Yokogawa has implemented in the CENTUM CS 3000 R3 a new, two-layer I/O architecture that integrates field networks into a distributed control system with general analog and digital inputs/outputs. This new architecture consists of two types of nodes: one directly connected to a controller CPU and capable of communicating and updating a large amount of data at a high speed, and the other connected to a controller CPU via a communication bus and placed near field devices. These two types of nodes allow flexible system configuration according to the applications while assuring a sufficiently short data updating period and reducing of wiring costs.
The recent increase of devices enabled for a field network such as the FOUNDATION™ Fieldbus is phenomenal. This has been amplifying a requirement for distributed control systems (DCSs) to be capable of constituting a more pliant and efficient production control system by means of flexible connections to field networks in addition to traditional 4 to 20 mA signals1. In response to this, Yokogawa has developed a new I/O architecture that allows existing I/O devices and field networks to be flexibly integrated under a CENTUM CS 3000 R3. This paper describes the requirements for integrating field networks into a CENTUM CS 3000 R3, and then introduces an example.
Figure 1 I/O System for CENTUM CS 3000 R3
An I/O system for a DCS is usually installed in the cabinet of a field control station (FCS) that carries out process control functions2. Figure 1 illustrates the positioning of an FCS in a CENTUM CS 3000 R3 system and outlines the I/O system.
The requirements for FCS's I/O system to integrate field networks and the existing I/O include:
Field Network Integration into DCS
Acquiring data via communication generally requires the use of a communication protocol, and integrating a field network is no exception. In particular, when connecting a field network to a DCS, it needs to be taken into account that the execution of a complex protocol may not largely affect the usual inputs or outputs, or control actions. Further, in the case of recently used, high-speed field network such as Profibus-DP is used, the DCS needs to be able to acquire a large amount of data within a short time for effective use of its merits. Hence, when separating a module that performs communications and a module that carries out controls, a sufficiently wide band should be provided for data exchange between those modules.
For both the inputs/outputs via field networks and usual analog and digital inputs/outputs, the I/O data needs to be updated at periods suitable for the respective applications. Namely, the data of those inputs and outputs for which high-speed control is required need to be updated at short periods, while, for those inputs whose data need not be updated at short periods but that are many in number, such as monitoring points, an update period that may not influence the execution of controls is sufficient. It is typical that these different types of inputs/outputs exist for a control object and thus each controller of a DCS is required to perform I/O data updates at different periods without affecting each other. Of course, also when connecting field networks, the update periods of the other data must be prevented from being influenced to the utmost.
Reduction of Field Wiring Costs
It is well known that the costs for field wiring are not a small part of the overall costs when employing a plant control system, and thus when considering the I/O system for a DCS, the deliberate reduction of field wiring costs is an inevitable aim.
Where diverse field networks and traditional analog and digital inputs/outputs co-exist, the I/O system for a DCS is required to have the capability of accommodating flexible field wiring schemes in accordance with the physical plots of plant equipment and plant characteristics.
Integration with CENTUM CS 3000 R3
Figure 2 Two-layer I/O System
The CENTUM CS 3000 R3 meets the aforesaid three requirements by adopting the I/O system architecture in which, in addition to I/O modules directly connected to a controller, remotely installed I/O modules can be connected to the same controller as illustrated in Figure 2. The following details this architecture.
Two-layer I/O System
To make the CENTUM CS 3000 R3 meet the aforementioned requirements, the two-layer I/O system was developed. In particular:
To facilitate integration of field networks, the following are also achieved in this architecture:
Next, the features and benefits of the new architecture are explained in comparison to the conventional architecture.
Reduction of the wiring costs by remote bus had already been realized before this new I/O architecture; however, connecting to a high-speed, large-capacity field network has encountered the problem that the communication band of the remote bus meant that the field network's merits could not be used optimally. Conversely, using only nodes connected directly to a CPU inevitably lengthened the field wiring cables, resulting in increased wiring costs. Now, these problems have been cleared by our new I/O system architecture, since it uses the remote bus and direct connections in the same controller to allow the method suitable for each application to be chosen flexibly.
The actual benefits include the following:
Adoption of the architecture described in this paper has allowed us to provide an I/O system that meets all of the requirements listed at the beginning:
We believe that the new architecture can cope with ever- diversified field networks emerging in the near future since it accommodates flexible configurations such as a broad band secured for communication between a controller's CPU and a field network communication module.
Field networks will surely be further developed, and are ever- becoming more important in the field. We believe that our I/O system architecture is capable of meeting the various requirements of the times. From now on, we would like to continuously satisfy user needs by keeping our eyes on field network trends and developing new field network modules for our DCSs.