Interconnect solutions for aircraft Ethernet data networks
Oct 1, 2007 12:00 PM By James Yoshitake
Design engineers face a considerable challenge in developing connectors for use in avionics Ethernet network applications. Initial evaluation of Ethernet networking system standards for commercial aircraft began with the formation of the aircraft data network (ADN) subcommittee of the ARINC Aircraft Electronic Engineering Committee (AEEC). The subcommittee created a multipart document addressing the ARINC 664 aircraft data network specification. The efforts of this subcommittee resulted in the creation of the Avionics full-duplex switch Ethernet (AFDX). The AFDX is based upon IEEE 803.2 Ethernet standards with the inclusion of specific deterministic network and bandwidth functionalities.
The most popular connector for use in standard Ethernet networks and functionality is an 8P8C plug and jack known as RJ45. While ideal for the telecommunications industry, the RJ45 connector is not suitable for the harsh environments found in aircraft, and in particular, military aircraft. The ADN subcommittee evaluated the rigorous environmental demands and identified the non-applicability of the RJ45 connector package. Further evaluation revealed the necessity for blind mating in ARINC 600 LRU applications. After obtaining substantial input from a sampling of connector manufacturers, the ADN subcommittee identified the appropriate connector package for Ethernet data network applications as the “Quadrax” connector, shown in Figure 1.
Quadrax connector specifications
The Quadrax connector comprises four inner signal contacts that are insulated from each other and encased in an outer shell. A polarization key assures proper mating into ARINC 600 or similar connectors. The polarization key also identifies the location of contact 2, directly next to this contact. Similarly, the wiring key identifies both the position of contact 1 and contact 2, as it is located directly between these two contacts. These two keys establish the proper mating of the Quadrax connector.
Concurrent with the development of the Quadrax connector was the development of an aircraft Ethernet cable assembly design, designated StarQuad. The StarQuad assembly consists of four conductors and four shielded wires that are wound together in a spiral pattern with the cable assembly shield terminating to the outer shell of the Quadrax connector.
The ARINC 664 standard identifies a specific color coding for the StarQuad cable assembly: red, green, blue and yellow. Per the standard, color coding in the cable twist may be visually used to identify both End A (clock-wise rotation of red to green to blue to yellow) and End B (clockwise rotation of red to yellow to blue to green). Table 1 outlines the color-coding portion of the standard.
The combination of the StarQuad and Quadrax designs increases the complexity associated with designing and handling military and defense aviation Ethernet cables. Furthermore, the twist or “lay” of the cables gives rise to specific geometric relationships that require knowledge of the cable ends, twist, and wire/contact relationship. This relationship is most critical when defining the final assembly and termination.
The interrelationship between the contact and the twist of the wire within that cable has not been an issue with traditional wire bundles that use standard (size 20 AWG or 22 AWG) pins at the connector junctions. However, in cases where an Ethernet link contained connectors with Quadrax contacts, new guidelines were developed.
To support this wire/contact relationship and aid in proper assembly procedures, the ARINC 664 committee defined the following color assignments: pin 1 = red, pin 2 = yellow, pin 3 = blue, and pin 4 = green, as noted in Table 1. Within this nomenclature, only four contact-number to wire-color combinations may be used.
The ARINC 600 specifications identify the external Quadrax contact geometry, the orientation of the Quadrax contact within the insert, and the appropriate mating interface location to ensure intermateability of the connectors between various suppliers. Furthermore, the ARINC 600 provides a list of electrical, mechanical, and thermal performance requirements.
While early Quadrax connector designs met the ARINC 664 standards, many were not capable of terminating a StarQuad cable. Additionally, Quadrax connectors were initially used in rack and panel products in different types of aircraft applications using circular connectors, but because different companies manufactured them, each Quadrax connector contained a different termination method. One such termination approach is shown in Figure 2.
ITT Electronic Components' termination process, for example, involves stripping the StarQuad cable and crimping the center contacts to the four discrete conductors. The center contacts are then placed into the holder. The user inserts the holder, with terminated contacts, into the Quadrax contact body assembly containing the inner insulator. The final step involves securing the StarQuad cable shield between the body and crimp ferrule using a standard hex crimp tool. The completed Quadrax termination assembly is shown in Figure 3.
The attachment of the cable shield to the Quadrax contacts provides several significant benefits including cable strain relief and a link to grounding the shield to the chassis via the connector housing. This is achieved by the retaining clip within the contact cavity. Current then flows from the shield through the Quadrax body, clip, metallic insert and connector housing to the chassis. These features make the connector perform as expected under the rigorous specifications of military Ethernet networks.
Emergence of military-grade connectors
Despite the fact that Quadrax contacts initially found their home in rack and panel applications for onboard Ethernet networks, over time networking applications migrated into other markets including industrial and military aircraft applications. This market migration necessitated the packaging of Quadrax contacts into other standardized connector configurations, such as MIL-DTL-38999 and ARINC 404.
While conventional Quadrax connectors typically consist of separate contact, insulator and housing components with the insulator retaining a separate contact from the shell, some Quadrax connectors integrate the insulator with the connector housing, providing maximum cable shielding to the housing. This type of connector also provides instant grounding of the cable shield upon locking of the Quadrax in the connector. ITT Electronic Components' 38999 Quadrax connector, for example, uses the latter design, providing the necessary robustness and reliability needed for military environments.
Furthermore, the cable to the Quadrax connector is often terminated but, in some cases, the mating connector is installed on a box. Inside the box are devices to which the connector must connect, either through the cable or via a printed circuit board behind the connector, which then requires soldering of the Quadrax to the board.
Alternative designs allow contacts to be removed and/or replaced without having to disassemble the entire box and all soldering points. This provides maximum ease in terms of field maintenance and contact reparability.
Figures 4 through 8 show different standard connector packages using the Quadrax contact configuration in a size 8 contact.
Supporting connector diversity
A number of contact arrangements are available for the MIL-DTL-38999, ARINC 600, and 404 rectangular connectors. These arrangements are presented in Tables 2, 3 and 4, respectively. All insert arrangements provide electrical bonding of the cable shield to the housing, and MIL-DTL-38999 inserts are an integral part of the shell.
As shown, the Quadrax connector design provides design engineers with a number of benefits when installed into Ethernet data bus network applications, from meeting standardization requirements to flexibility of design. The Quadrax contacts provide an industry-standard ARINC 600 interface, instant bonding of the cable shield upon installation into the insert/connector, and the MIL-DTL-38999 connectors prevent cracking of the insulators due to torsional loads when coupling.
The same Quadrax contacts may be used in a variety of connector types including ARINC 600, ARINC 404 and MIL-DTL-38999. Additionally, the rear-insertable, rear-removable Quadrax with crimp termination and front- insertable, front-removable Quadrax with PWB termination designs provide both ease of maintenance and reparability. Furthermore, by being able to use the same Quadrax contacts in a variety of connector designs, as well as the same connector being able to accommodate either pin or socket Quadrax with PWB terminations, both inventory and manufacturing costs are decreased, while simplicity and ease of use are increased.
Connectors that meet the increasing market demands of high performance, ease of use, termination requirements, reliability, and serviceability must be developed in order to satisfy both the manufacturer and application requirements. The wide range of styles and flexibility of the connector design in terms of solder and crimp-style contacts, shape, size and housing design allow the Quadrax design to be designed in to the continuously evolving applications using Ethernet functionality.
ABOUT THE AUTHOR
James Yoshitake is a product manager for ITT Electronic Components. He joined the product management team at ITT Cannon in 1985. He has been product manager of circular products at ITT since 2002. Yoshitake earned a B.S. in business marketing from California State University in 1982.
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