How to select the right connector

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Whether classic fieldbus, Industrial Ethernet or the new High-Speed Ethernet (which is now establishing itself), the cabling concept is decisive for the success of the system and its reliability. Its topology, dimensioning rules and components must be precisely adapted to the production environment.

The optimum plug connectors (see Figure alongside) must always be used depending on the particular application area, transmission medium and environmental requirements.

User organisations and standards specify the plug connector. In this case, important differentiating features include the transmission medium, either electrical or optical, as well as the protection against environmental stress, generally IP20 for protected and IP65/67 for unprotected environments.

Typical plug connectors for field-buses and for Industrial Ethernet permit electrical or optical communication in IP20 and IP65/67.They specify an appropriate connector profile for the particular use case (see Figure on the left). Based on this, the user organizations have made a selection for their particular system.

For instance, it was ensured that the protective housings are suitable for electrical and optical inserts, and that all fibre types are supported.

Also the definition of the plug connectors from a standards perspective is an important aspect, in order to facilitate that they establish themselves over a broad basis and they are plug compatible with one another.

When selecting components, users must take into account that industrial communications often have to cope with tough conditions.

Systemology of the MICE coding according to IEC TR 29106 for load/stress from mechanical (M), ingress (I), climatic (C) and EMC (E) areas.A systematic description of the environmental conditions is provided with the MICE concept according to IEC TR 29106 (see Figure alongside).

The typical stresses from the mechanical, ingress, climatic and EMC areas are defined with severity levels 1, 2 and 3 for office environments, and light and heavy for industrial areas.

As a consequence, the design engineer can precisely define the environmental stresses for the planned area of application and in turn specify suitable components; this avoids unpleasant surprises as a result of unsuitable components. For instance, a component according to M3I3C3E3 can be used in an environment according to M3I2C3E2.

Frequently, fieldbus organizations have varying definitions, more precisely for inside and outside the control cabinet, so that this concept has not been established across the board as a standard concept.

For electrical data transmission, fieldbus systems are configured as a line-type topology, and all of the signals are switched in parallel. The cable must be terminated at both ends with its impedance in order to avoid reflections. This is realised using terminating resistors.

Parallel signal

These components must be taken into account when planning the system.

The incoming and the outgoing cables for DeviceNet plug connectors are easily connected using spring clamp technology.With a parallel signal configuration, connectors are required at the device for locally connecting the incoming and outgoing cables (see Figure alongside).

In this case, plug connectors, which are optimally designed for the particular fieldbus as a result of how the cable and conductors are connected as well as their colour coding, are advantageous. Quick connection technology also represents a further advantage.

The terminating resistors required are already integrated in the plug connector and can be switched in, which means that the outgoing cable is simultaneously disconnected. As a consequence, when a change is made to the plant or system, instead of requiring a new installation, just one switch has to be actuated.

The bus can be systematically switched in and out, step by step – also during commissioning and troubleshooting. Here, pre-fabricated cables that are especially easy to use can be utilised, assuming that the device has a T distributor in which the incoming and outgoing cable can be inserted.

Route the cables and then connect the plug connectors; this is also required for Industrial Ethernet. Many commercial RJ45 plug connectors can only be connected up using a special tool.

They do not accept any AWG22 conductors, which are generally used in the industrial environment. This is the reason that plug connectors that are equipped with fast connection technology are more suitable, and where all conductor types can be connected (see Figure below).

Introduce the conductor, cut and close the flap - it's that simple to assemble an RJ45 plug connector in the field.360-degree shielding is also an important feature. This is required in order to minimise the EMC effects of adjacent devices, for example, a frequency converter. High-quality surfaces are required so that gases do not cause contact corrosion.

These prevent corrosion and therefore a creeping aging process, which can result in the connection failing due to higher transition resistances. Over the long-term, patch cables, which are generally used in the office environment, do not fulfil these reliability requirements.

RJ45 plug connectors with and without protective housing as well as M12 plug connectors have established themselves in the market here.

With Cat6A they permit data transmission rates of up to 10 Gigabit/s. A maximum of four plug connectors can be located in a cable section.

When a bulkhead is used, this is considered to be two plug connector socket transitions so that only two additional plug connections are possible.

However, when using high-quality versions, such a high signal quality is achieved that bulkheads are only considered to be a one plug connection. This gives the user a higher degree of freedom when designing a cable section.

Fibre-optic cable

Fibre-optic cables can also be used as a medium in all of the relevant systems. Different fibre types are used depending on the length of the cable section in industrial environments; the most significant difference between the various types is the diameter of the fibre core that conducts the light.

As a consequence, plug connectors should be suitable for all fibre types. Not only this, the required data transmission link must be implemented with a technology that is as convenient as possible to install.

The rugged SCRJ plug connectors have clearly established themselves in industrial applications. Their compact design allows them to be easily integrated into devices. The spring-loaded ferrules with their 2.5 mm diameter can easily accommodate the fibre.

This results in a rugged, optical plug connector that can be used with all fibre types. In addition, this system also facilitates fast and easy assembly in the field, which often represents a significant advantage.

Whether fieldbus or Industrial Ethernet, there exists an optimum installation concept for each and every system. And more precisely, for copper and fibre-optic cables as the media as well as for all environmental conditions.

Industrial communication solutions in machinery and plant construction can be simply implemented using well-conceived components for fast installation in the field.

[Bernd Horrmeyer is Specialist for Industrial Network Cabling, Phoenix Contact.]