Rs485 Using Hserin/hserout

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  1. - 25th January 2008, 20:21 #1
    koossa's Avatar
    koossa
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    Thank you very much Mister_e!

    I have implement it

    <hr>
    Code of My Master PIC
    <br>
    <img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2296&stc=1&d=120129233 2" border="1">
    <hr>
    Code of My Slave PIC
    <br>
    <img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2297&stc=1&d=120129234 2" border="1">



    But it still only returns:
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  2. - 25th January 2008, 20:33 #2
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    mister_e
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    what's your expected baudrate? I feel 2400 bauds right?

    so you should use...
    Code:
    DEFINE HSER_RCSTA 90h ' Enable serial port & continuous receive
DEFINE HSER_TXSTA 20h ' Enable transmit, BRGH = 0
DEFINE HSER_SPBRG 25  ' 2400 Baud @ 4MHz, 0.17%
DEFINE HSER_CLROERR 1 ' Clear overflow automatically
    OR
    Code:
    DEFINE HSER_RCSTA 90h ' Enable serial port & continuous receive
DEFINE HSER_TXSTA 24h ' Enable transmit, BRGH = 1
DEFINE HSER_SPBRG 103  ' 2400 Baud @ 4MHz, 0.17%
DEFINE HSER_CLROERR 1 ' Clear overflow automatically
    Unless your current USART setting show 600 baud.. and i'm not sure how god/bad it may works with your current hardware.

    I would tent to use Usart interrupt for that.. but you can still remove the TimeOut to see what happen. Your code work as design so far.

    Still looking...
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.
  3. - 25th January 2008, 20:52 #3
    Darrel Taylor's Avatar
    Darrel Taylor
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    Try adding "Fail-Safe" resistors to the RS-485 buss.

    2.2k from A to VDD
    2.2k from B to gnd

    When both ends are in receive mode, the buss is left floating.

    If you're going to have multiple slaves on long distance wires, you may also need 4 schottky diodes on each device.
    <br>
    DT
  4. - 26th January 2008, 13:33 #4
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    koossa
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    Thank you very much Mister_e, I have changed it to the second option, but still receives the :
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    WAITING FOR REPLY..
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    </b>


    Darrel, the 2.2k from A to VDD and 2.2k from B to gnd, must I put them on the Master and all the Slaves?

    I will have long distance cables and multiple slaves.
    Where will I put the diodes?

    Thank you
    Koossa
  5. - 26th January 2008, 20:39 #5
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    Darrel Taylor
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    The resistors and diodes should be on each device.

    I like the Schottky-Diode Termination method because it allows you the have a "Star" configuration on the 485 buss, and it uses less power than other methods.

    Other types of termination require the devices to be in a line with a single termination at the farthest end. This is not always practical, and invariably the farthest end device gets disconnected somewhere along the way, and there goes your termination. Unless someone remembers to terminate the next farthest device. (never happens)

    See figure 6 in this page for the diodes.
    http://www.maxim-ic.com/appnotes.cfm/an_pk/1090
    <br>
    DT
  6. - 27th January 2008, 09:11 #6
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    koossa
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    Darrel, thank you very much!!!

    I have updated my Schematic, could you please confirm that I understand it correctly (see attachment)?

    Koossa
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  7. - 27th January 2008, 21:45 #7
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    Yup!

    That looks good.
    <br>
    DT
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