I have a reuirement to protect RS485 transmission lines from up to 500v which may be prolonged. Basically I have 485 data within the same cable as 500v, and the cable is under water, so sometimes if a fault occurs, not only do i need to rectify the fault, but I always need to replace the RS485 transceivers at either or both ends. space is limited so I can not fit in adams units with this built in, I am down to chips and components, but I can't opto isolate like I can with 232 because 485 is a loop as opposed to an individual line for tX and RX. Has anyone got a good idea for me, ideally a chip that can be plug in would be best for speed and ease of replacement if they fail.
Electrical – RS485 transmission line high voltage protection
protectionrs485voltage
Related Solutions
Martin,
I'm by no means an expert at RS485, but I have run into some of the same problems you've had with independent power supplies on multi-drop RS485 networks.
Most frequently, in spite of claiming fail-safe operation, they don't play nice with ground loops/DC offsets. For me, in the past, the most direct route is to use fail-safe biasing on the two lines. E.g. 
.
By the way: Most of my learning has come from others here as well as some pretty good application notes from Texas Instruments (PDF), Maxim (HTML), and Analog Devices (PDF).
Hope to hear if this gets you in business.
Josh
You can find the required voltage difference at the sending and receiving ends in the RS485 standard. As a second best, read the specs of some RS485 transceiver.
What is probably the problem in your project (but it is difficult to reconstruct the original signal after a half-garbling filter) is that the common of your sender and receiver differ too much. That is, there are two requirements for successful RS485 communication:
- the voltage difference between D+ and D- must be OK. This is probably not your problem.
- the voltage difference between D+ and ground, and D- and ground must be in the range acceptable by the receiver. If they are not, you have a ground potential difference problem. To solve it, you can either reduce the difference, or find a receiver that can tolerate a higher difference.
The voltage levels for RS485, from The RS485 design guid :
Note that this picture is about the differential voltages, it omits the ground connection.
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Best Answer
I use a scheme that combines gas discharge tubes, inductors, TBU protection devices, TVS diodes, and pi filters to protect my 485 buses. There are two basic schemes I use: One for the supply lines and one for the data lines. Both are shown below:
Supply line protection:
Data line protection:
These are fairly common methods to protect RS-485 buses and you will find them in datasheets and app notes for the RS-485 transceivers.
You don't say what sort of space you do have. The above generally takes about 1 square inch per scheme on a PCB. You could put one set of protection on the top layer of a PCB and the other on the bottom side to only take up that ~1 sq. in. of space.