Connectors tie, link, and hold 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.
This article is an expanded version of the Back to Basics in the November 2002 Control Engineering. Click here to return to the original article .
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.
Selecting and implementing the most appropriate connectors depends on addressing physical requirements, application needs, layout and topology, and even philosophies governing different networking protocols. All connectors must satisfy the basic physical layer and cabling needs of their networks, but these different philosophies have resulted in different specific requirements for them.
For example, DeviceNet and the Open DeviceNet Vendors Assocation (Boca Raton, FL) define the thickness of the gold plating on the contacts on its connectors; Profibus-DP and the Profibus Trade Organization (Scottsdale, AZ) recognize many connectors, but not some others; and FOUNDATION fieldbus and the Fieldbus Foundation (Austin, TX) rests on the ISA SP-50.03 specification, which defines physical layer attenuation impedance in its cabling, but doesn't specify its connectors.
Because some protocol organizations define connectors and others don't mention them, users are often forced to seek out de facto market standards. So, it's usually still up to individual users to determine the connectors their network needs (see sidebar). For example, the biggest type of connector used in semiconductor manufacturing is the 7/8-in mini, which is considered small by users in the process field. Process applications often use M12 micro connectors on DeviceNet, Profibus, AS-i and SDS networks.
Evolving industrial technologies and trends also influence network cabling and connector requirements. For instance, the process field has long relied on screw terminations, and hasn't traditionally used many network connectors found in other applications. However, many process users are starting to use 7/8-in. mini connectors at 9 amps at with three, four or five pins, as well as M12 micro connectors with 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. Ironically, users are finding that newer connecting methods aren't just physically quicker to install, but they're also justified by the reduced time needed to schedule electricians or other technicians needed to install them. Meanwhile, in discrete industries and applications, M12 connectors are traditional because of the strict requirements of I/O points and devices. Mini connectors are used to supply power, which has been fueled by recent growth in PLC and related networks.
The reason users need to be careful their connectors have the right thickness of gold plating on their contacts is because networks need low contact resistance when going through each connection. You generally want that contact resistance to be less then 5 milliohms (mV) over the life of those contacts. This is why some traditional connectors don't work as well.
Low contact resistance is so important because, if you're comparing point-to-point on a DeviceNet network with 64 nodes, then your last device's signal could potentially have to 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 deal because industrial networks don't tend to draw much power. For example, FOUNDATION fieldbus typically draws about 20 milliamps (mA), while a mini connector can handle 9 amps. Likewise, DeviceNet is designed to handle 4 amps. It's also crucial to implement quality connectors because they're also subject to the same heat, vibration, abrasives and corrosives as the rest of the network. Good quality connectors mean their networks won't experience the drifts in voltage that occur in systems with connections that worsen more quickly over time.
Beyond these basic requirements, choosing connectors is also a matter of individual applications needs and personal taste. Technicians installing a network overhead with gloves will want bigger connectors they hold onto, while a system in a small panel will want smaller connectors. With the emergence of Ethernet, RJ-45 connectors have grown more prominent in industrial networks, and efforts have been made to help them survive and serve in plant-floor applications. RJ-45 can be industrially hardened, but its 1-in diameter and straight connections are still more subject to vibration and other forces in manufacturing applications. M12 is 0.5-in in diameter and has better contacts.
Because most engineers and other users like to have an accessible Plan B for handling unexpected network requirements, they often try to leave a couple of ports available when thinking through their network's connector schemes. These extra ports can accommodate adding an unanticipated device, or help provide a secondary node or input on the bus for an especially sensitive process. Planning up front can make many tasks easier down the road. Users often need to replace connectors in about the same time it takes for a coffee break.
The best advice is to choose connectors that serve your application's basic needs, but will also be able to handle previously unforeseen requirements.
-Ann Feitel, senior product manager for physical media, InterlinkBT (Minneapolis, MN),
and Jim Montague, news editor, email@example.com
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