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THVD1406: Bus line spike

user5163865
Genius 4110 points

Part Number: THVD1406
Other Parts Discussed in Thread: SN75176B,

Hi,

Previously, I contacted TI regarding a system using the SN75176B with a constantly ON TX (transmission) function, where there were issues with received data from external sources.

TI recommended the THVD1406 as the optimal product, and after evaluating it, the received data waveform became normal, resolving the problem.

However, due to the extremely low data rate of 1.92kbps, spikes occur in the received waveform of the transmit loopback.

This issue appears to be known, and a solution is described in the application note:
www.ti.com/.../slla574.pdf

It seems that connecting pull-up/down resistors to bus lines A and B might improve the situation.

I tried connecting 1.5k ohm resistors, and it appears to have improved.

 Cable length: 70mm, Data rate: 1.92bps
 Only two known RS485 slave devices will be connected.

Therefore, our customer plans to remove the termination resistors from this system.

Question

Based on the calculations in www.ti.com/.../slyt324.pdf, I calculated Rfs to be approximately 630 ohms. Is this correct? 

I used an input equivalent resistance of 12k ohm, but I'm not sure if this is correct.
I've included the evaluation data and calculation formula in the document.

THVD1406.pptx 

Rfs Calculate.xlsx 


I would appreciate any advice on whether it's correct.

Best regard,
Hiroshi

 

  • +1
    TI__Guru* 79571 points

    Hi Hiroshi,

    The 630 ohm value you calculated seems correct if you were to use a termination resistor. In your set up though, it sounds like you are going to remove it (slow data rate and short distance so removing it should be okay) which means that the pull up and pull down resistors don't really need to be that strong (low value). You can choose a larger value resistor, smaller is technically better but consumes more power. You are probably okay with just setting it to be 1.5k ohms.

    -Bobby