Connectors tie networks together
Network connectors have several tough jobs. They must make positive connections to transmit data; keep out unfriendly substances; withstand their environments for long periods, and often hold cables together under mechanical stress. And, as network data rates continue to increase from 9,600 bps to 12 Mbps and beyond, connectors are also being required to shield conducting pins and sleeves from ...
Network connectors have several tough jobs. They must make positive connections to transmit data; keep out unfriendly substances; withstand their environments for long periods, and often hold cables together under mechanical stress. And, as network data rates continue to increase from 9,600 bps to 12 Mbps and beyond, connectors are also being required to shield conducting pins and sleeves from electrical noise and interference.
To select and implement the most appropriate connectors, you must take into account physical requirements, application needs, layout/topology, and even different philosophies of each network protocol. Although all connectors must satisfy the basic physical layer and cabling needs, these different philosophies have resulted in different connector specifications.
Since some protocol specifications define connectors while others do not, users are often forced to seek out de facto industry standards. It's usually up to individual users to determine the appropriate connectors for their network (see sidebar). For example, typically the connector used in semiconductor manufacturing is the M8 pico, which is considered too small by users in the process industries. Process applications often use the 7/8-in. mini connectors for their FOUNDATION fieldbus, DeviceNet, Profibus, AS-interface and SDS networks installations.
Evolving industrial technologies and trends also influence network cabling and connector requirements. For instance, the process field has long relied on screw terminations, and has not traditionally used many network connectors found in other applications. However, many process users are starting to use 7/8-in. mini connectors rated at 9 amps, with three, four or five pins, as well as M12 micro connectors rated at 4 amps, with four, five or eight pins.
Secure plug-in and snap-on versions of many traditional threaded connectors, as well as adapters, are also increasingly prevalent in many applications. Meanwhile, in industrial factory applications, M12 micro connectors have become the 'tradition' because of the strict requirements for installation of I/O points and devices. The 7/8-in. mini connector is almost exclusively used to supply power, which has been fueled by the recent growth in PLC and related networks.
End-users need to take care that the connectors they are using have the right thickness of gold plating on their contacts. This is critical because networks need a low contact resistance when going through each connection. Generally, you want that contact resistance to be less then 5 milliohms (mV) over the life of those contacts. Most traditional connectors do not work as well.
Low contact resistance is extremely important. On a point-to-point DeviceNet network with 64 nodes, the signal on your last device could potentially go through 128 connections before returning. High contact resistance could spell real trouble for a bus system like this.
Current-carrying capacity is usually not a big concern since industrial networks generally do not draw much power. For example, FOUNDATION fieldbus typically draws about 20 milliamps (mA). Likewise, DeviceNet is designed to handle 4 amps. A mini connector can handle 9 amps.
It is equally crucial to use quality connectors because they are also subject to the same heat, vibration, abrasives and corrosives as the rest of the network. Good quality connectors mean your network installations will not experience the drifts in voltage that occur in systems with connections that decay more quickly over time.
Beyond these essential basic requirements, your choice of connectors is also a matter of your applications needs and individual preference. Technicians who are installing an overhead network using gloves will want large connectors for easier handling, while a small panel network system works better with smaller connectors.
With the emergence of Industrial Ethernet, RJ-45 connectors are being used, but much effort has been made to help them survive and serve well in plant-floor applications. Although the traditional RJ-45 connector can be industrially hardened, its 1-in. diameter and straight connections are still more subject to vibration and other forces in manufacturing applications. A better industrial alternative is the M12 micro connector, which has a 0.5-in. diameter and has better contacts.
Ann Feitel, senior product manager for physical media, InterlinkBT (Minneapolis, MN)
Basic Network Connector Methodologies
The majority of industrial buses use one or more of the following connectors or applicable variants:
9DB , which consist of nine-pin, subminiature, D-shell connectors. Daily use of approximately 250,000 connectors.
Micro (dc) or euro, which are two- through six-pin connectors based on M12 threads (approximately 14 mm outside diameter coupling unit). Daily use of approximately 150,000 connectors.
Mini , which include two- through seven-pin connectors based on a 7/8-in-diameter barrel with a 16 pitch. Daily use of approximately 50,000 connectors.