RS485 for 30m cable

Discussion in 'General Electronics Chat' started by aamirali, Aug 15, 2014.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    I am using a board with wired communication on RS485. I am using SN65HVD72, it specifies cable length to be 2000m.

    The board on other side which is in field, i.e 30m away has 4 wire connection, i.e Vcc, Gnd, A, B. (where A, B are RS485 signals).


    1. I am planning to use four core cable.
    2. My field board has 80mA current consumption. Vcc is +5V. This +5V goes to 3.3V regulator which is used for board finally.


    Question:
    1. Will +5V be able to show at filed board which is 30m away from centeral board. Also keeping in mind 80mA cuurent consumption.

    2. Will RS485 will be done fine?

    3. What type of four core cable I should select
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the voltage drop depends on the size of the wire. the rs485 driver you said you were using is speced for up to 2000 meters, 30 should be no problem
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    The cable must be twisted pairs. A and B signals must be paired.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Feed it with 6 volts.
     
  5. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    I have selected this 4 core cable 0.5mm^2.
    http://edgecdn.lappgroup.com/filead..._doku/datenblaetter/unitronic/DB0034302EN.pdf

    1. Since loop resistance is 38.9ohm/Km. & my requirement is 50m or 100m in loop so total resistance = 3.89ohm, so drop at 80mA is .32V. Will work fine according to me?

    2. Other is I didn't find characteristic
    impedance in datsheet for termination in RS485 connections.?

    I have only master & slave device (2 devices on Rs485)
    & at 9600bps. Do I need termination resistance or I can ignore
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Is the cable twisted pairs?

    You cannot ignore termination. Use 120Ω at both ends.
     
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