightning protection.

 

The fact that it works for "some days of communication" tells me that it's an ESD Problem.
The bare chip is rated for "15KV discharge human model" but in the real world there are sources of ESD that are far more energetic than the simple human model.
Just dragging a long cable around on a carpet can induce huge voltages, parts of the system can have much larger capacitance than that of the "human model" - creating far larger discharge currents that destroy even "ESD protected" chips. Bottom line is that a robust RS-485 com link that is expected to survive the real world needs some carefully chosen external ESD protection, the chip is too small to absorb the energy without blowing up.

I use a low capacitance bridge rectifier combined with 2 TVS diodes on all RS-485 lines, if the network is intended for long wire runs, I include a gas discharge tube in the protection circuit as well, for lightning protection.

Thanks a lot for sharing the experience & guiding us. Thanks for the ebook too

 

Yes. The DC commons are connected to earth in two different places. recommend to connect DC ground between devices to make a common ground. What is your view on this ?

If the DC commons are earth grounded at different points this could account for the result you are seeing.

Max.

 

If the DC commons are earth grounded at different points this could account for the result you are seeing.

Max.

Thanks a lot. You explain me very simple through this video. Can i conclude like this. Due to Earth connected to DC ground the current flow is high in this path so the chips are burning out ?

 

A common idea is that RS-485 is just a two wire interface, this line of thinking can get you into trouble; The receivers have a large but finite common mode input voltage range, all is fine as long as this range is not violated. Ignoring the issue of the ground potential can leave the things floating, the network might work for a while, then stop working for no apparent reason.
My point is that you must consider the third wire ground connection very carefully to make sure it's doing what it's supposed to do- keep the signals well within the receivers common mode voltage range. Additionally, when designing ESD protection circuits, one must consider both common mode and differential voltage transients.

 

Hi all
I have encountered the problem again after removing the DC ground to earth connectivity. I will put my observations at time of failure
1. The IC case( die) is getting heated up initially.
2. Gradually the the temperature is rising up and blows up the IC but even the IC is functioning.
3. After some time say 1 day the function of IC stops.
This is happening when we communicate through the IC in time period of 50ms

Can you put your views and help me out to figure it out the root cause

 

hi,
If I understand correctly, you say you have removed the ground connection.?
This means you are grounding/connecting the two items of equipment thru the IC signal pins.
Do you have a diagram showing the connections.?

E

 

No , Initially we had a connection between DC ground and earth. Now it has been removed. Pl find the attached for ref. In fig it is mentioned ADM 485.But we used Exar SP485EE.

 

Supplier RCA just arrived. He closes as EOS

 

What are the 100 ohm resistors in the dc common(ground?) paths for? I'm not used to seeing artificially added resistance in a ground path. Don't you generally want the least possible resresistance there?

 

What are the 100 ohm resistors in the dc common(ground?) paths for? I'm not used to seeing artificially added resistance in a ground path. Don't you generally want the least possible resresistance there?

100 ohm resistors are connected to protect the Master / Slave DC ground from any spikes or voltage addition in twisted cable from Master to slave or vise versa