Future proofing parasitic vs external power

[kw1816]

I need a bit of advice on planning a 1-wire network and using parasitic or external power. I already have a 1-wire network running but I've got some issues with lost sensors, bad readings etc. I've put this down to the mix of cable types and mix of topology I used when adding bits and pieces over time.

I have the opportunity to run new cable so I'm going to run a single dedicated CAT5e for all the sensors. I've always used parasitic power (mainly DS18S20's with a couple of DS2423's to be added). If in the future I decide to change to external power, what's the easiest way to do this ?

Is it acceptable to wire each sensor as external (i.e. using 3 wires) and simply ground the Vdd wire at the source or is it better to ground Vdd at each sensor ? I'm thinking that if I do switch to external power at a later date I don't really want to redo the connections on each sensor..

Many thanks
Kevin

 

[ericvic]

I would ground Vdd at the sensor. As far as I remember it is only the DS18S20 family that requires the grounding of the Vdd pin.

Eric

 

[sda]

The spec sheets for the DS1820 and DS18B20 both say
"When using the parasite power mode, the VDD pin must be tied to ground."

I initially had my DS18S20's in parasite mode, but had to wait 750ms for the conversion on each sensor.

"If the DS18S20
is powered by an external supply, the master can issue “read-time slots†(see the 1-WIRE BUS SYSTEM
section) after the Convert T command and the DS18S20 will respond by transmitting 0 while the
temperature conversion is in progress and 1 when the conversion is done."

Then I switched them to powered because I could process them faster if I checked to see if they were done.

' wait until temp is ready, not more than 2 secs
ready = False
t = Timer + 2
While Timer < t And ready = False
  If TMTouchBit(xxSessionHandle, &H1) = &H1 Then
	ready = True
  End If
  DoEvents
  If Not ready Then
	Sleep 25
  End If
Wend

Now I just send Skip Rom, Convert T, wait 750ms, and read each of them by romid.
I don't think you can use skip,convert in parasite mode. You'd have multiple sensors stealing
power from the bus at the same time.

My gut feeling is the 1-wire bus and sensors are more stable when used in powered mode.

If you don't have a hub or power injector, you can mickey mouse it by tapping off your computer power supply. I have a test connection set up like that. One time I shorted something, the cat5 cable and connector started melting and smoking, then the server power supply shut down. Melting cables smell nasty! Now there are 1A inline fuses for the 5V and 12V supplies.

 

[pete_c]

About 7 years ago I ran cat5e for the 1-wire sensors in the home and connected them in a star type configuration. Two of the 1-wire networks are mostly parasitic (temperature, very old combo sensors, weather status, rain guage, lightning sensor, etc). I know I will need to replace the battery in the lightning sensor sometime in the future but its still working today.