RE: DS1820

["Mike Griffiths" <mike@xxxxxxx>]

Hi Paul
I use a Star config without problems I'm running 5 sensors
Craig did give me this advice which I will just copy as sent:

When using the multi-drop mode, make sure you read each sensor's
address, entered them into the sensor table, and then downloaded the
schedule to the controller.  If you've done this and still have
problems, it's usually due to too much wiring capacitance.  However,
your wires aren't very long, and most users don't have any problems.
There are several ways to improve wiring performance:

*   - The first thing we recommend is to add a resistor between the temp
sensor input port and 5V.  A value of 2200 Ohms is ideal, but anything
between 2200 and 4700 should help.

*   - Minimize the total cable length.  One daisy chain usually works
better than many shorter wires "home-run" to HV (assuming the
total
cable length     is less).  However, we had one user report the opposite
results, so this isn't always an improvement


*   - Unshielded cable is better than shielded cable.

*   - If the cable is shielded, DON'T connect the shield, as it
increases capacitance.

*   - Non-twisted pair is better than twisted pair. This minimizes
capacitance.

*   - If you use twisted pair, use one pair for power and ground.  On
the other pair, use one for the signal and leave the other UNCONNECTED.
This     minimizes capacitance.

*   - Try connecting a signal diode (Schottky if available) from the
signal to ground at the DS1820 end of the line (cathode goes to signal).
This     will control "ringing" that can happen on long lines.

*   - Try connecting a 100 Ohm resistor in series with the signal line
at the Multifunction Expansion Board end of the cable. If you have more
than     one cable, use one resistor in each.



Also, we recommend you read the sensors in a periodic event running
every loop.

Check to see if the value is 255 (which indicates an error), and if it
is, don't put it into your temperature variable. That way, an
intermittent error will be ignored.

Here's an example:

Variable #0 (temporary variable) = Value of Digital Temperature Sensor 1

If

Variable #0 (temporary variable) <> 255

Then

Variable #1 (Temp Sensor 1) = Variable #0 (temporary variable)

End If

Let me know if you still have problems.

Thanks,



Craig Chadwick

President

Custom Solutions, Inc. (a.k.a., HomeVision)

mailto:csi@xxxxxxx

http://www.csi3.com


-----Original Message-----
From: Paul Gale [mailto:groups@xxxxxxx]
Sent: 18 August 2003 18:01
To: ukha_d@xxxxxxx

Thinking about mounting a few DS1820 temp sensors within Cat5 plugs
(they seem to crimp in nicely with no protrusions) and then plug them in
to various sockets around the house wherever it takes my fancy.

As I have a traditional structured wiring layout, I have two options in
patching them through to HomeVision -

a)	patch them in a 'star' topology at the patch panel and then take
one output to HV (means connecting up a few patch ways in parallel)
b)	Patch them so that they effectively all sit on one long length
of cable (needs less patch ways but more prone to a single point of
failure)

Anyone connected them up in a star topology before? Could it cause any
problems with timing and reading individual sensors etc? I've currently
got 5 DS1820's.

Ta,

Paul.





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