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THVD1450: RS485 signal pulsing high then dropping low as it carries through system

Part Number: THVD1450

I am currently validating a PCB that uses the THVD1450DRBR RS485 buffer chip and I am facing two issues. The first is that the signal, when toggled, will pulse very high or very low (>6V when change in magnitude should be 3.3V or less) for about 10 microseconds before reaching its intended output value. In the image below, see the green input toggling from low to high, upon which the yellow output pulses high before going to its target value of about 3.2V. The same happens on the falling edge of the input from 3.3V to 0V.

The second issue is that the signal is not maintaining its strength through the whole system. This RS485 signal travels out of its origin PCB and to another, but by the time it reaches the second PCB, the signal falls in magnitude. See below. The output signal probed right at the output of the buffer on the origin PCB (yellow) is higher in magnitude than the same signal (green) probed at the next PCB over. The signal is losing much of its strength over this period and continues to lose it across a third and final PCB (not pictured). Why is the signal not driving strong enough? Is there anything I can do to fix it?

  • Please show the schematic, including everything connected to the logic of the transceiver, and to the bus. And specify which pins you have measured.

  • Hi Courtney,

    A few questions:

    1. Is it possible to share schematic details of the THVD1450 and how it is used in the system? This can help determine if if the schematic is correct w.r.t. to the THVD1450 design requirements. Loading issues from the bus could be part of the issue - but I can't tell from the scope shot alone. 

    2. Could you indicate how the output was measured - what pin was measured or if it is the differential voltage that is measured? The pulse seems unexpected but I want to validate what is connected to output and where it is being measured. 

    3. For your second question - how long is the bus and how many nodes are connected, and does the bus have standard RS-485 termination (each end of bus terminated with 120 Ohm resistors - 1 resistor on each end so parallel is 60 Ohms (up to 10% tolerance is allowed by standard as minimum is 54 Ohms) ). The cabling and bus loading will attenuate the data signal - and it could be significant as the receiver thresholds are rather small - so understanding the implementation will help me determine if this expected or not and what possible improvements can be made.

    4. Finally - what is the desired target data-rate of the application - this also will affect the attenuation of the signal 

    Please let me know so I can start digging into the unexpected behavior and see if we can find the root cause of the issue.


    Parker Dodson