Bit-level Buses Compete on Speed, Connectivity

Down in the lower levels of the seven-layer International Organization for Standardization’s (www.iso.ch) Open System Interconnection Reference Model, two bit-level buses compete to control devices such as actuators and sensors: the Actuator Sensor Interface (AS-I) and CompoNet.

While the AS-International Association’s (www.as-interface.net) AS-I appeared about 15 years ago, CompoNet, from the Open DeviceNet Vendors Association (ODVA, www.odva.org), arrived in October 2006. Both offer high-speed communication. Both also offer some level of safety features and/or connectivity. The AS-I Safety At Work system is one example.

Both open networks use the master/slave communications scheme. With vendor-topology-independent AS-I, “that means a component—in our case, either a scanner or gateway—is the originator [master] of all conversations,” says Helge Hornis, manager of intelligent systems for Pepperl+Fuchs Inc. (www.pepperl-fuchs.com), a Twinsburg, Ohio, automation components supplier. The master calls or polls a slave, which is the input/output (I/O) node, and then sends the output, or the message; the slave must then reply.

Hornis says end-users have a larger number of smaller nodes in these systems. That means less cabling, thus less capital expense and fewer potential errors. He notes the system uses AS-I’s yellow flat cable, which carries both power and data, rather than traditional two-wire cabling.

To construct a system, technicians lay out the cable along the path of the I/O and then connect each one with AS-I’s snap-and-go piercing-technology connectors. Next, a scanner card or gateway is installed into a controller, and the network is plugged in and configured. “Once you set up the network, you can simply connect to the cabling anywhere along the cable,” Hornis explains.

At the network’s upper limit of 62 slaves, Hornis says, “you end up with update time for the entire system of 10 milliseconds.” That update time applies even if there are four inputs at each I/O node or slave, he adds. “But if we then double the number of inputs per module [to eight per node], then the update time goes up to 20 milliseconds.”

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Managed and licensed by ODVA, but created by Omron (www.omron.com), the Japan-based automation products vendor, CompoNet offers more speed than AS-I. It has the capability of 1-millisecond messaging in a 1,024-I/O-point system, asserts Jeff Jurs, manager of business development for Omron Electronics LLC, in Schaumburg, Ill. Designed to handle a 4-megabit-per-second (Mbps) backbone network, the bus also allows three other baud rates: 3 Mbps, 1.5 Mbps and 93.75 kilobits per second, or Kbps. “The differences allow maximum flexibility in topology,” Jurs comments.

At the upper end of the speed range, end-users may use a cable of 30 meters into which nodes can be connected. But at the low end of the range, cabling can be much longer, and “there are almost no rules on how you can configure the network,” says Jurs. For example, for materials handling, a low-end application, “if you need additional sensors, you can tap into the net using clamp-on nodes. You’re free to put on drops wherever you’d like,” Jurs remarks.

In point-to-point connections with CompoNet, the net works with a simple, low-cost, two-wire unshielded cable, he adds. “But you can also buy a four-wire industrial cable, a ribbon cable.” And repeaters used with the net “are actually nodes with some intelligence added to them,” Jurs adds. “The first segment is local, where the master is. But you can go up through two repeaters to give three segments.”

Because communications in an enterprise extend beyond sensors and actuators, one important feature both bit-level buses possess is connectivity to industrial networks. For example, CompoNet can access ODVA’s Common Industrial Protocol, or CIP, Network Library. That includes, among others, EtherNet/IP, DeviceNet and ControlNet. AS-I is compatible with ProfiBus, EtherNet/IP and industrial Ethernet, for example.

C. Kenna Amos, ckamosjr@earthlink.net, is an Automation World Contributing Editor.
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