Authored by: Mike Southworth, Curtiss-Wright Defense Solutions and Ken Braund, Meritec.
For many years, the favored rugged I/O connector for deployed aerospace and defense subsystems, such as ATR boxes, has been the familiar MIL-DTL-38999 standard circular connector. Over the years, connectors have evolved to meet the demand of high-speed interfaces.
Connector is approaching its 80th year in service, the 38999 first emerged in the 1930s to address the need for higher bandwidth and greater survivability in high vibration military environments. Since then, the connector’s basic design has evolved to keep up with requirements for better EMI shielding, fluid resistance, and endurance to temperature, moisture and dust exposure. Today, the MIL-DTL-38999 variation found most frequently in embedded systems is the Series III type, which connects via a triple-start, threaded coupling. Over the last fifteen years, though, a dramatic increase in bandwidth requirements-driven, for example, by Ethernet, DisplayPort, USB, SATA, and InfiniBand, has forced innovation, as the 38999’s familiar pin and socket contacts have proved inadequate to ensure signal integrity at the higher data rates at full cable length.
With the emergence of a new class of processors that natively support higher bandwidth interfaces, such as 10 Gigabit Ethernet, USB 3.0, PCI Express, and SATA 3.0, demand has emerged for a circular military-grade connector that supports their higher data rates. In response to this requirement, VITA adopted the VITA 76.0 standard (“High-Performance Cable – Ruggedized 10 Gbaud Bulkhead Connector for Cu and AOC Cables”) in 2015 to define a new 38999-style connector, one that marries a high-density, high-bandwidth mini-interconnect scheme and fits within the standard MIL-DTL-38999 circular shell. The new contact design was pioneered and first commercialized by Meritec of Painesville, Ohio. VITA 76.0 was ratified as an ANSI standard (ANSI/VITA -2016) in early 2016. Offering the same ruggedization and environmental ratings as the traditional D38999 series III (IP67; 175 degrees C; 500 mating cycles; 500-hour salt spray, etc), the VITA 76.0 connector embeds a “Hermi” contact system within a Qualified Production Listed (QPL) D38999 series III shellwork. Because it leverages standard, qualified circular shells, VITA 76.0 provides designers with the flexibility to use readily available market-standard backshells and accessories with which they are already familiar.
Driving the recent increase in demand for the VITA 76.0 connector is the challenge of ensuring exceptional signal integrity for high-speed interconnects over maximum cable length on size, weight, and power (SWaP) constrained ground, airborne and naval platforms. The problem is simple: the faster the signal and longer the cable, the greater the signal degradation, and critical applications can’t accept signal loss or distortion. The higher data rates supported by new high-performance processors, such as Intel’s Xeon-D SoC, aren’t well suited for use with traditional pin and socket 38999 connectors since they severely limit cables to only a few feet in length. Consider that the 10GBASET standard (IEEE 802.3an-2006) supports 10 Gb/s connections over unshielded or shielded twisted pair cables over CAT 6A or CAT7 twisted-pair with a maximum range of 100 meter (330 ft.). The VITA 76.0 connector has been successfully validated by Meritec to support the full 100-meter cable length of the 10GBASE-T standard.
Prior to the advent of VITA 76.0, system designers who needed improved signal integrity to support high-speed signals, but also wanted to stay with the traditional 38999 circular connector style, would turn to Quadrax connectors. The price of this alternative, though, was added SWaP and cost burden. The Quadrax contact provides increased bandwidth by rearranging the signal and ground contacts within the 38999 circular connector. Rather than re-designing the contact itself, as VITA 76.0 does, the Quadrax approach isolates the signals from each other to improve isolation from crosstalk. While increasing bandwidth capability, this design sacrifices pin count and pin density. For example, a 38999 size 23 shell can accommodate six (6) Quadrax contacts for a total of six bidirectional lanes each capable of approx. 2.5 Gb/s. This meant that four contacts were needed to support 10G Ethernet (2.5G x 4). In comparison, the VITA 76.0 connectors have been lab-proven to support 10Gbps data rates with differential pairs and grounds. Even better, with VITA 76.0 “Hercules” connectors, Meritec leverages shell work from QPL 38999 manufacturers and then integrates the Hermi contacts at substantially less cost than Mil-circular embedded Quadrax connectors.
Until the emergence of VITA 76.0, system integrators who wanted to take full advantage of a contemporary processor’s ability to support 10 GbE, USB 3.0, and/or SATA 3.0 have had to choose between three less-than-desirable tradeoffs. Either they have chosen to stick with the familiar and proven 38999 and accept a reduction in speeds and/or cable length, or adopt the Quadrax approach and its SWaP and cost penalties, or they’ve selected non-standard, and less cost-effective proprietary connector designs.
This signal integrity challenge has limited the design options available to embedded system designers. For example, the popular 38999 connector isn’t validated to support 10 GigE in copper-based BASE-T copper configurations. Instead, designers have had to turn to significantly more costly fiber optic interfaces in order to support 10 Gbps I/O from their chassis. Another area of frustration has come from system designers with requirements for video capture/encoders. The issue here is the recent growth in popularity of high-definition serial digital interface (HD-SDI) cameras, which were originally developed for the broadcast industry. Lately, they’ve been increasingly adopted for use in many deployed applications, such as onboard helicopters or ground vehicles. The problem is that while HD-SDI cameras are designed to transmit over a coax line, 38999 connectors don’t traditionally support coax. Instead, system integrators have had to use special inserts with their 38999 connector shells or use completely different non-military connectors in order to support such cameras.
With support for up to 145 total contacts, 44 differential pairs, and special shielding for maximum EMI/RFI protection, the VITA 76.0 connector was designed to address signal integrity and high-density interface issues while enabling the system integrator to continue to use the standard threaded-coupling 38999 circular shell. Given that VITA 76.0 connectors are optimized for use with 10 GbE copper interfaces, for example, and that industrial-temperature rated 10GBASET Ethernet transceivers are now available, it is anticipated that 10 GbE capable systems will begin to proliferate in deployed applications that have historically been dominated by optical architectures. What’s more, since VITA 76.0 also supports coaxial interconnect requirements, the new connectors will also be utilized in video and RF applications.
The new contact design used by VITA-76.0 connectors differs from traditional pin and socket designs in that it uses a flat hermaphroditic contact interface that is identical in both the cable plug and the receptacle. (FIGURE 2) When mated, the flat mating surface provides two points of contact. The wire termination techniques used by VITA 76.0 provide a virtually transparent signal impedance path. There are none of the electrical stubs typically found in pin and socket designs. And while many attempts have been made to embed commercial connectors such as Ethernet and USB within 38999 shells in order to make them usable in rugged military environments, the new hermaphroditic contact design is a superior solution, both smaller and lighter, that delivers the bandwidth capabilities needed to accommodate the various protocols.
VITA 76.0 defines six shell size variants; 23, 17, 15, 13, 11, and 9, four of which Meritec chose to tool - 23, 17, 13, and 9 - because the 11 and 15 shell sizes did not offer significant advantages over the 9 and 13 sizes, respectively (FIGURE 3). The shell size 23 connector supports 145 total contacts that will accommodate 22 bidirectional lanes at 10 Gb/s per lane for an aggregate bandwidth of 220 Gb/s per connector. It supports 44 differential pairs, each of which transfers 10Gbps. In comparison, the traditional 38999 connector in shell size 23 only supports 100 contacts and the slightly larger standard 38999 size 25 supports 128 contacts. With its 145 contacts, the size 23 VITA 76.0 can support 18x 10GbE lanes or an entire PCI Express x16 bus (82 contacts). The size 23 variant, with its smaller shell size, actually delivers 2.75x the differential pair density of Quadrax. Similarly, the shell size 17 VITA 76.0 connector provides 66 total contacts compared to the 55 supported by 38999 size 17. This enables the shell size 17 variant of VITA 76.0 to support 8 10G Ethernet ports or 18 differential pairs. The smallest variant of VITA 76.0, the shell size 9 connector, supports 8 total contacts, compared to the 6 supported by standard 38999 shell size 9, and is well suited to handle one port of 10GBASE-T Ethernet.
Four VITA 76.0 shell size variants are supported by Meritec, 23, 17, 13, and 9. The shell size 23 connector supports 145 total contacts for 22 bidirectional lanes at 10 Gb/s per lane and 44 differential pairs, each of which transfers 10Gbps.
Today, although Meritec is currently the only source of VITA 76.0 connector technology, multiple interested parties are working through licensing agreements, which will further accelerate implementation and geographic reach. Meanwhile, the status quo has not slowed adoption. VITA 76.0 has already been specified for use in major design wins by every major systems integrator. Having multiple vendors for the connector in the near future will only further reinforce the VITA standard’s open architecture and will provide broader worldwide geographic coverage from a supplier standpoint.
Curtiss-Wright’s recently introduced Parvus DuraCOR XD1500 Small Form Factor (SFF) modular mission computer provides an example of a contemporary rugged embedded system that takes full advantage of the 100 Ohm impedance matched VITA 76.0 connector to ensure the superior signal integrity of its multiple, high-speed 10 GbE, USB 3.0 and other I/O interfaces. This rugged COTS system was designed to deliver ultra-rugged mission computer capabilities in the smallest, lightest envelope possible. It features a 12-core Xeon-class floating-point processing supported with up to 128 GB of RAM, the highest capacity memory architecture available for embedded systems. With VITA 76.0 interconnect support for not only a comprehensive set of native Xeon-D high-speed I/O, but this system also provides 150+ spare pins for add-on expansion I/O functionality from its XMC and mini-PCIe sites for mission-specific avionics/vetronics payload interfaces. This modularity and scalability enable mission-tailored capabilities for C4ISR command and control, image processing, surveillance, virtual machine hypervisor, datacenter server processing, and network functional virtualization (NFV) applications in harsh deployed environments.
Curtiss-Wright’s Parvus DuraCOR XD1500 Small Form Factor (SFF) modular Xeon-class mission computer features a 100 Ohm impedance matched VITA 76.0 connector to ensure the superior signal integrity of its multiple, high-speed 10 GbE, USB 3.0, and other I/O interfaces.
Meanwhile, innovation in connector design continues apace: a field-replaceable version of the VITA 76.0 connector is scheduled for release in Q1 2018. The advantages to the VITA 76.0 contact design are numerous, including increased density, pin count, bandwidth per contact, and aggregate bandwidth per connector. System engineers looking for SWaP improvements within their systems can use the VITA standard that has been utilized in many designs.