Is Ethernet about to take off?
Original article date: January 1999
Do you know your sensorbus from your devicebus from your fieldbus? DICK CARO of Automation Research Corporation looks at what’s heppening at the lower end of networking.
There are three major driving forces at work in the industrial automation market: 1) the need for openness in the design of systems to avoid dependence on a single source of supply 2 the spread of intelligence downward in the automation hierarchy toward the field device itself and 3) the need to reduce cost. Field networks are a response to these driving forces. Many users from highly automated industries are moving away from proprietary systems at the control and field device levels using standardized field networks thereby eliminating the so called “islands of automation.”
The delays in production of field network standards have resulted in the availability of a variety of alternatives in choosing a field network protocol. As the capabilities of different protocols continue to overlap the scenario becomes increasingly confusing for the industrial user. Unfortunately for the user it is apparent that many different field network protocols will exist within industrial plant automation systems for the foreseeable future.
Field networks exist to connect sensors and actuators to a PLC DCS or other host controller replacing conventional I/O racks. In addition field networks are also used to connect one or more remote I/O or block I/O multiplexers to controllers when these elements are located in the field or plant floor.
While all field networks operate at the base level of the automation hierarchy differing functional requirements across applications and device types has led to the rise of three distinct classes of field networks: sensorbus devicebus and fieldbus. Additionally ISO 8802¡3 standard Ethernet is becoming popular for connecting remote I/O to controllers. Figure 1 illustrates the application of field networks to the levels of the hierarchy and the split between discrete factory automation and process control.
Sensorbuses are highly optimized to transmit very short amounts of binary data between sensors actuators and controllers where the data transfer is only in one direction for each device. The primary incentive to use sensorbuses is to decrease the cost of initial installation through multidrop (daisy-chain) wiring and addressable devices. The primary savings comes from elimination of the wiring from the device to the PLC or remote I/O rack and from the need to “ring-out” the wire pair. ASi-Bus Seriplex and WorldFIP-I/O are three leading sensorbuses.
Devicebuses were also created to reduce the cost of installation through multidrop wiring and addressable devices. However they are designed for bi-directional communications and have a longer message length. These characteristics enable the use of intelligence in the field device. Although practically no intelligent (often called “smart”) field devices for factory automation have yet emerged it will certainly happen. While LonWorks is the leading devicebus in worldwide sales its use has been concentrated on building automation and the less demanding water and waste treatment markets. Interbus-S Profibus-DP DeviceNet and SDS are the most popular devicebuses for factory automation.
The process industries have the same demands as factory automation but have been using smart field devices for over 10 years. The leading fieldbus to connect smart process sensors and actuators to controllers is HART. However Foundation Fieldbus and Profibus-PA standards have now emerged which will eventually replace HART.
While the Ethernet portion of the market today is very small it frequently represents the connection of many I/O points when used to connect remote I/O to PLCs and to other controllers. It often replaces a devicebus or more often a proprietary bus. This reflects the movement of the market to intelligent field devices more suitable to the use of devicebus Ethernet and fieldbus. Additionally with the low cost of Ethernet there is continuing interest in pushing it down to the sensor and actuator itself. With some adjustments to the Ethernet standard ARC expect it to play a major role in all segments of the industrial automation market over the next five to seven years.
January 1999