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Updated : 16/10/2016

One-Wire Introduction

Background

My interest on one-wire system stems not from any direct interest in electronics or the weather but from a need for information to support other interests such as home energy saving, software development and gardening. However, once I started with a simple one-wire weather system I became intrigued by the prospect of using relatively inexpensive simple devices to gather data on a whole host of things arround the home. Being naturally averse to expensive 'black-box' solutions ($20 for a temperature sensor from iButton versus $5 for the Maxim chip ) I was attracted to the DIY approach to making devices that one-wire makes possible.
My experience to date with wireless systems has been plagued with reliability and ESD issues (we get a lot of ligntning in Gauteng!) so yet another reason to go one-wire, provided the PC is isolated. So far I have only lost one or two in-expensive and readily replaced one-wire chips except that is for my bad-experience with an AAG weather system.

General

One-Wire is a device communications bus system designed by Dallas Semiconductor Corp. that provides low-speed data, signaling, and power over a single signal. One-Wire is similar in concept to IC, but with lower data rates and longer range. It is typically used to communicate with small inexpensive devices such as digital thermometers and weather instruments. A network of One-Wire devices with an associated master device is called a MicroLan.

One distinctive feature of the bus is the possibility of using only two wires: data and ground. To accomplish this, One-Wire devices include an 800 pF capacitor to store charge, and power the device during periods when the data line is active.

Dependent on function, native One-Wire devices are available as single components in integrated circuit and TO92 packaging, and in some cases a portable form called an iButton that resembles a watch battery. Manufacturers also produce devices more complex than a single component that use the One-Wire bus to communicate.

One-Wire devices may be one of many components on a circuit board within a product, or a single component within devices such as temperature probes, or attached to a device being monitored. Some laboratory systems and other data acquisition and control systems connect to One-Wire devices using cables with modular connectors or with CAT-5 cable, with the devices themselves mounted in a socket, incorporated in a small PCB, or attached to the object being monitored. In such systems, RJ11 (6P2C or 6P4C modular plugs, commonly used for telephones) are popular.

Systems of sensors and actuators can be built by wiring together One-Wire components. Each component contains all of the logic needed to operate on the One-Wire bus. Examples include temperature loggers, timers, voltage and current sensors, battery monitors, and memory. These can be connected to a PC using a bus converter. USB, RS-232 serial, and parallel port interfaces are popular solutions for connecting the MicroLan to the host PC. One-Wire devices can also be interfaced directly to microcontrollers from various vendors.

The iButton (also known as the Dallas Key) is a mechanical packaging standard that places a One-Wire component inside a small stainless steel "button" similar to a disk-shaped battery. iButtons are connected to One-Wire bus systems by means of sockets with contacts which touch the "lid" and "base" of the canister. Alternatively, the connection can be semi-permanent with a socket the iButton clips into, but is easily removed from.

Each One-Wire chip has a unique ID code. This feature makes the chips, especially in an iButton package, suitable for use as a key to open a lock, arm and deactivate burglar alarms, authenticate computer system users, operate time clock systems, etc.

Summary

One-Wire is a:

  1. communication protocol
  2. wiring scheme
  3. assortment of devices and "iButtons"

The design and control of the protocol is from Dallas Semiconductor (part of Maxim Semiconductor) which has application notes, software, and devices for sale.

The One-Wire protocol is marketing-speak. It is actually 2 wires, one for ground.

The active wire conducts data and power!

Advantages

  1. Simple connections
  2. Self configuring
  3. Cheap and flexible devices

Disadvantages?

  1. Relatively slow
  2. Needs an adapter (bus master) and software (like OWFS).
  3. Have to be a little careful in wire layout to avoid signal reflections

Home Monitoring

The simplicity of creating a MicroLan of One-Wire sensors combined with the accessibility of suitable software to interogate and control sensors makes the One-Wire system very attractive to hobbyists for the creation of such things as :

  1. Automated home weather stations
  2. Monotoring of temperatures within the home
  3. Home Energy Monitoring Systems
  4. Garden monitoring including soil temperature, humidity, Degree Days, Soil Moisture etc.
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