How CPU Execute Program in Siemens PLC?

In this article, we will learn about the execution of the program in the PLC.

Many people want to learn PLC programming directly without knowing what happens around it.

Before jumping into the programming, you must learn the basics of the PLC, how the program works, the flow of the program, and during execution how it uses memory, etc.

Here we are going to learn about how CPU executes a program in the Siemens PLC, although execution will remain the same in every PLC, in Siemens PLC it will take a little extra for an execution.

Normally in every PLC execution takes place like reading the Input, evaluate the logic and writing the output,

However, in the Siemens PLC, it reads the inputs from the input module then values are saved to PII (Process Image Input).

PII is a small memory part that resides in the CPU to store the status of the inputs coming from the input module.

Now you may have a question that why to save the value to PII? It’s because by saving input to PII it reduces the amount of the scan cycle. Which is helpful in a case where you have a large number of I/Os are available.

So, every time instead of looking back to the input module, CPU only looks at PII to make the execution of the program fast enough.

After PII, CPU then starts to process the program. It checks the status of the Inputs and based on their condition (ON/OFF) it processes the logic.

First, it goes for the first network and sees whether the input is active or not, if not then it moves to the next network and checks the input status. This process will last until the end of the program.

So, in the processing of the program flow goes from the top to bottom and left to right.

Now, for example in the processing part CPU sees that some of the inputs are true, what it does that it saves that output to the PIQ (Process Image Output).

After saving the output to the PIQ, it then goes to the output module to energize output resides in the field.

After this process CPU goes to the first step and repeats the same step again n again.

This is how PLC executes the logic in Siemens PLC.

Now, you might have one more question that is this works for both the analog and digital signal? The answer is NO.

Only for the digital inputs and outputs CPU looks at PII and PIQ, but in case of the analog signal, it takes a value directly from the module.

As the analog signal takes a value from the module itself, to recognize the analog inputs and outputs it uses P before writing any addresses like PIW100, PQW100, etc.

Still, you have a question? Then ask me through the below comment section.

Comments

PLC Program for Mixing Tank

Create a ladder diagram for controlling a batch mixing process. Implement a PLC program for mixing tank or Mixing Process using PLC Ladder Logic. PLC Program for Mixing Tank Fig : Mixing tank A tank is used to mix two liquids. The required control circuit operates as follows: A. When the START button is pressed, solenoids A and B energize. This permits the two liquids to begin ﬁlling the tank. B. When the tank is ﬁlled, the ﬂoat switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together. C. The motor is permitted to run for 1 minute. After 1 minute has elapsed, the motor turns off and solenoid C energizes to drain the tank. D. When the tank is empty, the ﬂoat switch de- energizes solenoid C. E. A STOP button can be used to stop the process at any point. F. If the motor becomes overloaded, the action of the entire circuit will stop. G. Once the circuit has been energized, it will continue to operate until it is manually stopped. Solution...

What is Relay? How it Works? Types, Applications, Testing

We use relays for a wide range of applications such as home automation, cars and bikes (automobiles), industrial applications, DIY Projects, test and measurement equipment, and many more. But what is Relay? How a Relay Works? What are the Applications of Relays? Let us explore more about relays in this guide. What is a Relay? A Relay is a simple electromechanical switch. While we use normal switches to close or open a circuit manually, a Relay is also a switch that connects or disconnects two circuits. But instead of a manual operation, a relay uses an electrical signal to control an electromagnet, which in turn connects or disconnects another circuit. Relays can be of different types like electromechanical, solid state. Electromechanical relays are frequently used. Let us see the internal parts of this relay before knowing about it working. Although many different types of relay were present, their working is same. Every electromechanical relay consists of an consists of an Elect...

Chlorine dioxide Analyzer Principle

Chlorine dioxide measurement Chlorine dioxide (ClO2) is an instable, non-storable, toxic gas with a characteristic scent. The molecule consists of one chlorine atom and two oxygen atoms – represented in the chemical formula ClO2. It is very reactive. To avoid the risk of spontaneous explosions of gaseous chlorine dioxide or concentrated solutions, it is generally handled in dilution with low concentrations. ClO2 is soluble in water, but tends to evaporate quickly. Typically it is prepared on site, for example from hydrochloric acid and sodium chlorite. The procedure provides solutions with approx. 2 g/l ClO2 that can be safely handled and stored for several days. Image Credits : krohne Sensor Parts : Reference electrode Applied chlorine dioxide specific potential Current needed to maintain the constant potential Counter electrode Measuring electrode The disinfection effect of ClO2 is due to the transfer of oxygen instead of chlorine, so that no chlorinated byproducts are formed. C...

TILI Automation

Search This Blog