Today, for internal communication networks in companies, generic networks compliant with IEC 11801/24702 are no longer the only option. In production environments, application-specific networks compliant with IEC 61918 are also important within automation islands. For Ethernet-based solutions, strategies range from using generic communication cabling in industrial settings to employing specialized components and selecting appropriate topologies. The most common fieldbuses are primarily based on RS-485 or CAN and always utilize specialized components.
Fieldbus organizations have their own definitions .
Industrial communications frequently encounter harsh application conditions. In addition to transmission-related properties, environmental conditions must also be considered when selecting components. These can be systematically described using the MICE model according to IEC TR 29106 for voltage related to mechanical effects, foreign substances, weather, and electromagnetic interference, as well as severity levels 1, 2, and 3 for office, light, and industrial areas, respectively.
On the other hand, fieldbus organizations often have their own definitions, such as those for inside and outside the control cabinet. Furthermore, the use of shielded cables and a grounding and potential equalization system has a significant impact on network reliability. This is also why many organizations adhere to their guidelines regarding system-proven components and appropriate cabling techniques. If a proper grounding system is not available or EMC effects generate unacceptable interference voltages, then optical connections should be selected.
Quick-connect technology is an advantage.
When transmitting electrical data, parallel-wired fieldbuses are physically designed so that once a signal has been fed into the bus, it is available at any location along the transmission path. Consequently, each participant can retrieve the signal at any location using a parallel passive circuit configuration in their receiver. The same applies to the signal read by a device on the bus. Here, the software protocol always ensures that only one transmitter is active at any given time. The cable must be terminated at both ends with a termination resistor to prevent feedback. Furthermore, branch lines should be kept as short as possible.
This physical configuration necessitates a linear topology. During the planning phase, termination resistors must be considered; these are either integrated into the devices or must be supplied as an external component.
The fact that all devices are connected in parallel necessitates the use of connectors on the device for the local connection of the incoming and outgoing cables (Figure 1).
Plug-in connectors, optimally designed for the specific fieldbus and featuring a quick-connect system, offer clear advantages in this respect.
On-site assembly is mandatory.
Often, the terminal resistors are integrated into the connector itself and can be activated (connected), which disconnects the output cable. Consequently, when a plant or system needs to be modified, only a switch needs to be activated
—no complex installation changes are required. The bus can be systematically connected and disconnected step by step—which is very important during commissioning and troubleshooting. Pre-assembled cables, which are particularly easy to handle, can be used if the equipment is equipped with a T-connector; the input and output cables can be inserted into this connector, thus eliminating any local assembly. This configuration is very common for M12 cabling.
On the other hand, Ethernet-based systems compliant with the IEEE 802.3 standard transmit electrical data using a point-to-point connection with a maximum total length of 100 m. Male-to-female connection systems are used, very similar to those employed in structured building cabling. Bulkheads with two female connectors can also be used. Here, there is a significant difference between industrial automation technology and communication cabling in office environments. While prefabricated cables are practically the only type used in offices, in automation technology, locally assembled cabling is absolutely essential.
Panel-mounted outlets are generally not used; instead, individual cables run directly from device to device. Therefore, RJ45 connectors must be mounted directly in the field. Standard RJ45 connectors can only be mounted using AWG26 conductors and a crimping tool. This is why cables with conductor cross-sections up to AWG22—generally used in industrial environments—cannot be used.
For this reason, plug-in connectors with a quick-connect system that allows for tool-free assembly are particularly advantageous for industrial applications (Figure 2).
Built to withstand mechanical stress,
all standard systems also allow the use of fiber optic cables as the transmission medium. Unlike office environments—where glass fiber dominates—industrial applications utilize various fiber types depending on the data link length. For single-mode glass fiber, the diameter of the light-transmitting fiber core is 9 µm, for multimode 50 µm, for PCF 230 µm, and for POF 980 µm. Therefore, precision requirements and installation costs play a crucial role in selecting the appropriate pluggable connector. Furthermore
, the required data link must be implemented using the simplest possible technology for installation. Loss due to connector attenuation must be considered in addition to the fiber's own attenuation.
Resistance to mechanical stress is also important, as it is more frequently required in industrial environments than in office settings. Shock, vibration, and temperature differences cause axial or radial stress on the optical axis. This results in an unacceptable increase in attenuation at the pluggable connection (Figure 3), which, in turn, reduces the transmitted optical intensity. Data transmission is interrupted when the attenuation loss margin of the data link is exceeded.
SCRJ plug-in connectors must be compatible.
This is why plug-in connectors used in office environments are not suitable for the specific requirements of industry.
On the other hand, apart from the office environment, rugged SCRJ connectors have also proven their effectiveness in industrial use and are specified for a wide range of networks (Figure 4). Their compact design as a small form factor (SFF) connector allows for easy integration into devices. The spring-loaded ferrules with a diameter of 2.5 mm easily accommodate the fiber. This results in a robust optical connector that can be used with all types of fiber.
Furthermore, this system also facilitates quick and easy assembly in the field, which is very practical. For example, for polymer fiber (POF), it only needs to be cut and fitted into the quick-release sleeves. The compatibility of the standard SCRJ connector with widely used SC connectors is an advantage in commissioning and diagnostics.
Summary:
Whether using fiber optic or copper cable, in a control cabinet or on a device, for a welding station or a chemical plant – industrial communication systems can be successfully used in automation applications. Reliable and flexible topologies can be easily implemented in plant and machine construction using adapted plug-in connectors and well-designed solutions for quick field installation.
Profinet overcomes the limitations of generic cabling
Flexible topologies in plant and machine construction can be easily implemented by following structured cabling principles. As a result, fieldbus networks overcome the limitations of generic cabling. For example, Profinet enables flexible topologies and simplifies cable selection and sizing between active devices. This means a network can be established following a few simple rules without requiring any calculations.
By using the defined cable types, a segment or channel length of 100 m can be established using any combination of flexible and fixed-installation cables. The plug-in connectors on each segment can be flexibly configured to meet industrial requirements, provided the limit of 4 connectors is not exceeded.
The simplified planning model can only be achieved by using a conductor section of AWG22, which is larger than AWG26 or AWG24 sections – the types normally used in structured building cabling.
Unlike generic cables consisting of four twisted pairs of conductors, Profinet uses an optimized "star quad" cable in which all four conductors are twisted together. All cables are shielded to ensure high immunity to interference and are optimally matched to the pluggable connectors specified by Profinet.
Author:
Eva Andueza, Product Manager at Phoenix Contact, SA.
