What Is 1-Wire?
1-Wire is a simple two-wire bus for temperature sensors and access keys -- before you start buying parts, it helps to understand what it actually does.
How it works
One wire for data, one for ground
The name comes from the fact that all communication -- both commands and data -- travels over a single data wire plus a shared ground (GND). Some setups add a third Vdd wire for power, or use so-called parasitic power, where the device draws energy straight from the data line's pulses.
Every device gets its own 64-bit ROM ID
So multiple devices can share one wire without getting confused, each one is burned at the factory with a unique 64-bit ROM ID (family code + serial number + checksum). No manual address configuration needed.
DS18B20 -- the temperature sensor
The most common 1-Wire device is the DS18B20 digital temperature sensor. It's a popular choice for putting one sensor per room, pipe, or boiler on the same shared bus.
iButton/DS1990 -- the access key
An iButton (such as the DS1990) is a 1-Wire chip housed in a small metal button that reads out its unique ROM ID when touched to a reader. It's used as a simple access key -- for doors, gates, or time tracking.
What to know before you shop
A unique ROM ID for every device
The factory-burned identifier lets the bus master tell dozens of sensors apart on the very same bus -- no manual addressing required.
A bus master polls the devices
One controller, the bus master, starts every conversation and asks each device in turn for its ROM ID. Devices never speak first -- they only answer.
Linear or star topology
In practice you'll see both a long shared bus run and a star layout branching from one point. Reliability depends on cable length, wire gauge, topology shape, and the quality of the bus master/pull-up resistor.
Often run over CAT5 cable
1-Wire installs commonly use CAT5/CAT5e twisted-pair cable -- one pair for data and ground, a second pair for extra Vdd power if you're not using parasitic mode.
Parasitic power -- convenient, with a tradeoff
This mode lets a device power itself from the data line, simplifying wiring, but it gets fussier on long runs or busy segments with several devices active at once -- worth treating as a reliability tradeoff, not a universal fix.
Works with Home Assistant and Loxone
Both platforms support 1-Wire through a suitable bus master/gateway device, which reads sensors by ROM ID and passes the values on to your automation.
What this page covers
The basics you need to know before deciding if 1-Wire fits your project
- {'t': 'Where 1-Wire comes from', 'd': 'The protocol was developed by Dallas Semiconductor; the brand and devices now belong to Maxim Integrated / Analog Devices.'}
- {'t': 'The physical layer', 'd': 'How data and commands travel over one wire, and why a third wire for power is sometimes added.'}
- {'t': 'Addressing with ROM ID', 'd': 'How the bus master tells devices apart without any manual configuration.'}
- {'t': 'The key devices', 'd': "DS18B20 for temperature and iButton/DS1990 for access -- what they are and where they're used."}
- {'t': 'Where 1-Wire does NOT fit', 'd': "An honest note that this isn't a general smart-home control bus for lighting or blinds."}
Frequently asked questions
Does 1-Wire work for lighting or blinds?
No. 1-Wire is built for slow measurements and ID reads -- mainly temperature and access control. For fast or critical control, buses like KNX or Modbus are a better fit.
How many sensors can share one bus?
There's no single fixed number -- it depends on cable length, wire gauge, topology, and the quality of the bus master/pull-up resistor. In practice you can run dozens of devices on one run.
Do I need a separate power wire?
Not necessarily -- so-called parasitic power draws energy straight from the data line. It simplifies wiring but can be less reliable on long or busy runs than a dedicated Vdd wire.
Does 1-Wire work with Home Assistant?
Yes, through a suitable bus master/gateway device that reads sensors by ROM ID and passes the values to Home Assistant or Loxone.
What cable should I use?
CAT5/CAT5e twisted-pair cable is a common choice -- one pair for data and ground, a second pair for extra power if you're not using parasitic mode.
Want to see 1-Wire components?
This page covers the basics -- head to the shop for actual DS18B20 sensors and other 1-Wire devices.