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THVD1551: Receiver Output Logic

Martin Wen
Prodigy 615 points
Part Number: THVD1551
Other Parts Discussed in Thread: THVD1552

Dear Brothers,

My customer using an 20M RS485 with below output logic: When RE is low and if (A – B) ≥ -40mV, RO is high. If (A – B) ≤ - 200mV, RO is low.

I try to promote our THVD1551, but our output logic is not the same as their using right now.

My question is whether we have related products have the same output logic. Or Whether THVD1551 will met some problem to instead what customer using? And any material for me to learning the output logic?

Thanks

  • 0
    TI__Intellectual 2090 points
    Hello Wen,

    Thank you for your questions. It looks like your customer is using an RS-485 transceiver with active-low receiver enable logic. Is that correct?

    Our THDV15XX family of devices includes transceivers both with and without enable logic. The THDV1551, which you promoted, does not have enable logic, but keeps the driver and receiver ON at all times. You may wish to promote a transceiver in the device family which includes the enable logic, such as the THDV1552. The datasheet will provide information for you about the enable output logic for each device.

    Could you answer a couple of clarifying questions? Do you know if your customer needs a half-duplex or a full-duplex device? Do you know the operating speed the customer requires? Answering these questions may help you to narrow down which device in the THDV15XX family to promote. You may also wish to consider the THDV1500, a robust 300kbps, half-duplex option for RS-485.

    Please let me know if I can clarify anything further.

    Best Regards,
    Max Megee
  • 0 in reply to Max Megee
    TI__Prodigy 615 points

    Hello Max,

    Thanks for your information. 

    Full-Duplex, 10MHz min, 20MHz will be better. 5V power supply.

    My customer using an 20M RS485 with below output logic: When RE is low and if (A – B) ≥ -40mV, RO is high. If (A – B) ≤ - 200mV, RO is low. The key question is whether we have related products have the same output logic. Or Whether THVD1551 will met some problem to instead what customer using? And any material for me to learning the output logic?

    Thanks.

  • 0 in reply to Martin Wen
    TI__Intellectual 2090 points

    Hello Wen,

    I apologize; I must have misunderstood your question.  Are you asking about the bus input range for the receiver?  Looking at the THDV15XX datasheet, the input thresholds for the receiver range from (A-B) ≤ -200mV (R is Low) to (A-B) ≤ -20mV (R is high).  This is a 20mV change on the high side from the thresholds that you have mentioned above.  

    From a system standpoint, having thresholds that are closer to 0V can help with receiver noise immunity and jitter performance.  On the other hand, the reason that receiver input thresholds are not centered around 0V is because of failsafe biasing that is integrated into the receiver.  Most modern RS-485 transceivers contain integrated failsafe biasing in order to ensure that if the A and B bus lines are in an idle state or a fault state (shorted, open, etc.), the output of the receiver will still be in a known state (High).  

    Overall, however, a 20mV difference should not make much of an impact on the application's performance.  For more information on failsafe biasing, receiver noise immunity, and how they depend on the receiver input thresholds, you can reference these:

    Receiver Noise Immunity and Hysteresis

    1-minute failsafe biasing

    RS-485 basic failsafe biasing

    From the information that you have given above, I would consider promoting the THVD1552, which is full-duplex with driver/receiver enables, and operates up to 50Mbps.

    Does this information help?

    Best Regards,

    Max

  • 0 in reply to Max Megee
    TI__Prodigy 615 points

    Hi Max,

    Great and thanks.