0
\$\begingroup\$

This is my first question.

I have a rs485 anemometer and a rs485 windvane purcased in Amazon from Cryfokt and Adsire-R vendors. I spent time trying to make them work so return time has expired. They are not giving me any support. (Perhaps two names for same vendor?)

Both items are supposed to be rs485 9600baud. They have four wires, taged: positive, negative, 485-A and 485-B

When connected to Arduino trough a rs485 converter both of them have an strange behaviour. Instead of wait and read the modbus frame (following instruction manual), both items start writing at high speed only to bus line A, and just at the first bit of the frame. Please find in the image the 'sum' of the modbus read frame an one of the the items answer.

Find line-A behaviour

The signal expanded:

expanded

It appears to be some short of serial protocol, but not rs485 Perhaps somebody have dealed with this devices and can help me to discover the protocol or tell me if I am doing anything wrong. If I can connect them to the Arduino I don't care whether they are rs485 or not.

Thanks.

\$\endgroup\$
2
  • \$\begingroup\$ In the future maybe you should purchase from reputable dealers. \$\endgroup\$
    – Andy aka
    Commented Aug 8, 2021 at 9:49
  • \$\begingroup\$ @Andyaka Maybe OP should, but that has nothing to do with RS485 voltage levels being sensed with logic levels of the logic analyzer. It just happens to resulting in the above picture, there's nothing that can be determined that's wrong. \$\endgroup\$
    – Justme
    Commented Aug 8, 2021 at 13:58

2 Answers 2

Reset to default
1
\$\begingroup\$

What you are seeing is what happens when trying to use a logic analyser to view slowly varying signals that are at voltage right between the levels of logic 0 and logic 1.

So that's not high speed data transmission during the start bit of a frame.

\$\endgroup\$
7
  • \$\begingroup\$ Check the bias resistor. There is a bias resistor, right? Not if both ends assume that the other end will bias the line. \$\endgroup\$
    – david
    Commented Aug 8, 2021 at 10:52
  • \$\begingroup\$ @david Bias resistors affect the bus very little when the RS485 transmitter is enabled. Besides it is received and transmitted differentially, so if one of the wires of the differential pairs spend a lot of time at voltage between logic threshold of the logic analyzer, it does not really matter much. \$\endgroup\$
    – Justme
    Commented Aug 8, 2021 at 12:50
  • \$\begingroup\$ Rs485 converter have 120ohm resistor between A and B, 20K resistor from A to VCC and 20K resistor from B to ground. It is a cheap converter but worked fine with a liquid level (also cheap) sensor. Then logic analizer also showed right signals. Instruction manual of the windvane have a bad CRC modbus sample frame wroten. Anyway, two faultly units are difficult for me to believe. So I think perhaps I'm doing something wrong \$\endgroup\$
    – Julio
    Commented Aug 8, 2021 at 14:28
  • \$\begingroup\$ @Julio Maybe, but we can't really see what you are doing except what you show and tell. Tried swapping A and B? Checked voltages with scope or multimeter to find out if you have triple termination? \$\endgroup\$
    – Justme
    Commented Aug 8, 2021 at 15:48
  • \$\begingroup\$ @Justme "The RS485 transmitter" is not a thing, and generic RS485 circuits may not do so, causing the problems shown in the OP and described in your answer. \$\endgroup\$
    – david
    Commented Aug 10, 2021 at 3:28
1
\$\begingroup\$

Finally I made them work. One of the manuals refers to address 2 but It is also wrong. Both of them have address 1. To summarize: The instruction manuals are crap. Wiring and addresses are wrong.

Thanks for comments and help.

Right wiring is as I described before, and both have default address 1. Also find below a link to a device like mine. Hope this will help in case someone runs in to the same problem.

Interfacing Arduino UNO with wind speed through RS485 problem

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.