How to Select the Correct Data Cable for Your Industrial Application

Not all data cables are the same. Many users do not realise this until they are experiencing disruptions and errors in the data transmission in their industrial applications. In this article, you will learn what the most common mistakes are when selecting an industrial Ethernet or bus cable and how to avoid them.

Technician working in industry

All cables and wires that contribute to communication in any way are commonly referred to as data cables. There are, however, significant differences, such as the completely different structures of copper and fibre optic cables. Copper data cables come in many different types, such as low-frequency cables, coaxial, telephone, and bus cables, diverse Ethernet systems, or microwave cables for special applications requiring transmission in the range of gigahertz. Selecting the wrong cable can quickly lead to costly disruptions and errors.

In general, data cables are low-capacity cables. This means that as little electrical energy is accumulated in the cable as possible when transferring data. This electrical energy has a negative impact on the signal quality. The capacity is in part dependent on the insulation material of the cable. Modern bus and Ethernet cables primarily use materials such as PE or PP, which provide especially good insulation, which is determined by measuring the dielectric constant (εr). The lower this value is, the better the material is at insulating, and the lower the capacity of the cable. This is why thinner insulation can be used when the same dielectric strength is used.

A Comparison of Cable Insulation Materials

MaterialVDE-LabelDielectric ConstantHalogen free
PVCY3.6-6no
PVC +90°CYw4-6.5no
PE2Y2.3yes
PE foamed02Y1.6-1.8yes
PP9Y2.3-2.4yes
PP foamed 1.6-1.8yes
PUR11Y4-7yes*
FEP6Y2.1no
*Dependent on flame retardants used

Pristine data transmission is achieved through the correct cable construction. A solid wire, which is perfectly round and has a uniform diameter, offers the best electrical performance. For industrial Ethernet and bus cables, construction according to the AWG is preferred since this flexible construction results in a round conductor. Metric cables are not suitable for this application since they have a bunched construction and are not round. This results in variable capacities which heavily impair high-frequency data transmission.

Common Errors when Selecting Industrial Ethernet and Bus Cables

1. Low-Frequency Cables for High-Frequency Applications

Selecting low-frequency cables for high-frequency Ethernet connections is a common cause for malfunctions in the transmission of data. These cables are also low capacity but exhibit a different characteristic impedance as what is required by the Ethernet standard. This results in a mismatch, or a discontinuity. In low-frequency data cables, all pairs are laid in parallel strands. This means that all four lay lengths are the same. Ethernet cables used in high-frequency applications must also be optimally decoupled. This is achieved by using four different lay lengths, which are individually measured. The positions of the twisted pairs within the overall construction must also be taken into account.

Poor and Good Decoupling
Poor decoupling (left) and good decoupling (right)

2. Classic Stranded Pairing instead of a Star Quad

Many industrial communications standards, such as PROFInet, EtherCAT, or SERCOS III, use cables with two twisted pairs, which are twisted into what is called a star quad, for data transmission. Here, all four cores are stranded in a perfectly round shape. The benefit of this is that it leads to no differences in transit time. This differs from the classic stranded pairing, where the individual twisted pairs must have two different stranded lay lengths due to the required decoupling. If the incorrect twisted-pair cable is used, it could lead to problems with transmission and transit times.

2 Pairs

2 Pairs

  • 4 x Core diameter
  • 3 x Stranding
  • Pairs according to illustration
  • Straightforward
  • Different signal transit times
Star Quad

Star Quad

  • 2.4 Core diameter
  • 1 x Stranding
  • Diagonal cores create the electrical pair: white/blue and orange/yellow
  • Identical signal transit times

In star quads, the cores that are diagonally opposite from each other form the electrical pair. If this rule is ignored when connecting the cable, the characteristic impedance and the near-end crosstalk (NEXT) of the cable are changed, which may reduce the transmission quality, as well. Even screened, four-core sensor cables are not suited for use as high-frequency industrial Ethernet or bus cables, despite their construction appearing comparable at first glance. The difference being that the core insulation strength is not intended for Ethernet, and the stranding is not perfectly round in construction. This results in an inadequately functioning cable due to unsuitable characteristic impedance, NEXT, and cable attenuation.

3. Too Long of Cable or too Small of Diameter

Another classic example are segments that are too long. According to the Ethernet standard, a repeater must be used after maximum 100 metres of length. This receives the weak signal and transmits it again at full strength. In practice, segments with lengths longer than 100 metres can be found, however these are not in accordance with the applicable standards. In these cases, rising temperatures, aging, and other factors could quickly lead to defects or failures. Thinner cables with AWG 26 diameters are limited to 60 - 70 metres. It is important to remember that every plug connector is a joint that causes loss via attenuation and reflection, which reduce the range.

4. Incorrect Plug

It is a common occurrence that non-standard and untested plugs are used in Ethernet applications, such as D-Sub or M12 plugs, which are A-coded with an 8-pin design. These plugs do transmit data, however the quality is significantly reduced due to the lower near-end crosstalk (NEXT). The reason for this is the positioning of the middle pin, which is not in accordance with standards and impedes data transmission.

Overview different connector faces

According to Ethernet standards, admissible connector faces for the best data cabling are screened plugs/sockets:

  • RJ45 4-pin (100 Mbit 4-pin)
  • RJ45 8-pin (Gbit 8-pin)
  • D-coded M8 & M12 (100 Mbit)
  • A-coded M8 4-pin (100 Mbit)
  • P-coded M12 (100 Mbit)
  • X-coded M12 (Gbit)
  • Ix Industrial (Gbit)
  • Mini-IO
  • SPE (Single Pair Ethernet)

Furthermore, there are varying standards such as PROFInet, EtherCAT, or SPE (Single Pair Ethernet) that can transmit data and power in one hybrid plug. These either adhere to IEC norms, have been evaluated by the appropriate organisations, or are being standardised.

Standardised Hybrid Ethernet Plugs

  • M8 SPE acc. to IEC 63171-6
  • M12 SPE acc. to IEC 63171-7
  • Y-coded M12 acc. to IEC 61076-2-113
  • M23 acc. to IEC 61076-2-117
  • RJ45 Hybrid acc. to IEC 61076-3-106
  • Ix Industrial acc. to IEC 61076-3-124

Certain plug manufacturers have their own hybrid solutions in their portfolios that are not standardised but have been tested and evaluated for Ethernet conformity. As a basic principle, it is suggested to always stick to standardised and evaluated Ethernet plug connectors.

Take away:

When selecting data cables for industrial applications, users should adhere to the appropriate standards in order to prevent failures and malfunctions. Users must also pay attention to the lengths of the individual segments, the number of plugs, and the different diameters of installation and patch cables. Furthermore, the aging of individual components can, in the long term, lead to a reduction in the transmission quality and to interruptions. With 45 years of experience, HELUKABEL is an expert in connection technology and is happy to support users in identifying the perfect industrial Ethernet or bus cables for their industrial application.

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