USB 3.0: Pushing I/O Performance
By Bob Hult, Bishop & Associates Inc.

It seems that connectors that are defined by an industry standard often fall into one of three categories: 

1. Initially popular interfaces that become obsolete as new technology is introduced. The bubble memory socket and the Device Bay connector are examples of pluggable interfaces that generated a lot of interest, but were quickly replaced as better solutions entered the market.

2. Connectors that remain basically unchanged over many years of use in a wide variety of applications. The common two-prong AC line cord plug and the standard, circular-Din are examples of connectors that find new applications with little expectation of design change.

3. Connectors that continuously evolve to address new mechanical or electrical performance requirements.

This third category is populated by a number of familiar connectors. Small Form Factor SFP/QSFP/SFP+ connectors are adapting to industry demand for greater speed in smaller packages. The PC/104 mezzanine card specification has experienced a series of upgrades in order to incorporate faster signaling protocols. These defined connectors have evolved over the years to accommodate high-speed serial differential signaling required by today’s embedded computers. But this omni-present Universal Serial Bus (USB) connector is perhaps the most visible example of an I/O connector that is constantly re-inventing itself to remain relevant in a rapidly-changing world.

The original objective of the USB interface was to reduce the wire congestion that typically existed at the back of personal computers. A collection of interfaces, including 50-pin Centronics ribbon connectors, mini Din, several sizes of D-subminiatures, RJ-45 Ethernet receptacles, telephone jacks, and phono jacks created a maze of cables and connector types that consumed the shrinking space available for I/O. Furthermore, the proliferation of portable devices made the development of a smaller universal interface imperative.

The USB 1.0 specification was introduced in 1996 by the USB Implementers Forum, a consortium of industry leaders that included semiconductor, component, and equipment manufacturers. This new interface was designed specifically for host computer-to-peripheral device applications, and offered 12 Mb/s transfer rates.

In addition to specifying a compact, 4-position connector, the standard facilitated hot-swap ability and plug & play connectivity, simplifying software configuration for new devices added to the system. A- and B-style configurations assured mating only to appropriate devices.

Up to 127 devices can be attached to the host controller. Low mating forces of the connector simplified mating connectors in cramped spaces. USB also introduced the ability to provide relatively low levels of 5-volt power to remote peripheral devices.

In 2001, USB 2.0 was approved to address the continuing need for faster I/O performance. Using the same interface and half-duplex signaling, transfer rates were boosted to 480 Mb/s. A key feature of USB 2.0 is its physical and electrical backward compatibility with USB 1.0. A device designed for USB 1.0 will plug into a USB 2.0 host, and continue to run at 12Mb/s, thus avoiding the problem of obsolescing an earlier generation of equipment. Equipment designers and consumers can be confident that their investments will have a long life, regardless of advances in technology.

Applications expanded beyond the host computer to include games, audio entertainment, and imaging. These typically portable devices provide even fewer slots available for I/O connections, so mini and micro USB 2.0 configurations were developed. The micro USB has become the defacto standard interface for charging mobile phones, as well as other portable devices.

In 2008, the USB 3.0 Promoter Group transferred the latest iteration of this globally accepted interface to the USB Implementers Forum. This time the transfer rate has been bumped 10 times—to 4.8 Gb/s—which required the addition of two differential pairs and a ground.

Maintaining backward plug compatibility with USB 1.0 and 2.0 was an essential objective of this new specification, so the additional five contacts necessary to boost the performance were added in a row located behind the standard 4-contact configuration. A standard-A USB 3.0 cable plug is extended somewhat to allow contact to this back row, while the shorter USB 1.0 or 2.0 connector will not reach these new contacts.

Standard-B connectors were more of a challenge, requiring the additional contacts to be placed on top of the existing plug. USB 3.0 Standard-B plugs will not mate with legacy Standard-B receptacles.

Also known as SuperSpeed USB, this upgraded interface offers a number of enhanced features. USB 3.0 now operates with full-duplex signaling over two differential pairs, improving host-directed simultaneous-by-directional communication.

The ability to provide power to remote peripherals is also improved. USB 2.0 allows a maximum of 500 mA to be supplied via the interface, while USB 3.0 can now deliver up to 900mA.

SuperSpeed USB cables are somewhat larger due to the increased number of shielded differential wires, but remain flexible and easy to route.

The maximum length of a USB 3.0 cable assembly is not defined in the specification. Any cable length that meets the performance requirements of the standard is allowed. From a practical perspective, engineers estimate that attenuation and crosstalk will likely limit USB 3.0 cables to about three meters. USB 2.0 cables are available up to five meters in length. Standard A and B connectors, along with a micro configuration, are part of the 3.0 specification.

As is the case with USB 1.0 and 2.0, SuperSpeed USB will not operate at its maximum capability of 4.8 Gb/s. USB 2.0 maximum sustained throughput in real-world applications is approximately 40 Mb/s. SuperSpeed USB will likely be limited to approximately 3.2 Gb/s, a major upgrade from it predecessors.

USB 3.0 is expected to find applications in set top boxes, media storage, video creation, mobile Internet devices, games, and media players. By its nature, USB 3.0 will become an instant commodity product, with the majority of product manufactured in Asia, by many local, as well as global, suppliers. Connector manufacturers will differentiate their products by quality of support and offering variations on the product, including stacked, vertical, and color-coded housings. Several major connector manufacturers, including Amphenol, Molex, and Tyco Electronics, have active product development or tooling programs in place, with additional sources likely to follow.

Designing an interface to perform at multi-gigabit speeds is not a trivial issue and will require high-speed engineering expertise and experience in signal integrity issues. One respondent commented that he fully expects non-conforming connectors to enter what will become a high-volume market, making vendor selection a critical decision to assure performance-to-specification.

Adoption by the industry will be paced by the introduction of USB 3.0 host controller chips, which are just now entering the market from NEC. Fujitsu introduced a USB 3.0 to SATA bridge chip. Microsoft announced that Windows 7, its new operating system, will support USB 3.0, probably via a service pack update. Volume production of consumer products is expected to begin ramping up in 2010, and hit peak penetration by 2012. SuperSpeed USB will be entering a market with established, as well as new, interfaces, vying for a piece of a huge pie.

The 6-contact IEEE 1394 (FireWire) serial interface has been competing with USB 1.0 and 2.0 for many years. Originally, Firewire offered much higher bandwidth and was focused on high-performance applications, while USB targeted low-cost consumer I/O.

Over the years, FireWire has upgraded its performance with FireWire 400, and more recently, FireWire 800, which is designed to provide a transfer rate of over 786 Mb/s. Due to the nature of the signal protocols of the two systems, FireWire is typically able to deliver closer to its theoretical maximum data transfer rate than its USB equivalent. Continuing iterations of SuperSpeed USB and FireWire are expected to continue serving their target markets.

The High-Definition Multimedia Interface (HDMI) video connector is another potential contender for the high-speed I/O market. It has rapidly dominated the flat-screen TV market with its ability to transmit high-definition video, as well as up to eight channels of digital audio. It has also evolved to address ever-higher performance with the most recent version; the 1.4 is capable of delivering over 8Gb/s of video bandwidth.

Molex recently introduced a new micro HDMI connector that targets mobile device applications. The HDMI Type D connector is only 6mm wide and 2.8mm high, but contains the same 19 pins as the standard HDMI connector.

eSATA is also a contender in this market. Primarily utilized as an interconnect to external storage devices, the latest version of these 7-pin eSATA cables offer raw bandwidth of up to 3Gb/s and hot-plug capability. The current specification has no provision for distributing power and cables are limited to no more than two meters.

All of these connectors are defined by a widely adopted industry standard, so the market will ultimately decide which will become the volume leader in next-generation equipment.

Bishop & Associates Comments

• Connectors defined by industry standards that recognize and adapt to the demands for increased performance require upgrades, while maintaining backward compatibility, will be adopted for long-term, widespread use.

• Universal Serial Bus 1.0 and 2.0 have become the industry standard I/O on the personal computer, as well as a wide variety of consumer and commercial products.

• The latest 3.0 iteration boosts the theoretical maximum transfer rate of 10 times to 4.8 Gb/s, and will support high-speed communications between video production/players, external storage, and other devices.

• The maximum cable length of SuperSpeed USB will be limited by distortion contributed by the cable and connectors. Current thinking estimates the maximum practical length may be three meters.

• USB 3.0 offers increased current delivery to support power-hungry peripherals, as well as management features that reduce power consumption when a remote device is idle.

• New electronics devices sporting the SuperSpeed USB interface are expected to reach the market in late 2009 to early 2010, and quickly ramp up through 2012. USB 3.0 compliant cable assemblies that are backward compatible with USB 2.0 are available from several suppliers now.

• Several other I/O standards, including FireWire and HDMI, are competing in this high-volume market segment. eSATA (external SATA) may be the most threatened I/O protocol by the adoption of USB 3.0.

• It remains to be seen if and when a USB 4.0 will be introduced, and if it will require the implementation of fiber optic links.

Robert Hult
Director of Product Technology, Bishop & Associates, Inc.
Robert Hult has been in the connector industry for more than 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel in Chandler, Arizona, USA. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a Bachelor of Science degree in electronics technology and a minor in business.