Design News, November 2010

Connector and Cable Choices for Gigabit Ethernet

The need for higher data communications speed is the driving force behind the evolution of Ethernet from 10MB/s up to 10GB/s bit rates. Currently, GB Ethernet has four types of physical layer signaling: 1000Base-SX (short wavelength optical fiber), 1000Base-LX (long wavelength fiber), 1000Base-CX (short run copper) and 1000Base-T (100m, four pair category 5 UTP).

Some applications that need the kind of speed delivered by GB Ethernet include:

Remote video monitoring of critical assets and operating equipment;

Machine vision systems for industrial production;

Real-time monitoring and control in processing and manufacturing industries; and

Remote data acquisition, test, measurement and data logging.

Industrial Automation Applications

On the industrial side, high-resolution video applications have been a leading force in seeking higher bit rates. This is due to dense data content in high frame rate video, especially applications involving full color. This applies to both visible light and infrared (IR) cameras. In fact, full-color cameras with both visible light and IR imaging capabilities are widely available, and can transmit images at full frame rates of 400Hz and higher. These frames include not only images, but also temperature measurements with 14-bit or higher resolution.

Most industrial video cameras used for machine vision automation and real-time remote monitoring of critical equipment come with GB Ethernet Vision capabilities. This includes the GB Ethernet Vision Control and Streaming Protocols (GVCP and GVSP, respectively). These capabilities allow an automated application to detect and enumerate the cameras and define how they obtain a valid IP address. In addition, the application can configure cameras and receive streaming data from them (bootstrap registers describe the cameras, including current IP address, serial number, manufacturer, etc.). Some of these applications take advantage of Power-over-Ethernet (PoE) to power the cameras.

Besides machine vision and other production automation systems, these capabilities are important in a variety of remote security, surveillance and monitoring applications, which include: asset monitoring and protection (warehouses, bulk material yards, etc.); process monitoring and control; machine condition monitoring; high-speed sorting; assembly verification and package inspection; and remote control of test and measurement systems.

Convergence of Office IT and Automation

Besides the need for high data transmission speeds, another driving force behind the expanding use of GB Ethernet (and Ethernet in general) is reduced capital investment costs. Instead of having separate redundant networks, there is one shared data communication system across IT and automation. But an automation environment has different cabling needs than an office IT environment. The types of connectors and cables used in the office are not robust enough for the factory floor. In order to successfully converge the two networks, passive component requirements for both sides need to be taken into account.

The ideal solution is to use cables and connectors that meet the data speed requirements of office IT networks, and provide the robustness, flexibility and ease of assembly needed on the factory floor. This approach allows the resulting network to combine applications in a way that saves cost, increases speed and improves efficiencies. The resulting flexibility also allows the system to keep up with changing automation needs and future data rate increases that office IT staff will be keen to adopt.

Cable and Connector Selection

CAT5 cables and connectors are fine at the lower Ethernet data rates. CAT6 hardware becomes a necessity for GB Ethernet. At GB Ethernet speeds, you can't view a cable/connector interface as a lumped-element, but must treat it as a transmission line. More specifically, the effects of cable and connector capacitance can degrade the system's signal performance. Excess capacitive loading can cause an impedance mismatch on the transmission line, signal reflections and distortion. Using cable and connectors that have minimal line-to-line and line-to-ground capacitance will help ensure an acceptable amount of signal distortion.

For example, Harting's line of RJ Industrial cables and connectors are suitable for 10GB Ethernet, and have features that facilitate the convergence of office IT and factory automation systems. These designs evolved from the company's work on 4-pole CAT5 applications several years ago. Through the use of patented IDC (insulation displacement connector) technologies, the company's line of RJ45 and M12 connectors allow for tool-less field assembly.

It's important to recognize that not all 8-wire cable is suitable for GB Ethernet. The jacket material, core structure, agency ratings, shielding, environmental ratings, etc. are all important considerations when selecting the correct cable for the application. Many 8-wire Ethernet cables are only rated for CAT5 applications, and may or may not have the ratings and specifications you need.

These considerations are particularly important when fabricating your own GB Ethernet cable assemblies. You don't want to invest in CAT6 connectors and put them on the wrong cable. Therefore, selection of Ethernet cables and connectors should be made at the same time to ensure compatibility. For example, if shielded cable is required, then you need to make sure the shielding continues through the connector, i.e., the connector termination has to be compatible with the cable.

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