Requirements in industry - whether for turnkey assembly lines, appliances or individual components - are often: compact - efficient - powerful. There are many reasons for this: smaller devices enable smaller systems that require less space. This results in savings due to lower raw material consumption during production, cost savings during transportation and smaller installation areas. In order to reduce device sizes, the interface to the outside is also considered.

Circular connectors are particularly popular in industrial environments - reasons include an IP-protected device design and thus the elimination of a space-consuming additional housing, as well as quick connection from the outside without having to open the respective device.

The M8 version of the M12 connector has established itself as the standard for particularly compact connections: A third smaller than the M12, the M8 nevertheless offers industrial-grade properties and is easy to handle. Whether for connecting compact sensors or supplying power to small devices - the M8 is already widely accepted in many areas.

The history of M8

With the increasing integration of industrial networks far into the machines, the M8 connector has to face another task: the reliable transmission of information packets in the context of network traffic.

Unlike the popular M12 size, no special coding has yet been provided for this application. Many users use four-pin M8 connectors with standard coding to integrate their devices into the network. Although this provides a functional solution, it comes at the cost of two disadvantages:

Optimized for high-frequency data transmission: In contrast to the previously used asymmetrical M8 mating face (right), the M8-D coding (left) has contacts with identical spacing between them.

© Phoenix Contact

• If devices are both supplied with power and connected to the network via two A-coded M8 connectors, these connectors are plug-compatible. If markings or labels are overlooked when connecting such a device, a supply cable connected to the data interface can cause damage by overloading the device electronics.
• In addition, the four contacts of the M8-A coding - an interface developed for connecting sensors and small actuators - were arranged asymmetrically to prevent mismating. In this way, it was possible to dispense with the very small coding lugs that ensure mismating reliability in the M12 with symmetrical contact arrangement. However, the asymmetrical arrangement has a negative impact on the transmission quality - caused by electromagnetic fields from the contacts. This mutual interference of electrical conductors is known as near-end crosstalk (NEXT). In practice, connections with M8 connectors for Ethernet or Profinet achieve a speed of up to 100 Mbit/s; however, the requirements of the CAT 5 / Class D transmission category are not fully met. This means that the transmission path may no longer have sufficient reserves to compensate for additional influences such as those caused by long cable lengths, EMC interference or other couplings. As a result, the transmission rate can fall below a stable 100 Mbit/s, meaning that data packets are lost and have to be resent. The connection becomes unreliable.

As the transmission rates required in industrial networks have so far been below 100 Mbit/s, it has so far hardly mattered if this speed is not always or not fully achieved. However, the amount of data to be transmitted and therefore the required transmission speed is increasing.

Larger data volumes in the future

Initial I-4.0 applications show that a significantly higher sensor density is required to ensure the high flexibility of production systems than was previously the case. The quality - and therefore quantity - of both the data collected and the data made available via central databases is also gradually increasing. All devices integrated into industrial networks must receive, process and send significantly more data.

These requirements can only be met with a powerful and reliable network. The Industry 4.0 principle also requires the industrial network to move closer to the process - right up to the integration of individual sensors into the network. As this is also implemented in compact machines and systems, a way was sought to optimize the small M8 interface to make it fit for the future.

Future-proof interfaces

The D-coding was developed and standardized in IEC 61076-2-114 to eliminate the two disadvantages of the standard M8 connector mentioned above.

The new mating face has four symmetrically arranged contacts. The opposing contacts form a pair to which either two opposite cores of a star quad cable or a pair of cores of a twisted pair cable are connected. As the electromagnetic fields generated during the transmission and reception of data cancel each other out thanks to the symmetrical contact arrangement, NEXT prevents data packet losses. This ensures that all CAT 5 / Class D requirements are met and that sufficient reserves are available. This provides the user with a reliable network connection.

Compared to the M12 connector, the space required on the device or control cabinet is reduced by a third by using the M8 size.

© Phoenix Contact

Due to the now symmetrical contact arrangement, additional coding elements are required in the mating face. On the one hand, the coding prevents mismating within the connector so that the contact sequence is always maintained. On the other hand, a mechanical distinction is made between connectors with the same contact arrangement but which are intended for other applications - such as the M8-P coding for Ethercat. Mechanical separation from coding that is intended for the power supply in the future is particularly important in order to prevent possible damage caused by incorrectly connected supply cables.

The arrangement of the coding lugs within the mating face is based on the coding elements of the D-coding in size M12. However, the elements have been optimized so that they can also be manufactured as plastic parts in the smaller size. By standardizing the new D-coding for M8 connectors as an IEC standard, users can ensure that a combination works reliably even across manufacturer boundaries.

Author:
Dirk Bunzel is Product Manager Passive Network Components, Industrial Field Connectivity, at Phoenix Contact in Blomberg.