Tag Archives: PLC Ladder Logic

Siemens S7-200 PLC programming examples – Belt conveyor control

To look many PLC Ladder Diagram is better for learn plc programming. you can learn the plc programming Thoughts and ideas. here is siemens s7200 plc rpgramming examples.
(1) control requirements
Motors M1, M2, M3 three drive belt conveyor.Press the start button SB1, start the order of M1, M2, M3, the interval of 3s.Press the stop button SB2, the order of the parking for M3, M2, M1, the interval of time is 3s. Three motors M1, M2, M3 respectively through the contactor KM1, KM2, KM3 connected three-phase AC power supply, with PLC control of the coil of the contactor.

(2)Address allocation table for programming elements
 Programming element
I/O terminals
(program address)
Circuit devices
(PT timer value)
 input relay
start button
Stop button
 output relay
M1 Contactor
M2 Contactor
M3 Contactor
Start first period of time
Start second period of time
Parking first period of time
Parking second period of time
Auxiliary relay
Parking holding first
period of time
Parking holding second period

(3)Ladder Diagram

PLC ladder programming

Siemens S7-300 Ladder Diagram PLC Programming Example-Control Belt

This PLC programming example is plc control belt

(1) Control requirements

Electric start belt,At the beginning of the belt has two push button switches: S1 for START and S2 for STOP;At the end of the belt also has two buttons switches: the S3 for START and STOP for the S4. You can start or stop the belt from either end. In addition, when an object on the belt reaches the end, the sensor S5 make  belt stop.

(2)Programming element symbolic address

Symbol Address
Absolute address
class type
Start button
Stop start button
Tail start button
Tail stop button

(3)Ladder diagram

PLC programming ladder diagram

Siemens PLC S7-300 Ladder Diagram Programming Example-pulse generator

Siemens plc S7-300 programming the timer can also be   a pulse generator.

(1) Control Requirements

The PLC program using a timer constitute a pulse generator,With two timers generate a pulse signal frequency of the duty cycle can be set

Timing diagram of the pulse generator

(2) Ladder Diagram

PLC Ladder Diagram

When the input I0.0 is 1, the output Q0.0 is 0 or 1 alternately, a periodic pulse signal for 3s, the pulse width is 1s.

Siemens PLC S7-300 controls the motor to start up Ladder Diagram programming examples

Today we’ll make a plc ladder diagram for siemens plc s7-300 control the motor start up. plc ladder diagram  is very important to learn plc programming.
(1) Control requirements.
There are three motors M1, M2, M3, press the start button to start M1, M2 starts after delay 5s, M3 starts again after 16s.
(2) Programming element symbol address

Programming element symbol address

(3) PLC Ladder Diagram

PLC Ladder Diagram

Siemens S7-300 plc dial switch setting Heating time programming examples

here is plc program examples about siemens s7-300 plc , You can learn about Timer instruction。

(1)Control requirements.

the operator can set the heating time with a dial switch, and the operator set the value to be displayed in a second unit in a BCD format.

(2) Defining the programming element symbol address

  System components
Start button
Single digit dial switch
Tens digit dial switch
Hundreds digit dial switch
Start heating

(3) Ladder diagram

plc ladder diagram


Siemens S7-300 plc using functional blocks To realize the clock pulse generator programming examples

Now  we learn plc programming examples for  siemens s7-300 plc,here some ladder diagram program.

(1) Control requirements.

using a timer freedom to set the clock pulse generator function (pulse duty factor 1: 1)

(2) FC1 variable

FC1 variable

(3) FC1 ladder diagram program

FC1 ladder diagram program

(4) OB1 calls ladder diagram program

OB1 call Ladder diagram program


What is a ladder diagram? Ladder diagram programming

Ladder diagram is the most used graphic programming language, it is called the first programming language of  PLC. Ladder diagram and electrical control system circuit diagram is very similar, with intuitive and easy to understand the advantages, it is easy to be mastered by the factory electrical personnel, especially suitable for switching logic control. Ladder diagram is often referred to as the circuit or program, the design of ladder diagram is called programming.
Ladder diagram programming, use the following four basic names:

1 ,Soft Relay

Some of PLC ladder diagram programming element follows the name of the relay, such as input relay,output relay, internal auxiliary relay, etc., they are not real physical relays, that are  some storage unit (soft relay), a storage unit for each soft relay and PLC memory image register. The storage unit if the “1” state, the ladder diagram of the corresponding soft relay coil energized, the normally open contact is turned on, normally closed contact is disconnected, this is the soft relay “1” or “on” state. If the memory cell is “0”, the state of the coil and contact of the corresponding soft relay is in contrast to the above, this is  soft relay is “0” or “OFF”. The use of these “soft relay” is often called to as the programming element.

2,Flow Power”

Figure, contacts 1 and 2 is switched on, there is a supposed concept of current or Power Flow), flowing from left to right, the direction and execution when a user program is consistent with the order of the logical operations.

Power flow can only flow from left to right. Using the concept of energy flow, can help us to better understand and analyze the ladder diagram.

Figure, maybe two  directions of power flow through the contacts 5 (through contact points 1, 5, 4 or contact points 3, 5, 2), which do not meet the principles of energy current can only flow from left to right

A) the wrong ladder diagram b) the correct ladder diagram

3,Bus bar

Ladder on both sides of the vertical line is called public (Bus bar) ,. In the analysis of ladder logic, in order to borrow the relay circuit analysis method, there are conceivable between the left and right side bus (left bus bar and right bus bar) a left and right positive negative DC supply voltage, the “power flow between bus bar from left to right. You can not draw right bus bar.

4,The ladder logic solution

According to the state and the logic relation of each contact in the ladder diagram, the state of the programming elements corresponding to each coil in the diagram is obtained,is called  logical solution of ladder diagram.

Ladder logic solutions were considered by from left to right, top to bottom sequence. The results of the resolver, can be immediately utilized by the logic solver behind. – Logic be based on the input image register values, rather than resolver the instantaneous state of the external input contacts carried out.

PLC ladder programming sequence control

Learning PLC programming, we often encounter the sequential control, but how to make plc ladder diagram
there is  example Shuttle program.

The initial state of the car parked in the middle (limit switch X0 is pressed, the normally open contact closure),Press the Start button (X3 closed), the car began to follow the direction shown to reciprocate when you need to stop, press the stop button (X4 closed), stop automatically when the car moves to the middle position. Here all the buttons and travel switches are normally open contacts are connected to the input terminals of the relay.

how to program long delay PLC ladder diagram

When plc programming often need several hours, or even longer delay, the best way is  series timer, or  PLC internal counter.Using the timer series Can be a long delay, in essence, it is to allow multiple timers turn ON, delay time is Accumulating multiple timer settings.

Serial time delay

Set time of the timer T1 is a counter C100 input pulse signal, so that the delay time is several times of  T1 setpoint.

Timers and counters associated with long time delay
The internal special relay R901E (1 min clock) as the counter C100 input signal, so that the time delay is a few minutes. If a counter does not meet the requirements, the counter can be used in series, that is, the contact of the front counter is used as the input pulse signal of the back of a counter to achieve a longer time delay.
Counter long time delay

how to program Power Delay on and Power loss delay off PLC ladder diagram

When the X0 is turned on, the timer T1 starts to delay, after 2S Timer T1 normally open contact closure (set point signal), hold instruction make the Y0 power and hold; When the X0 is off, the timer switch on T2 (normally open contacts at Y0) start delay, 4S timer T2 normally open contacts (reset signal), Y0 power loss


How to make the system start PLC ladder diagram Program

when your learn plc programming , you must need plc system start up. PLC system start up  include    Start self-locking, without self-locking.
but how you choose the ladder diagram?
If the hardware buttons circuit with a reset function you need to use with self-locking startup programs;

If the hardware buttons circuit without  a reset function you need to use without self-locking startup programs;


with self-locking startup program ladder diagram

without self-locking startup program  ladder diagram


How to use PLC control Motor starting voltage Ladder Diagram

Press the start button SB2, contactor KM1 coil get electricity and self lock, KM3 coil power, which normally open the main contact closure, motor access power supply, and Y connection to start running. At the same time, the time relay KT coil power, after the delay, the normally closed contact is disconnected, cut off the power supply of the KM3 coil, making the KM3 power loss, normally open contact off, normally closed contact closure. KT of the normally open contact is closed and a contactor km2 coil power, km2 main contact closure makes motor by Y connection switching method for delta connection operation.

Relay control circuit

Y-△ Buck start control Schematic diagram

PLC control program design

PLC control I/O

Ladder Logic


how to get a signal PLC ladder program

PLC programming often need to use different commands to obtain different information on the switch
this is plc programming examples

1. Get information on the rising edge

To obtain information on the rising edge of the input signal by the transition instruction, contacts and trigger rising edge pulse

2. Get information falling edge

To obtain information on falling edge the input signal drops by contact and falling edge pulse trigger

3. Get level information

     If the button corresponding to the input port is normally open, the program can be obtained through a high level of information normally open contact, normally closed contact information to obtain Low level

PLC Ladder Diagram same button multiple functions progrmming

this is PLC Ladder Diagram about  the same button needs to achieve a variety of functions, that is, the button 1 times to achieve the first function, according to the second to achieve second functions, according to the third to achieve third functions

The function procedure is triggered by the rising edge of the input signal, so the rising edge of the input signal needs to be acquired. At the same time, it is necessary to count the number of the input signals, and the different values will perform different functions. When the counter reaches a certain value, it is necessary to reset the counter.

PLC ladder programming Instruction control bits

        ① set and reset. When the condition becomes ON”, the set will be put in operation ON”, but not as output (OUT), when the condition becomes OFF”, the reset will not reset on the operation of OFF”. When the condition becomes ON”, the reduction in operation position reset OFF”, but does not output non-directive (OUT NOT), when the condition becomes OFF”, the reset will not reset on the operation ON”. In the following example, no matter how IR00100, IR00100 reset ON” IR20000 will reset when ON”, and can remain ON” until the IR00101 reset ON”. When IR00101 set ON”, the reset IR20000 reset OFF”. Set and

No function code set and reset. From the handheld programmer to enter them as FUN and SET key or FUN and RSET button, followed by the address.

       ② rise differential differential and the falling edge. Rise differential and the falling edge of the so-called differential refers to the input signal front porch and back operations. Rise differential differential command is used to enable and falling in a cycle time of the built-in ON”. In implementing the conditions OFF” to ON”, the rising differential instruction makes a cycle of operation ON”, in the implementation of conditions ON” to OFF”, the falling differential instruction makes a cycle of operation ON”. Both of these instructions requires only one line mnemonics.

Here, in IR00000 to ON”, IR20001 in a cycle will reset ON”. Then perform DIFU (13) 20001, no matter how IR00000, IR20001 will be OFF”. Falling edge differential instruction IR00001 to OFF” after (IR20002 OFF” until this time), the IR20002 will be built in a cycle ON”, and will be implemented in the next DIFD (14) 20002 post OFF”.
Rise differential and the falling edge of differential

       ③ maintain. Use directives to keep based on two perform conditional operations. Doing so will keep the instructions attached to two on the command line. When the first command line execution at the end of the condition for ON”, keep instructions operating position ON”. When the end of the second command line execution as ON”, keep instructions operating position OFF”. Even if it is located in the internal interlocking parts of the program, keeping the operation of the directive will save it to the ON” or OFF” State.
In the example shown in Figure 1-33, when IR00002 is ON” and IR00003 to OFF”, the HR0000 will be set ON”. HR0000 remain ON” until another IR00004 or IR00005 reset ON”. Keep instructions as the others, need more than one command line, the command line in the instruction encoding before they control.

Siemens S7-200 PLC’s Belt conveyor Control Programming Example

Three conveyor belts, respectively, by the motor M1, M2, M3 driven.

Requirements: After pressing the start button SB1, a startup sequence for M1, M2, M3, interval of 3s.
After the stop button SB2, Sequential stopping for M3, M2, M1, interval of 3s.
Three motors M1, M2, M3, respectively, through the contactor KM1, KM2, KM3 three-phase AC power supply is turned on, PLC control coil contactor