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THVD1406: Echo on the Rx pin.

Krzysztof Dluzynski
Prodigy 10 points
Part Number: THVD1406
Other Parts Discussed in Thread: THVD1400

I'm using THVD1406 Auto Direction feature. SHDN and RE connected and both pins are pulled up to VCC (3V3).Also UART RX is pulled to 3V3 by 10k resistor (not visible on this schematic)

The issue is that, to be compatible with MAX13487 I'm using 5V for chip supply, and 3V3 on logic side. But I've tested it with 3V3 supply voltage, and the issue persisted.

My expectation is that it would work similarily to a standard transceiver - when TX is active, the receiver is inactived, and we don't receive echo of what we are sending on TX pin.

However In my case, I always receive an echo on Rx after about 6 us. (Baud rate is 38400).

EDIT: 1. Yellow - RX, Blue - TX

Also, the signal on AB is "weird"... High bits are driven correctly, but low bits are driven for not full period. This works for my the receiver device, but seems unstable.

EDIT: Yellow - A vs B

Is there a way to eliminate this echo on Rx pin?

  • 0
    Guru 284990 points

    The THVD1406 and MAX13487E work in the same way.

    Table 8-2 says that when /RE is high, you see a low on R only when an external device drives the bus. Please specify exactly which waveform corresponds to which pin.

    A high data input is driven only for a short time; this is necessary for the auto-dir function to work.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    1.Yellow - RX, Blue - TX

    2.Yellow - A vs B

  • 0 in reply to Krzysztof Dluzynski
    TI__Guru 60540 points

    Hi Krzysztof,

    This Echo behavior you're seeing is called out in the datasheet and is part of how the device works (see image below).

    As Clemen's pointed out, this behavior is linked to the auto direction feature for the device. 

    The THVD1400 is a similar device to this one without the auto direction feature, this would remove the echo you are seeing when the #RE pin is held HIGH or left floating. See table 7-2 in the THVD1400 datasheet.

    Krzysztof Dluzynski said:

    Also, the signal on AB is "weird"... High bits are driven correctly, but low bits are driven for not full period. This works for my the receiver device, but seems unstable.

    EDIT: Yellow - A vs B

    Can you overlay the TX waveform and the A-B waveform on the same scope? This would make it a bit easier to compare the periods. This looks like the differential signal is driving Low for a short time before going HI-Z. During the High-z state, the differential receiver is likely seeing this as a positive signal because A-B is still larger than V+ threshold (looks like the idle state for this has a positive 1V A-B value).

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Thank you for your answers. It looks that "it's not a bug, it's a feature" thing.