PLC operations

Now we are discussing about PLC configuration, its internal major parts and working. After this we will start Software practice.

Let us discuss about configuration. In PLC we have three major parts as below…

1. Input circuit
2. CPU (With memory)
3. Output circuit

Input circuits are those from which any programmer gives input command for controlled output result. As we discussed its wiring in previous blog, we use it different types of inputs. In popularity we roughly listed few as below 

• 12-24 Vdc
• 100-120 Vac
• 10-60 Vdc
• 12-24 Vac/dc
• 5 Vdc (TTL)……We study about TTL below
• 200-240 Vac
• 48 Vdc
• 24 Vac

PLC input cards rarely supply power, this means that an external power supply is needed to supply power for the inputs and sensors. We have wiring concept of it in previous blog.

Inputs concept from diagram

Here is example to show you how to write on software as we have simple circuit.

There are different names of inputs as according to different PLC’s  company.

SW indicates switch and Coil as load.

How plc works (CPU OPERATION)

When we complete our wiring and write program in software then it run with few steps as shown in figure. 

At this PLC check the input status applied by programmer then it downloads program inside its memory. After this we run program, execute it.

As program executed and it is correct then it update output status of the program.

In this program we have NO switch (0000) and NC switches (0001) where as coil (0500). NO means normally open switch it is open normally but it can be turned on by programmer. NC means normally closed switch which is closed normally but can be opened as requirement by programmer.

We have some pictures that show how plc works to execute program and also the interrupts. To run two different programs, there is some time required for scans as shown in figures.

                                                

                                                 

This is the signal diagrammatic operation occurs in PLC.

                                                           

                                                      Fig: Response time 

The total response time is time taken by program to give outputs from applied inputs.

 So

Total response time= Input response time+program execution time+Output response time

OUTPUTS

There is two major types of outputs that is

1. Transistor output

 This is presentation of NPN transistor output. For PNP + terminal is connected with common and other process is similar in both. PLC has internal circuits like CPU, REGISTERS ETC. which is connected with outputs. Photo coupler is the major part for its process. We should remind one thing that transistor operates with DC supply so never go for AC wiring.

2. Relay output

Relay is mechanical switch. We have many relays in plc for I/Os which operates as according to the programmer. In the execution of program all the relays, sensors and other parts are checked for updating outputs.

                                                                   Fig: Relay output

The above figure is description of PLC with Relay output control. AC is connected with common terminal which helps to operate on loads by programmer. Below is a kind of relay available in market.

                                                             Fig: Simple Relay

Examples

                                                 

                                                                        Fig: I/Os

This is simple circuit for a process. Two switches are taken to handle simple coil.

When First switch (SW1) is ON (True) and Second switch (SW2) is ON (True) then coil gets supply. Any of these switches is OFF (False) then coil does not operate.

We have timers and counters which delays our operation and also to work in automatic systematic process by priorities by programmer.

Below we have some programs which helps you to understand the process better way.

 0000, 0001, 1000 are the input address for the program where as 0500 and 1000 are outputs address of program. We can change the address as programmer requirement.

                                                                 Fig: plc scanning

Guys till now we studied general theories for PLC operation. From tomorrow we will go for Software practice.

Have fun. thanks for reading.