connecting up 1 wire ?

[Marcus Warrington <marcus.warrington@xxxxxxxxxx>]

Having been playing around with a couple of 1 wire thermometers for
the last few weeks I'm beginning to think of how to wire up something more
permanent

At the moment I just have a USB adapter (DS9490R) plugged into my NSLU2
(unslung and running OWFS) and have a couple of thermometers (DS18B20)
strung together on a couple of lengths of cat5 cable. One thermometer is in
Node0 and the other just outside in the hallway. The connections are
crudely done by twisting the wire around the legs of the DS18B20 and
securing in place with electricians tape. Both sensors are running on
parasitic power only.

Using a PERL script and RRD I have been able to graph the temps in Node0
and hallway, but OWFS does seems to crash quite a lot (although it can run
days at a time with no issues) although this may be down to my rather dodgy
connections. I have read that I should have a 4.7K ohm resistor on the wire
but I'm unsure if this is already included in the USB adapter? I've also
read articles that state I should put a small resistor (100ohm)  on the
data leg of each sensor?

Now my 1 wire counter board has arrived from Hobbyboards I'm looking at
pulling a wire from node0 to the front of the house where the Gas meter is
(a distance of about 10 meters). This has gotten me thinking about how best
to connect sensors to a one wire system. Looking around on the net it seems
most people simple crimp RJ45 plugs on the end and then use a 3 port
"Y" converter of some form e.g. http://www.ebay.co.uk/itm/280558030831?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

The trouble (for me) with this is that as far as I understand the sensor
should only be a maximum of 2inches away from the bus i.e. the convertor
must be near the sensor and so I would need to run two cat5e cables (feed
and return from the main bus wire) to each place I wanted a sensor. Looking
at the "standard" use of cat5 cable for 1 wire implies that even
with 12v and 5v feeds only 6 of the 8 wires in cat5 are used i.e.

1  5 volt
2  5 volt GND
3  GND            ** I'm not entirely sure why pin 3 is  GND or indeed
which GND it refers to?
4  1 wire Data
5  1 wire GND
6  N/C
7  12 volt
8  12 volt GND

Given the above (and assuming I ignore the ground on pin3) could I not wire
up a Y connector such that 1 wire data was sent up on pin 4 but return on
pin 6 and then connected back to pin 4 on the return socket of the Y
connector. The same could be done for pin 5/3 for GND.  All the other pins
would be connected straight through to all 3 ports. This would mean that
plugging in a sensor would actually extend the bus rather than  T'ing off
it.

.---.
: .-:-.
V : : V


1 2 3 4 5 6 7 8
@ @ @ @ @ @ @ @




--in----> 1 @-----+---------------@ 1 ----out--> 5v
--in----> 2 @-------+--------------@ 2 ----out--> 5v gnd
--in----> 3 @           ---        @ 3 ----out-->
--in----> 4 @----------'  `----------@ 4 ----out--> data
--in----> 5 @------------'     .------@ 5 ----out--> data gnd
--in----> 6 @         `---------'      @ 6 ----out-->
--in----> 7 @-----------------+---------@ 7 ----out--> 12v
--in----> 8 @-------------------+--------@ 8 ----out--> 12v gnd

The big advantage of this is that I could run a single "bus" wire
around any room that I might want a sensor in and then when I want to
install one  I can simply cut the wire , insert a Y connector and the run a
cat 5 cable (of any length) to the exact point I want the sensor.

Marcus



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