Logical Function Blocks available in DCS

Generally in DCS Functional Block Diagram (FBD) is used for programming and Sequential Flow Charts (SFC) is preferred for batch control applications.

Functional Block Diagram (FBD)

This language basically is a graphical language wherein the users are allowed to program elements in the form of blocks. The blocks then can be wired together like electrical circuits. It describes a function between input and output variables. It is a graphical method of programming DCS. Functional algorithms and control strategies for a particular plant can be implemented by FBD. The control modules are treated as unique entity in DCS system. Each function block contains specific algorithm such as AI, AO, DI, DO, PID and the parameters required for the algorithm. Function blocks ranges from simple input conversion block to a complex control strategy.

Generally available Function blocks are

Input/ Output (I/O)
Math
Logical
Timer/ Counter
Analog Control
Advanced control

A Typical Function block

The description of the various parameters associated with this block:

Definition name

This is the name by which the block is identified in the library.

Usage name

This name can be user defined as per his/her convention. Parameters: These are part of the function block available for standard connection.

Execution number

The number by which the function block is executed.

Following functions blocks are generally available in the libraries.

Input Output library

Analog Input
Analog Output
Discrete Input
Discrete Output
Pulse Input

Math Library

Add
Comparator
Divide
Multiply
Subtract
Absolute value
Integrator

Logical Library

And
Or
Condition
Not
Negative Edge Trigger
Positive Edge Trigger
Multiplexer
De multiplexer

Timer / Counter Library

On Delay Timer
Off Delay Timer
Counter
Retentive Timer
Timed Pulse

Analog Control Library

Input Selector
Limit
PID
Ratio
Ramp
Filter
Scalar
Signal Characterizer
Signal Generator
Signal Selector
Splitter

Analog Input function Block

Analog Input (AI) Function Block accesses a single Analog measurement value and its status on the AI channel. In manual mode, when AI block is configured, input has to be in engineering units as well as output is indicated in engineering units on the screen. In automatic mode, output of the block will reflect process variable value. Input to the block could be 4-20mA or digitally communicated variable from HART based transmitter.

Analog Output function Block

Analog output (AO) function block assigns any output value to a field devices like control valve through a specific AO channel. In manual mode, the output parameter value can be set manually. In automatic mode, OUT is set automatically based on set point and process variable value in engineering units. A feedback can also be set for the output parameter.

Discrete (digital) Input (DI) Function Block

The Discrete Input (DI) Function blocks accesses a single digital input value with its status between two states of the field devices. The block gives processed physical input for other function blocks. In automatic mode, Process variable is copied to output which can be either 0 or 1. In manual mode, the digital input can be toggled manually and output follows the same.

Discrete (digital) Output (DO) Function Block

Digital Output block takes value from the process and writes it to specific digital output channel. One can confirm physical output operation by configuring a hardware discrete input to it. In automatic mode, when the signal from process changes the output of the block which will be connected to AO channel which in turn connects to field outputs e.g. a solenoid valve, changes.

Logical Library

AND Function Block

AND function block is used as an AND operator. For 2 input AND gate if both the inputs are high the output of the function block is high. The function block and the truth table is as given below

OR Function Block

OR function block is used as an OR operator. For 2 input OR gate if any one inputs is high, the output of the functions block is high. The function block and the truth table is as given below

NOT Function Block

NOT function block is used as NOT operator, wherein if the input to the block is high or active the output is opposite i.e. low and vice versa. The function block and the truth table are as given below