Connecting 1-Wire temperature sensors

1-wire is an interface that allows you to build networks from devices with a common bus topology, one master - many slaves. Each device has its own unique 64-bit address.

When connecting end devices, such as switches, digital sensors, dangerous interference can occur on long lines, so we use additional protection and improvement of the inputs using optocouplers. By default, all devices with 12 inputs are configured in “Optocoupler” mode using jumpers . If you use interface or analog sensors on the controller inputs, do not forget to disable this mode.


The Atom series controllers give a possibility to connect DS18B20 temperature sensors to multifunctional inputs.


The DS18B20 is a high-precision digital temperature sensor.Main characteristics:● Range: -55 .. 125 ° C● Accuracy: 0.5 ° C● Power supply: 3-5.5VThree wires are used to transmit the sensor signal: the first is common\ ground, the second is for data transmission, and the third is for power supply.
The main theoretical parameters of the 1-Wire interface (provided by the manufacturer):
● supply voltage of devices — 3.3-5 V;
● the maximum length of the bus when using twisted pair under ideal conditions is no more than 300 m. (We recommend using a bus no longer than 100 m);
● the maximum number of users on the bus of the maximum length can theoretically be up to 250;
●  speed of exchange of the bus of the maximum length is up to 16.3 kbit/s.

Important! Important! In practice, a maximum of 10 sensors can be connected to one input of the controller. If the number of sensors is more than 3 and the line length is long, it may be necessary to connect an additional pull-up resistor between the 3.3 V power output and the controller input with a resistance of 1.5 kΩ.


1-wire network topology

Although 1-Wire networks often have a fairly "free topology", they usually fall into several generalized categories based on the distribution of 1-Wire devices and the organization of connecting wires.

1. Linear topology: the 1-Wire bus is formed as a single line, starting from the controller and extending to the most remote slave device. Other slave devices are connected to the 1-Wire bus with small (<3 m) branches.


2. Topology "Star": the 1-Wire bus is divided near the controller and extends to several branches of different lengths. There are subordinate devices along or at the ends of the branches.


Testing has shown that the topology of the star network is the least stable. Different branches have inconsistent electrical parameters, which cause spurious signal reflections in the bus, which cause data errors. For this reason, the use of a star topology is not recommended. If several 1-wire physical lines are installed in the system, the best solution is to connect them to different inputs of the controller.

Interferences in the 1-Wire network

The 1-wire bus is very sensitive to interference, the number of sensors, the length and organization of connecting wires. Each additional sensor or additional length of wire increases the parasitic capacitance of the bus, which worsens its overall stability and can lead to its complete failure.
Here are some tips for installing a bus:
● Lay the 1-Wire bus at the maximum distance from high-voltage wires, wires of dimmable lamps, LED lighting, motors, solenoids, electromagnets, pumps, which create significant interference when controlling them.● Mount the 1-Wire bus away from other power and switching equipment that may create electromagnetic and spark interference.● Pay special attention to the installation of the wiring of LED strips, as they are usually dimmable, so they are a source of impulse interference for the 1-Wire bus, which is highly dependent on the length, cross-section, wire material, wattage and length of the strip used. Wires with a larger cross-section and shorter length reduce voltage losses on the connecting wires and reduce the interference they cause. Also, be sure to lay them away from signal wires, such as the 1-Wire bus, button inputs or sensors. In no case do not lay power and signal lines in the same cable.● If possible, use separate power supplies to power devices and LED strips. This is especially relevant in 9U devices, where the DC outputs are galvanically isolated, which significantly reduces the impact of interference on the input interfaces.


● For the 1-Wire bus, use high-quality twisted-pair cables with good insulation, copper cores and a high-quality screen.● Shielded twisted pairs reduce interference from power lines and equipment, so it is recommended to use them when laying in places where LED strips, dimmable lighting, motors, inductive loads are used. Do not forget to connect the shields in the places where the sensors are connected, so that the cable is shielded along its entire length. However it should be understood that shielded cables increase the total capacity of the bus, which negatively affects its maximum length and the number of sensors on it. That is, the noise immunity of the bus increases, and the number of sensors and length decreases. Use one pair to transmit power to the sensors, and the other pair to transmit the signal and minus of the sensors.


● Branches from the bus with a length of 1-2 meters do not significantly impair the quality of the system, but can significantly simplify installation.● Buy only genuine encoders, there are now a lot of fake DS18B20s that either have very poor performance or don't work at all and can cause other encoders on this bus to malfunction.● If you need to connect several sensors at a long distance and there is equipment on the site that creates significant electromagnetic interference, then you should use sensors with an RS-485 interface, since this interface is much less sensitive to interference.● NTC sensors can also be used as an option, they are less sensitive to interference, but then a separate signal line must be used for each sensor.

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