Function Blocks - Programmable Logic controllers

The term function block diagram (FBD) is used for PLC programs described in terms of graphical blocks. It is described as a graphical language for depicting signal and data flows through blocks, which are reusable software elements. A function block is a program instruction unit that, when executed, yields one or more output values. Thus a block is represented in the manner shown in Figure with the function name written in the box.

The IEC 1113-3 standard for drawing such blocks is shown in Figure. A function block is depicted as a rectangular block with inputs entering from the left and outputs emerging from the right. The function block type name is shown in the block, such as AND, with the name of the function block in the system shown above it, for example Timer 1. Names off unction block inputs are shown within the block at the appropriate input and output points.

Cross-diagram connectors are used to indicate where graphical lines would be difficult to draw without cluttering up or complicating a diagram and show where an output at one point is used as an input at another. Figure shows an example of a function block diagram.

Function blocks can have standard functions, such as those of the logic gates, counters, or timers, or have functions defined by the user, such as a block to obtain an average value of inputs.

Logic Gates

Programs are often concerned with logic gates. Two forms of standard circuit symbols are used for logic gates, one originating in the United States and the other an international standard form (IEEE/ANSI) that uses a rectangle with the logic function written inside it.

The 1 in a box indicates that there is an output when the input is 1. The OR function is given by !1 because there is an output if an input is greater than or equal to 1. A negated input is represented by a small circle on the input, a negative output by a small circle on the output. Figure shows the symbols. In FBD diagrams the notation used in the IEEE/ANSI form is often encountered. Figure shows the effect of such functional blocks in PLC programs.

To illustrate the form of such a diagram and its relationship to a ladder diagram, Figure shows an OR gate. When either the A or B input is 1, there is an output.

Figure shows a ladder diagram and its function block equivalent in Siemens notation.The ¼ block is used to indicate an output from the system.Figure shows a ladder diagram involving the output with contacts acting as an input.

The function block diagram equivalent can be shown as a feedback loop.

Consider the development of a function block diagram and ladder diagram for an application in which a pump is required to be activated and pump liquid into a tank when the start switch is closed, the level of liquid in the tank is below the required level, and there is liquid in the reservoir from which it is to be pumped. What is required is an AND logic situation between the start switch input and a sensor input that is on when the liquid in the tank is below the required level. We might have a switch that is on until the liquid is at the required level. These two elements are then in an AND logic situation with a switch indicating that there is liquid in the reservoir. Suppose this switch gives an input when there is liquid. The function block diagram and the equivalent ladder diagram are then of the form shown in Figure.

Boolean Algebra

Ladder programs can be derived from Boolean expressions since we are concerned with a mathematical system of logic. In Boolean algebra there are just two digits, 0 and 1. When we have an AND operation for inputs A and B, we can write:

where Q is the output. Thus Q is equal to 1 only when A ¼ 1 and B ¼ 1. The OR operation for inputs A and B is written as:

Thus Q is equal to 1 when A ¼ 1 or B ¼ 1. The NOT operation for an input 1 A is written as:

As an illustration of how we can relate Boolean expressions with ladder diagrams, consider the expression:

This tells us that we have A or the term B and C giving the output Q. Figure shows the ladder and functional block diagrams. Written in terms of Mitsubishi notation, the preceding expression might be:

In Siemens notation it might be:

As a further illustration, consider the Boolean expression

Figure shows the ladder and functional block diagrams. Written in terms of Mitsubishi notation, the expression might be:and in Siemens notation:

Consider the XOR gate and its assembly from NOT, AND, and OR gates, as shown in Figure.

The input to the bottom AND gate is:

and so its output is:

The input to the T top AND gate is:

The ladder diagram to represent this idea is shown in Figure.