THVD1406: Rx(R) waveform will same as Tx(D) when I transmitt data.

Will Yan

Part Number: THVD1406

Tool/software:

Hi TI supporter,

I use THVD1406 in auto-direction mode for a RS485 device.

Althought the communication result is ok, but I measure the waveform, I found the Rx(R) waveform will same as Tx(D) when I transmitt data.

Is it correct? I thought Receiver should be disable when driver is working.

Thank you.

6 months ago

0 Clemens Ladisch 6 months ago

Guru 317280 points

What is the voltage at the /RE pin?

0 Will Yan 6 months ago in reply to Clemens Ladisch

Prodigy 10 points

Hi Clemens,

/RE pin Voltage is about 3.2~3.3V, I had pulled up 10kohm to 3.3V at the /RE pin.

0 BOBBY 6 months ago in reply to Will Yan

TI__Guru 73651 points

Hi Will,

I believe this is a feature built into the device:

You probably need to put something infront of the R pin to block it when RE# is set to a logic high or ignore the echo that is received on RX when RE# is high.

-Bobby

0 Will Yan 6 months ago in reply to BOBBY

Prodigy 10 points

Hi Bobby,

Thanks for the explantion.

0 Will Yan 6 months ago in reply to BOBBY

Prodigy 10 points

Hi Bobby,

Is the schmitt trigger(Receiver) a root cause of echo?

0 BOBBY 6 months ago in reply to Will Yan

TI__Guru 73651 points

It's not specifically the schmitt trigger, it's how the device was designed. After the t_device_direction_dir timer passes, the RE# input gets ignored and the signals on the RS485 gets received.

I'm not entirely sure why the systems engineer for this device designed it this way since customers who pull RE# low specifically don't want to see the echo.....

-Bobby

Interface

Interface forum

THVD1406: Rx(R) waveform will same as Tx(D) when I transmitt data.