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From: "John Jardine"
Subject: Re: What is the Rs 485 current return path on its data lines A and B ???
Date: Tue, 3 Dec 2002 14:54:49 -0000
NNTP-Posting-Date: 3 Dec 2002 14:42:44 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Zaheer wrote in message
> I have a working knowledge of RS 485 but there is one aspect of it
> that I am a bit confused.
> If you connect 2 devices via Rs 485 and you connect a common ground
> between them, then there is return path for the Rs485 A or B signals.
> This I understand.
> BUT in an application WITHOUT a common ground and based on the many
> report that A || B is NOT a return path for A || B. Then were is the
> return path ????
> Surely even in differential mode the receiver compared signals A and B
> and in doing so has to allows current to flow (even if minimal) into
> its self and this current will only flow if there is a return path
> back to its source.
> I connected a Rs485 camera on a 12v battery (to totally isolate this
> device) to a PC (without a earth connection). There was just 2 wires
> connecting the Rs485 converter to the camera ( A and B).
> YET it worked. The camera responded to the messages on the Rs485 .!!!!
> I expected it NOT to work.
> Looking at the internal configuration of a receiver. There is no
> current path, back to the transmitting device. So how does it work
> without a return path ???
> Any help would be greatly appreciated.
Most RS485 are run *without* a common ground. The only connection between TX
and RX is by the single twisted pair.
At the receiver, just think of the A-B lines as being fed from a (say) 9V
battery that reverses backwards and forwards in tune with the data. You
should see that *no* extra current return is needed back to the transmitter
as all the circuit actuation current can be supplied from the battery
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