Plugs and connectors for sensor cables

Plugs, connectors, and sensor cables affect the reliability and efficiency of systems in modern industrial automation. But what types of sensors are available and how do you choose the right sensor cable? How is the connection of sensor connectors optimally implemented to ensure secure data transmission? In this blog post, we’ll explore the different connector types, highlight commonly used connectors, and explain some variations of coding M12 sensor connectors – everything you need to know to choose the right connection for your applications.

What are sensor leads or Sensor cables?

A sensor cable is an electrical connection between a sensor and a controller or other evaluation system. It transmits measurement signals and supply voltage and must be protected against external interferences such as electromagnetic fields, mechanical stress or chemical effects, depending on the application. Sensor cables are available in various versions, e.g. shielded for noise-proof analog signals, multi-wire for digital bus systems or highly flexible for moving applications. They are often equipped with standardized connectors such as M8 or M12 for easy and safe installation. The correct selection of a sensor cable depends on the number of wires required, the signal type, the environmental conditions and the mechanical requirements. How sensor cables work together with additional mechanical components in a complete system is shown in our blog post on mechanical components in automation.

Types of Sensors by Output Signal

Sensors can be classified into different categories based on their output type.

Analog sensors provide a continuous output signal that is directly proportional to the measured physical quantity. The signal can be output in the form of voltage (0-10 V) or current (4-20 mA). Examples include pressure sensors, temperature sensors, or distance sensors.

Digital sensors, on the other hand, output their measured values as data packets via serial interfaces or fieldbus systems. This not only allows them to transmit measurements, but also to send status information and diagnostic data. This includes, but is not limited to, CAN bus sensors or Ethernet-based sensors.

Binary sensors, in turn, provide an on/off signal (0 or 1), which is typically plus-switched as a PNP discrete output or negative-switched (ground-switched) as an NPN discrete output. They respond to a specific physical quantity and either turn on or off without providing intermediate values. This includes, for example, inductive proximity sensors or capacitive sensors.

Our article on Sensors - selection and importance for automation technology deepens the topic of sensor selection in automation and offers an ideal complement to the sensor types presented here.

Proper selection of sensor cables

Sensors provide valuable measurement data that is essential for controlling and monitoring industrial processes. However, to ensure these signals are transmitted reliably and without interference to controllers, displays or networks, choosing the right sensor cable is as important as the sensor itself. Depending on the application, sensor cables vary in their number of wires, insulation and shielding to meet specific requirements.

The number of wires in a sensor cable depends on the type of sensor and its output signal, with simple binary sensors requiring fewer wires than digital or analog measurement systems. Insulation plays an important role in mechanical strength and temperature resistance, especially in demanding industrial environments. To ensure trouble-free signal transmission, many sensor cables are additionally shielded. This is to minimize electromagnetic interference. Depending on the application, sensor cables are specially designed to meet the requirements of flexibility, chemical resistance, and mechanical stress. This ensures a reliable and long-lasting connection between sensors and control systems.

Connecting Sensor Connectors and Sensor Cables

Sensor connectors are used to connect sensor cables and to securely connect the sensor cable to the sensor or controller. They provide fast and secure connection and disconnection of cables without risk of damage or improper wiring. Sensor connectors are offered in different styles depending on the application, e.g., for screw or quick-release technology. They are often protected against moisture, dust or other environmental influences. In many cases, sensor connectors allow for simplified component integration, which is especially important in maintenance or component replacement.

In principle, two main connection types are distinguished: Wire-to-wire and wire-to-board. A wire-to-wire connection is the connection of two cables through one connector – often used in modular cabling systems where wire harnesses between different devices or components must be connected without loss or interference. In particular, crimp connections are used here in practice, in which the conductors of the cable are crimped with contacts and then inserted into a connector housing. This method allows for a pluggable, mechanically resilient connection, as is widely used with M8 and M12 connectors. Alternatively, screw terminals are also used, which are appreciated as a releasable connection, particularly for wire-to-wire solutions.

In contrast, in a wire-to-board connection, a sensor cable is connected directly to a printed circuit board (PCB) - either through a matching connector or by directly contacting the wires with solder pads or terminals on the board. These applications primarily use clamp connections, which enable quick and tool-free assembly. For particularly compact or permanently stable connections, soldering is often also used in this context, for example when space constraints or mechanical stability require a fixed soldered solution.

Common Sensor Plug Types

Round plug connectors are the most commonly used connector types for industrial sensors because they provide a compact, robust and standardized solution. In particular, the M8 sensor connectors and M12 sensor connectors have established themselves as industry standards, as they are suitable for various signal types and offer high protection classes for harsh environments.

Smaller and more compact with a diameter of 8 mm, the M8 connector is ideal for applications where space is limited, such as small, compact sensors. Typically, M8 connectors are designed for sensors with fewer wires (often 3 or 4) and lower power requirements, such as those found in simple digital or analog measurements. The 12 mm diameter M12 sensor connector is significantly more robust and offers more space for additional wires (pins), making it a universal standard for a wide range of sensors. M12 connectors can also be used for higher currents and signal strengths, as they allow for a larger conductor cross section depending on the number of conductors. M12 connectors are available in many configurations and encodings and are believed to be the most widely used variant of connectors internationally. At the same time, they impress with their compact design and various shielded versions.

M12 connectors - Pin assignment, colors, standards

The M12 connector is the industry standard for sensors, actuators and bus systems in automation technology. It provides a compact, robust and reliable connection for analog and digital signals as well as industrial networks. The M12 connector’s versatile encoding (A, B, D, X encoding) makes it suitable for both classic sensor-actuator connections and data communication in Ethernet and fieldbus systems. A key factor for safe wiring is the correct assignment and color coding of the contacts, which varies depending on the signal type. To ensure standardized and error-free installation, M12 connectors are regulated by international standards such as DIN EN 61076-2-101. DIN EN 61076-2-101 defines in particular the mechanical, electrical and environmental requirements for connectors used in automated systems and machines.

Coding using the example of M12 connectors

To avoid accidentally incorrect insertion of connectors, they are mechanically coded. The encoding determines for which signal type or communication interface the plug is suitable.

• M12 A coded: M12 connectors with A-coding are the standard for sensors and actuators in industrial automation. They are often used for analog and digital signals as well as for IO-Link communication. Available in a variety of variants, the A-coded M12 connectors provide a reliable and robust connection for industrial environments such as mechanical and plant engineering and robotics.
• M12 B coded: M12 connectors with B-coding are mainly used for fieldbus communication, in particular Profibus and serial interfaces (RS485). They are typically 4 or 5 poles and ensure secure, trouble-free data transmission in networked automation systems. Mechanical coding ensures that they cannot be confused with other M12 variants or inserted incorrectly.
• M12 D coded: D-coded M12 connectors are specifically designed for Industrial Ethernet and Profinet and allow data transfer of up to 100 Mbps. With four poles, they are ideal for network connectivity of sensors, actuators and controllers in industrial applications. Thanks to their often high protection class, they are resistant to moisture, dust and mechanical stresses.
• M12 X coded: X-coded M12 connectors are designed for high-speed Ethernet applications with Gigabit data transfer (up to 10 Gbps). Their 8-pin configuration and optimized shielding make them ideal for Industry 4.0 and high-speed data communication. They provide a secure and reliable solution for modern automation and networking systems in harsh industrial environments.

In addition to these commonly used codings, there are other M12 connector variants such as C, S, T, L and Y codings, which are specifically designed for AC and DC power supply, hybrid solutions or special industrial applications.