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 #-Link-Snipped-# 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).
[​IMG]

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:

[​IMG]
[​IMG]
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:

[​IMG]
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?

Replies

  • Harshad Italiya
    Harshad Italiya
    #-Link-Snipped-# :
    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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