Almost all manufacturer makes the 5 level DCS system. The 5 levels are described below.
Level 0 is the field device level. All the field devices such as control valves, transducers, transmitters etc, are at this level.
Input devices (transmitters etc.) and output devices (control valves, etc.) are connected to input/output units (I/O units). I/O units convert the 4-20mA or digital signals to specially coded signals for the Fieldbus. And also converts the coded signal to 4-20mA.
This is the control level. The microcontroller takes data from the Fieldbus to control individual control loops. At a time more than one can be controlled. All control actions are to be done with this level.
The data on the Fieldbus contains all the information for each loop input and output. The CP, using only milliseconds of time, controls each in turn. The PID of each is separately programmed. To the operator, it looks as if all the loops are controlled at the same time.
This is the plant supervisory unit. All the information about the control loop is displayed in this unit using Video display unit (VDU).
Large control systems may have many workstations which display the distributed control units around the plant.
From this unit, an operator can adjust the setpoint or he can change from manual to automatic etc.
This is the Production control level. The application processors takes some values from the micro processor and send to Central Control Room (CCR).
Normally, you cannot change control operations from this level. It can only display information for management overview.
The Applications Processor (AP) takes some of the signals from the CP and puts them into a digital code (protocol) so that they can be sent over a higher level data highway.
This is the production sheduling or Group management level. The group management level. Some signals for the AP are converted so that they can be sent (by microwave link, satellite, etc.) to a distant headquarters
The workstation at headquarters can not make changes at plant level. However, the workstation displays up to date information on production operations for planning purposes.