PLCs: Siemens

Basic Ladder Diagram Programming for Siemens PLCs 280

This class explains how basic ladder diagram programming is used to program PLCs. It examines the basic rules that are used to construct a ladder diagram program, including Boolean logic functions. It then illustrates these rules and how they relate to hard-wired circuitry by showing the various methods used to create a start-stop control application.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 15

  • Language English


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Course Outline
  • Objectives
  • Ladder Diagram Programming
  • Instructions and Operands
  • Power Flow
  • Contact and Coil Instructions
  • Network Configurations
  • The AND, OR, and Exclusive OR (XOR) Functions
  • Inversion
  • Instructions that Set and Reset Bits
  • Hardwired Motor Start-Stop Circuit
  • PLC Motor Start-Stop Control
  • PLC Motor Start-Stop Control Program
  • PLC Motor Start-Stop Control Program Operation
  • Why Use a PLC?
  • Summary
  • Describe ladder diagram (LAD) programming.
  • Understand the use of binary instructions and operands in LAD.
  • Explain common terminology used in the LAD programming language.
  • Explain the concept of power flow in a LAD program.
  • Detect logical conflicts in complex LAD programming network configurations.
  • Explain the AND, OR, and XOR functions.
  • Explain inversion in LAD programming.
  • Understand the instructions that set and reset bits.
  • Explain how start-stop control is done using hardwired circuits.
  • Describe the operation of a typical PLC motor start-stop control application.
  • Explain how to program a PLC motor start-stop control application.
  • Explain the operation of a typical motor start-stop LAD program.
  • Understand when PLCs are used for motor control.
vocabulary term

absolute operand

An operand that can be used by any code block in a program. In STEP 7 (TIA Portal), absolute operands are preceded by the % symbol.


A function in Boolean logic that requires all input status bits to be 1 for the output status bit to be 1.


A number system used for PLCs that has either 0 or 1 as a value.


A single numerical unit in the binary number system. Bit is the smallest unit of information on a computer.

bit logic instructions

A category of Instructions in the LAD programming language that performs logical operations using single bit memory locations.

bit memory

M. A part of the global memory of a Siemens PLC used to store the immediate results of program operations.

Boolean logic

A branch of mathematics that deals with the use of logical operators that define relationships between two or more entities. Examples of Boolean functions include AND, OR, and Exclusive OR.


8 consecutive binary bits.

circuit breaker

An overcurrent protection device that also can be used to manually disconnect power from a circuit.

code blocks

Program block. A grouping of PLC instructions. A Siemens PLC can have three types of program blocks: organization blocks (OBs), function blocks (FBs), and functions (FCs).


A device that uses a small control current to energize or de-energize a load. Contactors can handle high amounts of current and are combined with overload relays to create motor starters.


A conductive part that opens or closes an electrical circuit by either separating from or contacting with the main conductor. The term contact can also be used to identify normally open or normally closed contact instructions in a LAD program.

control circuits

A type of circuit that uses control devices to determine when loads are energized or de-energized by controlling current flow. Control circuits usually operate with lower voltages and currents than power circuits.

control transformer

An electromagnetic device that steps down voltage levels useable by loads in a control circuit. A control transformer powers the control circuit.

global memory

A type of PLC memory that is available to all code blocks. Global memory includes both the input and output bits of the process image and the bit memory.

IEC 61131

A standard of the International Electrotechical Commission that provides common approaches and concepts for PLCs. Part three of the standard describes the LAD programming language for PLCs.

input signal module

A Siemens PLC input module. An input signal module converts the signal from an input device to the low voltage DC used by the CPU.

instance data blocks

Store data for a specific function block.


An operation that changes a logic 1 to a logic 0 or vice versa.

ladder diagram programming

LAD. A graphical programming language that is defined in 61131-3 and derived from the diagrams used in electromechanical control circuits, though it has expanded to include more advanced capabilities. Ladder diagram programming is also known as ladder logic.

ladder logic

LAD. A graphical programming language that is defined in IEC 61131-3 and derived from the diagrams used in electromechanical control circuits, though it has expanded to include more advanced capabilities. Ladder logic is also known as ladder diagram programming.

linear program

An LAD program whose instructions are contained in one main program block, called organization block 1, or OB1. Linear programs are executed with each PLC scan.

local memory

L. A type of PLC memory that temporarily holds information but does not store it. Local memory is allocated to code blocks when they are executed.

motor control centers

MCCs. A modular assembly designed to contain plug-in motor control units. MCCs can also contain networked control and protection devices.

motor control units

Plug-in assemblies of a motor control center that each control a motor circuit.

motor starter

A device that starts and stops a motor circuit.


A Boolean logic operation that inverts a logic 1 to a logic 0 and vice versa.


A part of a PLC instruction that identifies a memory location associated with the instruction.


A function in Boolean logic for which the output status bit will be 1 if one or more input status bits are 1.

output signal module

A Siemens PLC output module. An output signal module controls an output device in response to the signal provided from the CPU.


A level of current that exceeds the recommended level for a device or circuit, but is less than the level of a short circuit.

overload relay

A device that disconnects a motor from its power circuit when the overload relay senses an overload for a specified time.

PLC tag

A symbolic identifier used in a PLC program to associate an instruction with a CPU memory area.

power circuit

A type of circuit that carries power to electrical loads.

power flow

A condition in a LAD network for which the result of logic operation to that point in the network is a logic 1.

power rail

The vertical line on the left of each network in an LAD program. Networks in a LAD program are executed from left to right beginning with the instructions closest to the power rail.

process image

The area of CPU memory where input and output status information is stored.

reset set flip-flop

An circuit instruction with Set and Reset inputs that controls its output based on the condition of these inputs.

result of logic operation

RLO. The solution to a configuration of instructions. The RLO at any point in a network affects the operation of instructions that come after that point in the network.


One complete cycle of a PLC's cyclical operation that includes checking inputs, executing its programs, updating the status of its outputs, and performing various other required processes.

Set Reset Flip-Flop

An instruction with Set and Reset inputs that controls its output based on the condition of these inputs.

soft starters

Solid-state, reduced-voltage starters.


Software used for the configuration, programming, testing, and diagnosis of Siemens SIMATIC PLCs.

structured program

A PLC program with instructions divided into multiple program blocks. Program blocks in a structured program can be executed with each PLC scan or on a conditional basis.

three-phase AC motor

A type of electric motor that runs on alternating current. Three-phase AC motors are the most common type of industrial motor.

TIA Portal

Siemens Totally Integrated Automation Portal. Software used for designing, commissioning, operating, maintaining, and upgrading automation systems.

truth table

A table that shows all the possible true and false combinations for a logic function or configuration and the corresponding state of the output for each combination.

variable frequency drives

A device that controls the speed of an AC motor.


16 consecutive binary bits.


A function in Boolean logic that requires an odd number of input status bits to be 1 for the output status bit to be 1.