The Industrial Internet of Things or Industry 4.0 is leading to an increase in sensor and drive connections and requires smaller connector units - while at the same time placing higher demands on reliability, as lower latency times must be achieved. In industrial IP20 applications, RJ45 connectors are often used, which were originally developed for unshielded telephone lines with contacting via piercing contacts. These connectors are not considered the most stable and reliable electrical connections in corrosive environments with vibrations and moisture.

An alternative for industrial Ethernet is TE Connectivity's 'Mini I/O' connector system, where the mating face is only 25% the size of an RJ45 jack. How do the systems compare? TE asked itself this question and subjected both systems to special test procedures. The RJ45 and the Mini I/O were tested, both in versions with piercing connections (piercing) and with solder connection technology as well as for field installation. Five different manufacturers were considered.

The test conditions

Initially, the requirements in accordance with IEC 60603-7-X were used for benchmarking. However, to compare the performance of connector systems, it is not enough to simply test whether certain specification criteria can be met. Rather, the systems must be loaded beyond the requirements of the product release or the IEC specifications. For example, IEC 60603-7-3 requires a harmful gas test lasting only four days, a vibration resistance of 50 m/s², i.e. approx. 5 g, and no limit values for shock/impact loads. Standards for connectors for industrial use, on the other hand, include test sequences that prove their suitability for the corresponding environments. For example, ISO/IEC 11801-1 describes the 'MICE' environment classes (Mechanical, Ingress, Climatic & Chemical and Electromagnetic); the M3 environment class, for example, requires resistance to a shock/impact of 250 m/s².

Therefore, for a meaningful comparison, test combinations are required to generate a stress that corresponds to the actual conditions in industrial applications. Vibration levels can easily be increased, but only a sequence of load simulations for durability (mating cycles), harmful gas tests (mixed flowing gas MFG), high temperatures and vibrations can cause wear and corrosion. This provides a better picture of the performance of a contact system in a harsh environment.

IEC 60603-7 specifies a test to check for discontinuities during vibration that will give a pass or fail result. For the TE test, an alternative test has been defined in which the number of short discontinuities during vibration is counted.

The RJ45 connector system

The RJ45 is standardized as a modular 8P8C connector in accordance with IEC 60603-7, ANSI/TIA-1096-A and ISO-8877. With T568A and T568B contacting (in accordance with the TIA/EIA-568 standard), the RJ45 is suitable for both telephone and Ethernet. Its piercing contact is the only known principle that enables reliable connections with stranded wires used in fixed-line telephone spiral cables. The modular design is particularly suitable for connections to flat cables of different widths and offers unsurpassed connection speed, as all conductors are penetrated in one step and there is also double cable strain relief. However, as the wiring diagram of T568B requires the wires of the round cable to be crossed, the connection is far more complicated here. With additional shielding and a more robust housing, the RJ45 can also be used in industrial environments.

The mini I/O connector system

TE's mini I/O system, specially developed for industrial applications, is designed to be smaller than the RJ45. This reduction in size of the mating face has been achieved by means of fully enclosed metal housings that absorb the forces and provide tight tolerances and stable mating surfaces. With the help of several additional solder pads and anchor pins, the Mini I/O can withstand a tensile force of up to 98 N in accordance with the product specification without having to forego full SMD assembly. Due to the hermaphroditic contact layout, both plug and socket contacts have spring pressure and two independent contact points. An IEC standardization project for Mini I/O with speeds of up to 1 Gbit/s was completed in 2017 with standard IEC 61076-3-122:2017. Another manufacturer of the Mini I/O is Amphenol FCI.

Tested in pairs

Figure 1: Comparison of the cross-sections of the mating surfaces of two Ethernet connector systems.

© TE Connectivity

For the test, the connectors are connected to a standard industrial Cat5e cable with 26 AWG wires (0.13 mm²). The RJ45 jacks are - with the exception of one jack in reflow design - right-angled SMD jacks designed for through-hole mounting; the RJ45 plugs are piercing or field installation versions. All connector products are tested in pairs; with the exception of one pair, the plug and socket are of the same brand. For right-angle mini I/O jacks, an SMD version with type 2 coding was combined with solder plugs. Several defective RJ45 piercing connections (Rtotal - Rbulk >20 mΩ) were already detected during the first LLCR measurements in the laboratory after preparing the cable sets. These defects were not detected during the regular quality inspection of the connections using an electrical continuity testing tool. The tests showed that the wire insertion depth and/or cable management depth is crucial for good piercing connections(see Fig. 1). This means that with RJ45 piercing connector products, care must be taken to ensure the correct wire insertion depth when pressing the cable manager into the connector.

Analysis of the contact physics

Both the RJ45 and the mini I/O contact systems are based on gold plating. The main differences between the two connector types with regard to the influence on connection stability can be found in the layout:

For example, the dimensions and tolerances of the IEC 60603 series allow for large movements of the contact point (in the worst case up to 1.08 mm in the mating direction) with the RJ45, as well as deviations in the normal force of the contact. Contact systems with only one spring element and contact point, on the other hand, can be extremely stable. However, in combination with wear caused by poor contact point stability, the redundancy of parallel contact points is missing, which can lead to resistance fluctuations. This is particularly true if the test for wear caused by mating cycles is followed by a test for stress caused by environmental influences. The punching direction of the piercing contact leads to a sheared edge of the contact mating surface, whereby the contact surfaces of some manufacturers' RJ45 plugs are smoothed. The principle of cable connection with piercing contacts can also cause problems, as the tool presses against the contact surface when penetrating, which can be deformed or damaged.

In the layout of the Mini I/O, the tolerance chain of the contact point is well monitored; the metal housings enable tighter tolerances and more rigidity. In the plug with solder connection technology, the contacts of the plug and SMD socket have smooth surfaces. The overall layout is more complex than with the RJ45 and requires an elastic spring material on both the plug and socket sides.

Performance and reliability

The test groups were defined on the basis of specific power ranges. Each group consists of four mated pairs, resulting in 32 signal contact measurements per group.

For two test groups, 375 mating cycles and either a humidity test (21 cycles at -10/25/65 °C) or an MFG test (4 gases/21 days) were followed by a temperature resistance and vibration test with a maximum of 25 g. This was to simulate environmental stress after wear. The aim was to simulate environmental stress after wear in order to determine differences between the contact systems without weak tests or destroyed connections. One of the results was an increased contact resistance at low current levels (ΔLLCR). While all signal connections are OK at the beginning of the test sequence (<20 mΩ), a large increase in static resistance (>300 mΩ) was observed after the first 375 mating cycles.