The figure below shows a generalized architecture of disturbed control system.
The explanation of each block is given below:
Local control unit (LCU)
The smallest collection of hardware in the system that can do closed – loop control. The LCU interfaces directly to the process.
Low level human interface (LLHI)
A device that allows the operator or instrument engineer to interact with the local control unit (e.g to change set points, control nodes, control configuration or tunning parameters) using a direct connection. LLHI’s can also interface directly to the process. Operator oriented hardware at this level is called low level operator interface, instrument engineer – orientated hardware is called a low level operator interface, instrument engineering interface.
Data Input / output unit (DI/OU)
A device that interfaces to the process solely for the purpose of acquiring or outputting data. It performs no control function.
High level human interface (HLHI)
A collection of hardware that perform functions similar to the LLHI but with increased capability and user friendliness. It interfaces to other devices only over the shared communication fatuities. Operator – oriented hardware at this level is called high level operator interface, instrument engineer oriented hardware is called high level engineering interface.
High level computing device (HLCD)
A collection of micro processor based hardware that performs plant management functions traditionally performed by a plant computer. It interfaces to other devices only other the shared communication facilities.
Computer interface devices (CID)
A collection of hardware that allows an external generate purpose computer to interact with other devices in the distributed control system using the shared communication facilities.
Shared communication facilities
One or more levels of communication hardware and association software that allows the sharing of data among all devices in the distributed system should communicate channels between specific devices as between hardware elements within a device.