THVD1452: fail for test and check the schematic

Hi Zoe,

I don't see a schematic provided.  Can you please try to re-post it?

Thanks,
Max

Hi Max,

Sorry for missing.

Please see the schematic as below.

Thank you!

Zoe,

After reviewing the schematic, I have several questions. 1) Why is the DE pin pulled up to 3.3V while the supply is 5V? 2) There is an option for 120Ohm termination for RX channel, how about TX side? 3) How is the 10kOhm (R1099/R1100) pull up/down selected? 4) What's the half-duplex test failing? Can you elaborate?

Regards,

Hao

Hi Hao

1) Why is the DE pin pulled up to 3.3V while the supply is 5V?
=>Because the ADM1491 VDD used 5V, but the DE pin range can support 3.3V, and our signal source is 3.3V level.

2) There is an option for 120Ohm termination for RX channel, how about TX side?
=>When we used half-duplex, we used jumper to short the TX/RX (JP18(2-3) & JP20 (2-3))

3) How is the 10kOhm (R1099/R1100) pull up/down selected?
=>It's an old design, I can't answer.

4) What's the half-duplex test failing? Can you elaborate?
=>We used Beartech card to test RS485, when i used 1491 the test is PASS, when i change to THVD1452 show Fail.

*I have tried removing the peripheral resistors, but it still doesn’t work.
*If i want to used THVD1452, how can i modify?(new design)

Zoe,

Thanks for your feedback. I don't see the reason that THVD1452 would not work. From what I can tell, the major difference between the two devices is that THVD1452 has faster rise/fall time.

How far is the communication distance? What's the cable type (is it 120Ohm impedance)? What's the data rate? Is it possible to capture the bus waveforms using these two devices, especially at the receiver input? If the issue is due to signal integrity, we might get the clue by comparing the waveforms.

Regards,

Hao 

Hi Hao,

How far is the communication distance?  

Ans: Around 30CM

What's the cable type (is it 120Ohm impedance)?

Ans: General cable and the 120 ohm end resistor is depends onJumper (Jp15)，not using for test

What's the data rate? Is it possible to capture the bus waveforms using these two devices, especially at the receiver input? If the issue is due to signal integrity, we might get the clue by comparing the waveforms

Ans: these questions will reply you after testing the waveforms and provide to you

Also, from the schematic, do you think any we can adjust or add/remove?

Do you have the THVD1452 reference schematic to provide to us since we didn't see on TI website?

We really would like to know move to TI solution where has to adjust?

Thank you! 

Zoe,

One more thing to check: is the ground of the transmitter side the same as the ground of the receiver side?

Did you mean 120Ohm termination is not placed in the testing? Can you try redoing the test with the termination? Again, I suspect the problem might be due to the fast transition time of THVD1452. If the reflection is worse with THVD1452, the error bits could show up on the bus. I'm looking forward to the waveforms.

Regards,

Hao