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SN75LBC176: Matching resistance between output pin A and B

Wendy Wang1
Genius 13720 points
Part Number: SN75LBC176
Other Parts Discussed in Thread: THVD1420, THVD1520

Hi team,

I have a question of matching resistance between output A and B of RS485 transceiver ‘SN75LBC176’. If we do not use the value of 110 Ohm, which is shown in the datasheet, does 220 Ohm have a big impact on the signal quality? Is there a reference design for RC matching?

What is the newest alternative recommendation for SN75LBC176?

Thanks a lot for your help!

Best regards,

Wendy

  • +1
    TI__Guru 69662 points

    Wendy,

    The drivers for an RS485 compliant transceiver are designed to generate 1.5V over a 54 ohm load., with RS422 expecting 100 ohms. I'm thinking the 110 ohm load is meant to represent just one side of the bus, so that if the other side of the bus has 110 ohms, it equates to 55 ohms in parallel. Either way, having 220 ohms on the bus will increase the overall amplitude of the differential signal on the bus in the positive direction, and may cause some asymmetry in the signals. If you email me directly I can share a presentation that explains termination and other aspects of RS485 in more detail.

    As far as a replacement for SN75LBC176, the THVD1520 or THVD1420 will work, depending on what supply voltage you are using.

    Regards,

    Eric Hackett 

  • +1 in reply to Eric Hackett
    TI__Guru 69662 points

    Wendy,

    I just found this app note that may be helpful as well.

    Regards,

    Eric Hackett