I use
my
system in Star config but have had to add inline resistors and
Diodes
It’s
a while
back Craig from HomeVision gave me all the info I will repost it here for
you I’m
sure Craig won’t object.
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
Mike Griffiths
-----Original
Message-----
From: Dr John Tankard
[mailto:john@xxxxxxx]
Sent: Fri 11 May 2001
15:58
To: ukha_d@xxxxxxx
Subject: RE: [ukha_d] Dallas
1820
WAS:Homevision - one more time...
> Actually, I'm telling pork pies, I know of someone who
has
> set a few up in a
> star config, but according to Dallas, that shouldn't work
:-)
>
I gather it will work but does cause problems if all the legs are long,
I
cannot remember the reason but it is in the data book. Dallas
recommend
using switches to isolate legs to improve propagation and reduce the
load
on
the network, the I idea is you switch on a leg then access the device
on
the
leg, when you have finished with a device you instruct the switch to
open
again cutting of the leg, from memory its a DS2405.
In a home vision set-up you are unlikely to be able to use this
approach.
John
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