1-Wire

1-Wire is a name for a communication bus designed by Dallas Semiconductor company, that is designed for the low-speed transmission of digital signals. As the name suggests 1-Wire sensors can theoretically use only a single conductor for both power voltage and data. The negative pole can be grounded. In practice, however, at least two conductors are used. 1-Wire sensors available in the Unipi stock then use a total of four conductors for better reliability and enhanced serialisation options.

A distinct advantage of the 1-Wire bus is the low component price; thermometers available at Unipi e-shop are offered from €10 apiece. A single bus can be up to 200 m long with up to 15 sensors connected at once. Each sensor is also provided with its own HW address for its addressing on the bus.

Mind the interference The 1-Wire bus is prone to electromagnetic interference, it is thus recommended to use a shielded cable (ie. FTP). The cable must be routed through places with the least interference possible and must be well clear of any cables supplying induction loads. The most frequent sources of interference are frequency converters, inverters etc.

Follow the correct topology It is also important to follow the bus topology and to avoid creating any branches. To meet this condition we recommend using Unipi 1-Wire thermometers featuring a pair of data conductors - one leading back to the PLC or the previous sensor on the bus, one leading to the following sensor in the line. On the end of the bus, a sensor must be placed to avoid any signal bounces.

Mind the limitations Communication speed is pre-defined, eg. cannot be decreased as with the RS485 interface to mitigate the interference. Any issues with the bus should be fixed only by adjusting the wiring, decreasing the number of sensors or dividing the network between multiple controllers. We are currently developing a Modbus RTU extension with eight 1-Wire ports that will be able to prevent such issues.

Use the 1-Wire only in suitable installations Given the sensitivity towards EM interference, it is not recommended to use the 1-Wire in industrial automation. Its low cost, simplicity and wide range of compatible products makes the bus an ideal solution for smart home applications, especially when combined with the versatility and broad connectivity of Unipi controllers.

The Unipi 1.1 board, all Unipi Neuron controllers and nearly all Unipi Axon models are by default provided with a single 1-Wire port. The Unipi range of goods also includes a range of 1-Wire sensors for measuring various quantities; alternatively, you can use other 1-Wire sensors not offered by Unipi.

1-Wire on UniPi Neuron (RJ45)

On the Unipi Neuron range of programmable logic controllers, the 1-Wire bus is a default feature present on all Neuron models. Originally, Neurons have been using RJ45-type 1-Wire ports. This connector is, however, being slowly phased out in favour of a 3-pin screw terminal port, which became a standard for both Neuron and Axon controllers (see below).

The standard DS18B20-based 1-Wire thermometer is by default using the RJ45 connector. Connecting it to older Unipi Neuron PLCs equipped with the RJ45 port is straightforward - simply connect the connector to the port. The RJ45 plug pin map can be seen on the picture below

1-Wire on UniPi Axon and newer Unipi Neuron

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The UniPi Axon as well as newer Unipi Neuron controllers are equipped with pluggable screw terminal for 1-Wire. The pinout of this connector is as follows and you can check our wiring guidelines.

Connecting a single 1-Wire sensor to the Unipi controller

The 1-Wire thermometer can be connected to a 3-pin screw terminal after a simple modification of its connector

  1. cut off the RJ45 connector
  2. fasten ferrules onto the conductors
  3. screw the conductors into the plug-in screw terminal according to the picture below (the plug-in terminal is included in the controller's package)

Connecting a pair of 1-Wire sensors to the Unipi controller via RJ45 splitter

No modification of the thermometer itself are needed in order to connect two thermometers to a single controller. All you need to do is to purchase a corresponding variant of the 2-port 1-Wire splitter featuring a pair of RJ45 ports; the splitter is available in both connector variants (RJ45 / 3-pin screw terminal)

Connecting multiple 1-Wire sensors through a 1-Wire hub

A compact 8-port 1-Wire hub features eight RJ45 ports and allows you to connect multiple 1-Wire thermometers or other sensors with the RJ45 connector. To connect the hub to a controller you can either use a suitable patch cord (on controllers with the RJ45 port) or the cable reduction (on controllers with the newer 3-pin screw terminal).

It is necessary to follow the correct port order when connecting devices to the hub! Port 1 must be always used as an input port, the remaining ports must not be skipped; otherwise, any devices connected after the skipped port would be cut off from the 1-Wire bus.

Creating a multi-hub extensive network of 1-Wire sensors

It is possible to create a chain of multiple hubs if an extensive network of 1-Wire sensors or devices is needed. When interconnecting two hubs the topology stays the same - Port 1 serves as an input while the output is located on the last port in order.

Any other 1-Wire device connected after the output port will be cut off from the bus!