Ladder Logic | A programming Language

• All the outputs are aligned at right side Q1,Q2,Q3,Q4
• I1 & I2 inputs are aligned at left side.
• NO contacts of coil output Q1 & Q2 are also aligned left as an input to the coil outputs Q3 & Q4.
• Q1 output energizes when input I1(NO) get closed.
• Q2 output energizes directly because I2 is NC.
• Q3 output energizes when Q1 output is energized.
• Q4 output energizes when Q2 output is energized.

1. Circuit works from left to right & from up to down.
2. It has two rails ‘positive’ & ‘negative’. Left rail is positive while the right rail is negative.
3. Current flows from positive rail to negative rail.
4. Current do not flows through an open contact, It only flows when contact is closed.
5. The output defines the logic Not input !

In Ladder language, There are two contacts NO & NC

1. NO Stands for ‘Normally Open’
2. NC Stands for ‘Normally Closed’

1. NO represent a switch which is OPEN at default state or at initial state. But when this switch get pressed, The open NO contact get closed. After that if we release that switch, It comes back to its initial open state. A bell switch is the example of NO contact.
2. NC represent a switch which is CLOSED or connected at default initial state. When this switch get pressed, the closed NC contact gets opened. Further if we release that pressed switch; it regain its initial closed state. A refrigerator’s door limit switch which is operating the refrigerator’s light is the example of NC contact Which makes the circuit closed when refrigerator’s door get opened.

A  ladder diagram representing the DOL starter is been shown above. All the components are listed below :

• Inputs- I1, I2
• Output- Q1
• Output’s contact- Q1(NO)

Wiring for DOL across the PLC is represented below :

• 24V DC power supply is connected across the PLC.
• 2 inputs are used as Start & Stop respectively.
• 1 contactor coil is connected across the output.

 

In an image–3, The first contact is I1 which is NO & type of input. When input I1 get present than NO contact I1 get closed & current starts flowing & reaches to Q1 output after crossing the NC contact I2. Which results output coil get energized which further energizes contactor.

When output get energized, at that same time NO contact of coil’s output (Q1) get closed. At this condition, Output remains energized even if we remove the input I1 because a Latch circuit is been created through a NO contact of output. The output get deenergized when I2 input get power supply & I2 NC get opened Which results hold circuit get break.

NO & NC contacts, output coils are the basic components of ladder logic which are explained through an example above.

Special components includes timers & counters.  For understanding of timer & counter, We are taking an example of DOL Starter again !

The idea is, Starting of motor after some delay. Starter remains the same.

Complete wiring remains the same, Only a timer & Its NO contact is being connected in the circuit.

Trigger signal to the timer is connected through I1(NO ) input & timer’s NO contact is connected in between I1(NO ) & I2(NC). (shown in an image–5)

When input push button I1 get pressed, Delay timer get a input pulse than timer starts counting. After stored time value, timer produces a pulse output. Then timer’s NO contact get closed which results Coil output Q1 get energized. The output remains energized because of latch circuit through Q1(NO ) contact. The output get deenergized when I2 input is being pressed.

To understand the working, A counter is added in DOL starter. The idea is, output energizes & get hold  until input I1 get pressed multiple times.(logic shown in an image–6)

I1 input produces a signal to the counter. After complete counter’s counting; Counter produces an output. Here counter’s value is 2; When Input I1 signaled 2 times than counter produces an output. After complete counter’s counting, NO contact of counter get closed & than output coil get energized & remains energized until I2 input get a signal As The counter get reset through I2 input.

• Ladder logic is a type of programming language which is used to create a functional program(logic) in programmable logic controller (PLC).
• This language is being formed in between the two rails of positive & negative supply seems like a ladder that’s why known as ladder logic. Inputs & Outputs are being represented in the form of symbols in between the two rails.
• Ladder logic is the soft digital version of hard wired relay logic system.
• NO & NC contacts & Output coils are the basic components of ladder logic While timers & counters are the special components.

1. Circuit works from left to right & from up to down.
2. It has two rails positive & negative, Left rail is positive while the right rail is negative.
3. Current flows from positive rail to negative rail.
4. Current do not flows in an open contact, It only flows when circuit is closed.
5. The output defines the logic Not input.