RS485 Signal Attenuated

Development Boards Arduino
1

Hi,

I have an AST-CAN485 Dev Board. I am trying to read RS485 data bytes from a steering angle sensor.

When I scope the signal from the steering angle sensor without any other RS485 transceivers connected I get a clean signal.

But when I connect the AST-CAN485 Dev Board with termination resistors and program the RS485 receive example in the Arduino IDE, the signal get attenuated.

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With the attenuated signal I’m not able to read the data bytes from the steering angle sensor.

I’ve tried with and without termination resistors and the signal looks the same. Any other things I can try to get it working?

Thanks for your help.

2

What does the soldering look like?
Are the baud settings identical?

3

Hi,

Soldering joints look ok.

The baudrate is 38400 kbps when I measure with a scope.

Also, I have a DTECH USB-to-RS485 converter and I am able to read data from the steering angle sensor when I connect to it. The signal gets slightly attenuated when I connect to the converter.

Sorry about all the screenshots in one pic. I can only post one media file.

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Lastly, even when I connect lines A and B of the steering angle sensor to the AST-CAN485-DEV Board with no power to the dev board, the signal still shows a large attenuation.

Any help getting this working is greatly appreciated.

4

This is a long shot but try cleaning the flux off your solder joints. Sometimes flux can be conductive and cause weird issues like this.

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5

Wild guess: Triple check the sensor/source is actually RS485.

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6

Hi, I cleaned the solder joints but the signal still gets attenuated when I connect the ST-CAN485 dev board to the steering angle sensor. Also, I checked and confirmed the signal is RS485. The steering angle sensor uses a Denso SE485 transceiver.

https://www.jotrin.com/product/parts/SE485

I did notice the resistance across lines A/B of the ST-CAN485 dev board is around 127k Ω. Whereas, the resistance across lines A/B of the DTECH USB-to-RS485 converter is 16k Ω.

This is strange because I can read data from the steering angle sensor when I connect the DTECH USB-to-RS485 converter even though it has a lower resistance which would load the circuit more.

Do you think if I remove the pull-up and pull-down resistors on the RS485 lines of the ST-CAN485 dev board that this would reduce the attenuation? I only need to receive and not transmit.

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Thanks for your help

7

What I really meant was to make sure you don’t have an RS485 and CAN485 mismatch.

8

Yes, I ran the CAN example and can see the CAN messages being transmitted with my CAN tools.

I double checked to make sure I connected the RS485 lines of steering angle sensor to correct A/B pins on the AST-CAN485 dev board. I also made sure the ground connection from the sensor are connected to the dev board.

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9

Just wanted to follow up. I removed the pull-up and pull-down resistors on the RS-485 lines and the signal was no longer attenuated. I was able to read the steering signal with no issues after that.

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