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RS485 Chip

Discussion in 'Electronic Basics' started by Richard, Nov 16, 2005.

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  1. Richard

    Richard Guest

    I have a 75176 RS485 Transceiver chip here. Looking at the application
    notes, it seems pretty straight forward to hook up, however I'm wondering.

    Should I add a Jumper and a termination 120 ohm Resistor Between A and B?
    Should I install any pull ups or Pull downs on the 485 data signal lines?
    Does A pull down and B pulls Up?

    Just wanting to do this 1 time. I will be using it strickly in half duplex
    (For ModBus Mode).

    Thanks for any pointers.
  2. Dan Hollands

    Dan Hollands Guest

    Don't add any pull up or pull down

    There needs to be a termination resistor at the end of the cable run.


    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  3. Check the TI, National, Maxim and Linear Technology websites for application
    notes related to RS485, termination and failsafe.
  4. tlbs

    tlbs Guest

    You need to terminate the transmission line (across A and B) with the
    characteristic impedance of the twisted pair transmission line you are
    using. If that characteristic impedance is unknown, any value from 98
    to 125 Ohms will usually do the trick.

    The only reason to use pullup and/or pulldown resistors is for ESD
    protection of the 75176. Values between 10 kOhms and 50 kOhms will add
    protection to the RS485 receiver. You could also use switching diodes
    or MOV devices. If you pullUP the non-inverted input, and pullDOWN the
    inverted input, then the fail-safe value of the logic output will be
  5. Richard

    Richard Guest

    Thanks for all the advice. I noticed while looking thru many schematics
    that each person seems to implement the 485 chips in many different ways. I
    think I will put the termination, pull up, and pull downs all on there being
    jumper selectable and that way I can have it either way.

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