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THVD1452: TX signals on RX lines

Tim De Bondt
Prodigy 140 points

Part Number: THVD1452

Hello all,

We use THVD1452DR (full duplex). When measuring with oscope we can see (part of) the signal we put on the transmit line directly back on the receive lines (also after the tranceiver, so logically translated). 
For our use it is not a big issue as we never send and receive at the same time (more polling system), but still I'm interested what could cause this "issue".


Greetings,

Tim

  • 0
    TI__Guru* 81381 points

    Hi Tim,

    For full duplex, this doesn't sound like it should happen unless you have the A/B pins shorted to the Y/Z pins. 

    Does this happen when you don't have the device connected to a cable?

    Do you have a schematic I can review?

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 140 points

    Hi Bobby,

    No they are not shorted. In the picture you can see the signal measured with the oscope. The signal read is the RX line (between DSP and Transceiver). It shows both the weird influence of the TX and an expected "ack" from the device connected. 

    signal left is the weird, right is normal):

    zoomed in (left is the weird, right is normal):




    I will test tomorow without a device connected to it Slight smile

    Schematics:

    Greetings,

    Tim

  • 0 in reply to Tim De Bondt
    TI__Guru* 81381 points
    Tim De Bondt said:
    I will test tomorow without a device connected to it

    I'm thinking there might be a downstream board that could have the pins shorted to the receiver. 

    Tim De Bondt said:

    No they are not shorted. In the picture you can see the signal measured with the oscope. The signal read is the RX line (between DSP and Transceiver). It shows both the weird influence of the TX and an expected "ack" from the device connected. 

    signal left is the weird, right is normal):

    zoomed in (left is the weird, right is normal):

    The concern you have isn't the signals on the left side, correct? 

    This might be crosstalk. Especially if the Receiver is disabled (RE is held a logic high causing R to be high-Z).

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 140 points

    Hi Bobby,

    I checked RX line when TX was sending data without a device present with the oscope. I saw 0 "weird" attunations on the line. 
    So might this be crosstalk on the cable? Or do you have any other ideas?

    And yes the signals on the left are my concern.


    Greetings,

    Tim

  • 0 in reply to Tim De Bondt
    TI__Guru* 81381 points
    Tim De Bondt said:
    I checked RX line when TX was sending data without a device present with the oscope. I saw 0 "weird" attunations on the line. 
    So might this be crosstalk on the cable? Or do you have any other ideas?

    The scopeshot shows the TTL Rx (R output pin)? or is it channel A? (pin 12 of the device)

    If it's the R pin, do the ripples show up if you place an external pull up resistor (like 4.7k) or a capacitor (100pF) on it?

    I do think it's some kind of noise that's coupling in somewhere. If it's the A pin, it might be the Y/Z pins driving and coupling into the channel.

    -Bobby