rs485 resistor and noise test question

Discussion in 'Embedded Systems and Microcontrollers' started by losturcos, Apr 16, 2010.

  1. losturcos

    Thread Starter New Member

    Apr 16, 2010
    3
    0
    Hello,

    I am testing my rs485 communication thru a 60 meters of cable, it seems working at 9600 baud rate. I have one master and one slave.I am using SN75176. I have put 1k resistor between A and B pins. There is a 4,7k from A pin to +5V and 4,7k from B to ground. My maximum communication distance will be about 60 m. Do you think this is the optimal circuit, or what corrections can be done?

    I also want to ask, how I can apply noise to this 60 meters cable for test.

    Thank you very much.
     
  2. losturcos

    Thread Starter New Member

    Apr 16, 2010
    3
    0
    Thank you, I will correct the resistors. I will write the results here.
    Also I will write the results after I apply the noise to the circuit.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    If it is single direction, the 120 Ohm should be at the receive end, if it's bi-directional you need termination at both ends. You could terminate both ends anyway to minimise noise & transients.

    The other ultra-critical thing that many people miss is that a common ground is needed between all drivers and receivers. The safest thing is to use screened cable, or other cores in the same cable - CAT5 ethernet is useful for that, use one pair for data and another for ground. Failing that, you can use other grounds as long as the voltage difference never exceeds +/- 5V at any point along the bus cable.

    For improved noise immunity, you can modify the termination to two 56 or 68 ohm resistors in series, with a small cap (1nF?) from the center point to chassis ground.
    That helps cut down induced noise spikes that could exceed the common mode range of the drivers/receivers, without affecting the fundamental balance or impedance of the system.
     
    losturcos likes this.
  4. losturcos

    Thread Starter New Member

    Apr 16, 2010
    3
    0
    So I should use one twisted pair for a and b, and one for ground for a better solution. Is there a cable type, that only includes two twisted pair cables and one ground? Because as I see, CAT5 cable include 4 twisted cables some of which I don't need.
    Thank you.

    edit: I am noting down the advanced noise immunity tip, I will try it by making some manual corrections on the pcb, I may make correction in the future progress.

    edit2: You have written 1nF?, the capacitor should be 1nF or that is changable by another condition. (This may be a dumb question, but I should be counted at circuit design :) ).
     
    Last edited: Apr 17, 2010
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