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TIDA-010035: HVD1786

sajul babu
Prodigy 70 points
Part Number: TIDA-010035

Hi,

   I am working on a project where two board are connected with 24V DC over RS485 TP cable. The first board powers second which draws close to 60mA from 24Von the RS485 bus. The circuits are working except for an additional extra byte being present in my data read.

I tried disconnecting the controller pins at the receiver end where RE pin of driver IC is asserted low to forcefully to put second device in receive mode without controller selecting the driver to be in RX mode.

The same firmware works on these boards if I do not couple 24V power( I am not seeing the additional byte in normal RS485 communication and both boards are powered separately).

Circuit used is same as shown in application note as we use a baud rate of 38400 which is similar to minimum baud of 32000 mentioned. The inductors used are rated 300mA

Logic analyzer captures for TX and RX data

Waveform capture showing the A-B signal 

Schematic

Please suggest what could be done to rectify the issue

A short video on the issue is attached, you can see the issue occurring from 3 min onwards . for this video the RS485 baud rate was reduced to 9600

https://vimeo.com/711147088/78af8d1a00

  • 0
    TI__Guru 62920 points

    Hi Sajul,

    It looks like both the A and B lines have a large pulse at the end of the transmission, and there may be some period where the differential signal on the bus is interpreted as a logic "low" because of that. Does this pulse occur when the DE line transitions from high to low on the active driver node? If so it may be due to a change in current through the coupling inductances when the driver is disabled.

    Max

  • 0
    Prodigy 70 points

    Hi,

      The spikes in A & B line with coupled DC are common mode and they cancel each other when checking A-B math, But one thing noticed is that after the signal is decoupled, there seems to be an undershoot in the A signal which is not there in B and this cause the A-B function to go below 2V for a brief period of time which is detected by receiver.

    Data packet read right

    Data packet with additional byte detected

    Can you suggest some way to get rid of the undershoot

  • 0 in reply to sajul babu
    TI__Prodigy 600 points

    Hi Sajul,

    What is the series resistance of the inductors being used?  Could you provide the part number?  Is this just a point to point connection of the RS485 bus, are there any other loads or transceivers being connected to the bus besides the two transceivers?

    David