Distributed control systems are known. A typical distributed control system includes a host controller and a plurality of isolated controllers which communicate with the host controller through a local area network. The isolated controllers may include controllers embedded in machinery. Each of the isolated controllers and/or embedded controllers has the capability of relaying information gathered from sensing devices and/or actuating devices to the host controller which usually acts as an operator interface. The information may also be relayed to a monitoring system which can act as a redundant low-level operator interface.
Different local area networks have been employed in industrial environments, i.e., surroundings with above normal electrical noise and relatively harsh physical conditions that can significantly degrade the performance of local area networks. Examples of different versions of industrial local area networks are disclosed in U.S. Pat. Nos. 4,517,637; 4,888,726; and 4,901,218.
In some known distributed control systems which use industrial local area networks, the "instantaneous" network response time may range from a few seconds to several minutes because of software overhead associated with the host controller and/or isolated controllers of the particular distributed control system. A problem associated with these known distributed control systems is that the relatively long network response times are unacceptable in real time control applications requiring response times measured in milliseconds. Another problem associated with some known distributed control systems is their lack of modularity which would provide flexibility and variety in use.
U.S. Pat. No. 4,517,637 discloses a single control processor which handles both communications and control functions. The design of the local area network disclosed in U.S. Pat. No. 4,517,637 lacks modularity as well as distributed intelligence in network handling. Also, the cost and time to install the local area network are relatively high since two shielded twisted pairs of wire cable are used for a communication medium.
U.S. Pat. No. 4,888,726 discloses a controller including a controller section and a communications section which shares memory with the controller section. The design of the local area network disclosed in U.S. Pat. No. 4,888,726 provides distributed intelligence but lacks modularity since the design cannot be applied to a control arrangement in which memory resources cannot be shared.
U.S. Pat. No. 4,901,218 discloses a communications adaptor which provides distributed intelligence and modularity. A controller is operatively connected to a machine which is controlled by the controller. The controller has a processor and is connected through a serial link to the communications adaptor which, in turn, is connected to a local area network. The communications adaptor is modular since it has its own processor which is separate from the processor of the controller. The processor of the controller provides the control functions to control the machine operatively connected to the controller. The processor of the communications adaptor provides the communications functions between the controller and the local area network. Although the distributed control system disclosed in U.S. Pat. No. 4,901,218 has distributed intelligence and modularity, the network response time is significantly degraded because of the relatively slower transmission rates through the serial link between the controller and the communications adaptor. The transmission rates through this serial link is much slower than transmission rates through the industrial local area network. Moreover, the industrial local area network disclosed in U.S. Pat. No. 4,901,218 requires a network manager, thereby further increasing the cost to implement the industrial local area network.