Re: RS485 is bidirectional does it mean it is fullduplex?

On 2005-06-16, kunil <kunilkuda@xxxxxxxxx> wrote:
> May I summary this ?
>
> I think RS485 problem is 2 :
> *) Noise over long distance cable and
> *) Faulty data because of different node voltage reference
>
> To prevent noise over long distance cable, we can earth one side of the
> cable shield (refer to Ott, Henry, Noise Reduction Techiques in
> Electronic Systems).
>
> To prevent faulty data because of different node voltage, we can use
> common line.

Yes.

> But since RS485 is a differential mode protocol, we can use
> either A or B line as our common line.

I don't see how you can use A or B as a "common" line.

> However, if we connect using this fashion, when the master is
> in the idle mode, there will be floating voltage between A or
> B line (since nobody is driving the bus). Therefore, we
> connect pull-up/pull-down resistors in the A and B line (to
> give at least definite voltage level when nobody's driving the
> bus).

What happens when nobody is driving the bus is a (mostly)
different issue. Usually solved by pulling one line to the
reference/common node and the other to 5V (with respect to the
reference node).

The reference node connection between the two ends is required
to keep the A/B signal values being output by the transmitter
within the common-mode voltage range spec for the receiver.

> To make this "definite voltage level" same at the receiver /
> transceiver point, we need to earth their voltage reference
> node at the both side.

You don't need to earth either one, as long as the
transmitter/receiver reference nodes at the two ends are tied
together.

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Grant Edwards grante Yow! Bo Derek ruined
at my life!
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