PLC PROGRAM SCAN

During each operating cycle, the processor reads all inputs, takes these values, and energizes or de-energizes the outputs according to the user program.  This process is known as a scan.  Because the inputs can change at any time, the PLC must carry on this process continuously.

1.     I/O scan – records status data of input devices.  Energizes output devices that have their associated status bits set to ON (1)                 

2.     Program scan – instruction are executed sequentially.

PLC OUTPUT TABLE FILE OPERATION

Processor continually activates or deactivates ouput status according to output image table file status

PLC INPUT TABLE FILE OPERATION:

Processor continually reads current input status and updates input image table file.

INPUT TABLE FILE OPERATION:

PLC PROCESSOR MEMORY ORGANIZATION

PLC PROCESSOR MEMORY ORGANIZATION

The memory of a PLC is organized by types.

The memory space can be divided into two broad categories:

Program and Data Memory:

Advanced ladder logic functins allow controllers to perform calculatins, make decisions and do other complex tasks. Timers and counters are examples of ladder logic functions. They are more comples than basic inputs contacts and output coils and relay heavily upon data stored in the memory of the PLC.

The user program will account for most of the memory of a PLC system.

Program files contain the logic controlling machine operation.

This logic consistes of instructions that are programmed in a ladder logic format.

DATA FILES:

The data file protion of memory stores input and output status, processor status, the status of various bits and numerical data.

PLC Architecture

What is a PLC?

PLC is a device, which is used to control a machine or process as per the human control sequence. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine.

Advantages

• Smaller physical size than hard-wire solutions

• Easier and faster to make changes

• PLCs have integrated diagnostics and override functions

• Diagnostics are centrally available

• Applications can be immediately documented

• Applications can be duplicated faster and less expensively

PLC HARDWIRED CONTROL

Prior to PLCs, many of these control tasks were solved with contactor or relay controls. This is often referred to as hard-wired control. Circuit diagrams had to be designed, electrical components specified and installed, and wiring lists created. Electricians would then wire the components necessary to perform a specific task. If an error was made the wires had to be reconnected correctly. A change in function or system expansion required extensive component changes and rewiring.

DRAWBACKS:

Bulky and complex wiring.

Difficult to change the logic.

Unreliable.

RELAY :

RELAY LOGIC – AND GATE:

ELECTRONICS CONTROL

Logic gates and Electronic Circuits are used

Ease of programming

Ease of maintenance

Drawbacks:

Difficult to Troubleshoot while Change the Logic of the Process

Difficult to expand

Not suitable for industrial conditions