The name 'One-Wire' is mis-leading in that in fact two-wires are needed:- one for the voltage supply and data and the second as a 0v / ground return. In automotive applications or any application which involve a metallic chasis this second ground return is the chasis itself and hence in such applications the device does in fact only require one wire.
All one-wire devices have an in-built communication interface which enables the device to be controlled or interogated and whilst this device may be parasitically powered via the data line it is highly likely that any sensor used in combination with the device will have additional powering requirements that cannot be satisfied by the data line. Therefore it is advisable to wire the one-wire network or microlan using multiple twisted pair cables to provide the power for sensors in addition to the one-wire pair, The most common choice is the 4 twisted pair CAT5 network cable with RJ45 connectors.
Not surprisingly many retailers of one-wire enabled sensors have adopted differing wiring conventions and therefore it is essential that a specific wiring convention is selected at the outset and rigorously maintained throughout the microlan
RJ45 8-wire Conventions
The above wiring convention is that used by most of the HobbyBoard devices and one which I selected to use throughout my microlan.
When working with one-wire a common problem is maintaining the correct polarity. A useful device for testing for polarity can be made by crimping a two-color LED such as the Fairchild MV5491A into a RJ45 connector (Note the flat denotes the green cathode so slide this into pin 5 and the other leg into pin 4).
Types of One-Wire Networks
As for any other kind of network, there are many different topologies possible but main 1-Wire network types are bus and star and combinations of the two.
A bus network is the simplest kind of network. It consists of a length of cable
with sensors tapped into it along the way. The above figure shows an example.
An advantage of a bus network is itís electrical simplicity and transmission
property performance. Bus networks generally exhibit very little electrical noise,
and thus the communications are more robust than star networks. Bus networks
are easy to electrically implement as well, however, it is not always convenient to
string a cable around to the locations of all sensors required on a bus. Also, bus
networks are like the old Christmas light strings Ė any break anywhere in the bus
will cause a fault downstream from the break. Any short on the bus will affect all
sensors on the bus.
A star network is somewhat more complex than a bus network. A typical star
network is shown above.
An advantage of a star network is that it can be easier to physically wire. Many
sensor locations can be reached, and added, by stringing a cable from the main
point to the sensor. Also, a break of one sensorís wiring will not affect other
The biggest disadvantage of a star network, however, is the electrical noise that
can be introduced if not properly connected. Electrical noise will result from One-
Wire communication pulses traveling down different branches of the star, which
generally have different lengths, and returning at different times. This is called
reflection. Fortunately, adding a series resistor to all the branches of the network
can minimize the reflections. These resistors reduce the reflections, and hence
the electrical noise, and therefore make communications somewhat more robust.
This figure shows where the resistors should be place. Generally, a 100-ohm resistor is
recommended for this.
Of course, the two main types of network topologies can be combined to achieve
required results. The above figure shows an example of a properly connected Bus and
Star network. Note the locations of the 100-Ohm termination resistors, which are
required to minimize noise in the network. Generally, the resistors should be
placed at every branch of the network.
For complex network the layout can be simplified significantly by incorporating a DS2409 MicroLAN Coupler16 as a network hub thus avoiding the need for any series resistors.
For further detail information on one-wire networks please consult the One-Wire-Design Guide v1.0 from Springbok Digitronics