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How can i test RS-485

Dsoky

Hello, i am looking for a way to test MAX-485 , i do not have any idea how to do it , anyone can help me?

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  • If you want to test RS485, then really you should use THREE or more devices.

    Since RS485 is a multi-drop bus, you'll want to ensure that the data can be transmitted from any one node to every other node successfully.

    One node will becomde the bus driving force by asserting the DE signal on the Max485 and de-asserting it on all the other nodes.
    You can assert the !RE (Receive enable) signal on ALL the nodes, even the transmitting node at any time, even when transmitting. This allows the transmitting node to "read back" it's own transmissions. This is useful for detecting collissions on the bus. 

    For example if you send 0xff,0xff,0xff but read back something other than that, you can deduce that there is eithew a) A fault on the bus, or b)some other node transmitted at the same time.

    The bus should be wired point to point, from one node to the next, but not as a "star".
    Appropriate terminating resistors should be used on the nodes at the far extents of the bus, but not on the intermediate nodes.

  • in reply to KennyMillar

    I should add... 
    Use the appropriate size and guage of wire, connecting all the A's together and all the B's together.
    A common ground is not necessary, since RS485 is a differential signal, however using a common ground can reduce current consumption.

  • in reply to KennyMillar

    Thanks for all that can you please send me a link or pdf for more explamation?

  • in reply to Dsoky

    Yeah no problem, here it is:https://www.analog.com/media/jp/technical-documentation/data-sheets/1111.pdf

    If you have any questions please feel free to ask - I use these chips in loads of things.

    Also here's a diagram showing how to wire it up.
    The double-s symbol means "long bit of wire"

    image

    and the A wire and B Wire should be a twisted pair.

  • in reply to KennyMillar

    Oh.. and there's one more thing...
    You only need to connect 1 GPIO pin to DE and !RE - because DE is active-high and RE is active low, it means you can connect them both to the same pin. When you drive the pin HIGH you have a trasnmitter, when you drive the pin LOW you have a receiver ;-)

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