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Salla May
Salla May • Jun 21, 2017

What's the problem of the RS485 communication?

I've designed a board with microcontroller (NXP, K64) to control i/o's. The uart port of the uC is connected to Maxim 3362 which converts the signal to RS485 half duplex communication. Using modbus protocol in the uController, the communication flows towards the connector on the right in the scheme, from there to an outside rs-485 to USB dongle (which connects to a master PC).

I'm experiencing communication problems and using a differential probe and oscilloscope to sample the Rx and Tx lines of the differential rs485 communication (at the board output connector, pins 4 and 5 in the drawing) I've seen the following picture:

The picture shows a message sent from the PC to the uController, before the uCOntroller answered - and answered wrongly (with RTS high, as expected).

As you can see inside the message sent there are one's and zero's. The zero logic has a lot of oscillating noise on it and the one logic seems quiet.

Furthermore, before the message starts the zero seems okay. This seems a bit strange because if the ground had noise on it I would have suspected that zero logic inside the message and the zero logic before the message starts would have noise (also '1' would have noise, I guess) and not the zero logic inside the message alone...

Furthermore, I had one time during all my experiments which it did worked as expected (it was working ok for few days until now), see the following:

As you can see in the second picture the '0' and the '1' seems okay and these pictures are from the same board.

is this a board problem, component problem?

I'm guessing it does not concern with the firmware... Any ideas?
Harshad Italiya
Harshad Italiya • Jun 23, 2017
@Salla May :
If you haven't change anything in code from the time it worked once it must be some component issue then.
Can you check series resistor on A and B lines?

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