IIRC the spec for current loop is that either end can detect a broken cable,
and will not happen with multidrop IE the TX will not notice say 1 load out
of 50 missing.

I know one unit we used in the security industry had dual current loops and
if one failed each unit either side of the break would loop through the 2nd
one to restore comms :o)


Regards,

Kat.

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----- Original Message -----
From: "Christian Dorner" <doc@AUSTROSOL.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, September 03, 2001 10:04 PM
Subject: Re: [EE]: RS-485 vs Current-Loop


> Hi!
>
>
> > To my Knowledge, there is no multidrop current loop. IT is all daisy
> chained
>
> Thank you for your answer.
>
> This was my knowledge, too, befor i've seen this kind of bus. Maybe it's
not
> really a current loop, but the construction seem's to be.
> I've found a trasistor for TX (pulling one line down to gnd) and on the
> other side a optocoupler to receive the signal. This seems to be a classic
> current-loop. But i found also they just connect some receivers on the
line
> and do not daisy chain it.
>
> I think the construction should work in this way to lit/drive just more
than
> one led (opto coupler) as in a classic current-loop. But with every
receiver
> (opto coupler) the current will raise.
>
> I found this contrustion in some rather old (around 10 years) data
terminals
> but i like due the low part count and price.
>
> Maybe it will work fine with some modification, like low current opto
> couplers or with a other receiver part which will not draw so much
current.
>
> cu, Christian
>
> --
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>
>

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