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Updated : 16/10/2016

One-Wire Wiring


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.

Other Conventions

RJ45 (8-connector)
Description 1 2 3 4 5 6 7 8
Hobby Boards standard P GND 5V P GND DQ GND NC 12-24V unreg
12V 12V 5V P GND P GND
AAG WAI602A RS485A+ RS485- 5V 12V 12V 5V P GND P GND
EDS RJC Cable       DQ GND Power
LINK45 (pin numbering adjusted) NC NC
LinkHub (pin numbering adjusted) P GND 5V
iButtonlink Multisensor P GND 5V AUX GND DQ
HobbyBoards Power Injector GND NC->5V GND DQ GND pass thru NC->12-24V unreg GND
iButtonLink Power Injector NC->5V GND NC->5V AUX GND DQ GND AUX NC->12V NC->12V GND

One-Wire Tester

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.

Bus Network

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.

Star Network

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 sensors.

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.

Combination Network

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.

Complex 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.

Further Information

For further detail information on one-wire networks please consult the One-Wire-Design Guide v1.0 from Springbok Digitronics

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