RE: DS18S20 temperature ic sockets

[Mike Griffiths <mike@xxxxxxx>]

Stephen 
This is Craigs notes on connecting the sensors I'm sure he won't mind
me
reposting them

66
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

99

Mike

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