Control system technology is migrating toward an open concept where networks accommodate factory floor devices from different manufacturers, and control intelligence is distributed among these devices. Two new open networks — the Smart Distributed System (SDS) from Honeywell’s Micro Switch Div. and DeviceNet from Allen-Bradley Co. — offer a giant step forward in this move toward distributed machine control.

From a broad technological standpoint, these communication networks or buses share several common features. They:

• Communicate over a single cable into which all devices connect, Figure 1. Plug-in connections reduce wiring and make it easier to expand or modify the system.

• Let factory floor devices communicate with each other even though they come from various manufacturers.

• Encourage other vendors to develop devices that are compatible with the network, such as intelligent sensors and actuators.

• Are both based on a communication technology called Controller Area Network (CAN).

The two companies took different paths, however, in developing these networks. Honeywell launched a threephase program about 21/2 years ago. In Phase I, they established a network concept for connecting sensors, limit switches, and actuators. This was followed by development of PLC interfaces, sensors with CAN microprocessor chips to provide communication capabilities, and interfaces for sensors and other devices without CAN chips. Then, they initiated Beta sites to field test and refine the system.

Phase II, started within the past year, added smart sensors and actuators with advanced functions and diagnostic capabilities. Phase III, scheduled for next year, will expand system control and diagnostics. Throughout this program, Honeywell developed most of the hardware in-house, though partner companies also participated in the development.

Allen-Bradley, on the other hand, began about 1½ years ago to develop a network that links a wide variety of factory floor devices — including drives and starters as well as sensors and actuators — with each other and with PCs and PLCs. The company is developing some compatible sensors and drives in-house. But, their major thrust is to encourage other vendors to develop compatible devices within their respective areas of expertise. Numerous vendors have started this development effort.

Both of these systems have the potential to become a de facto device network standard. End users need to carefully evaluate each system and determine which is best for their applications. In the meantime, this overview will help you understand how the two systems work.

SMART DISTRIBUTED SYSTEM (SDS)

The SDS network, or bus, lets intelligent factory floor devices communicate with each other, and with PLCs or PCs, regardless of who manufactured the devices. Up to 64 sensors and other devices are linked over a single cable that carries both data and control power.

Unlike conventional 2-wire systems, this digital network provides system and device diagnostics through CAN chips embedded in PCs, PLCs, and other control components. Diagnostic capabilities cut installation and repair time, and aid troubleshooting.

Honeywell selected CAN because of its cost, robustness, and speed. “CAN is a proven communications technology” says Jeff Beal, SDS Marketing Manager. “Other systems are too slow for discrete manufacturing, where real time is measured in msec.”

To date, Honeywell has developed nearly all of the intelligent devices that communicate over the network. But, the system allows other vendors to develop and manufacture compatible devices, a feature that will expand the end user’s choice of I/O devices and controllers (PCs and PLCs). Accordingly, partner companies are developing devices such as valves, motor starters, and analog I/O.

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