Reading and understanding ladder logic
Once the hardwire relay logic concepts are understood then its easy to comprehend ladder logic.
The term ‘Programmable Logic Controllers’ (PLCs) originated from relay-based control systems.
In a PLC, there is full flexibility to change the sequence of operations and interlocks for different conditions.
There are integrated circuits and internal logic in the PLC, in place of discrete relays, coils, timers, counters, and other such devices.
PLCs provide greater computational capabilities and accuracy, to achieve increased flexibility and reliability, than hard-wired relays.
The symbols and control concepts used in PLCs come from relay-based control and form the basis of ladder logic programing (Figure 2.10).
In the following sections, commonly used terminology for ladder logic is dealt with. The terminology used in commercially available PLCs from various manufacturers may differ slightly but the concepts remain the same.
PLC terminology may differ from relay terminology, but the control concepts are the same.
working bit NO contact of relay condition Normally open NC contact of relay condition Normally closed As such, there is no equivalence between these terms. The term ‘condition’ is only used to describe ladder logic diagram programs in general and is equivalent to a set of basic instructions. The terms input/output are used for reference to I/O bits assigned to input and output signals.
In ladder logic programing, the following two types of instructions are used:
1. Instructions that correspond to the conditions of the ladder logic diagram.
They are used in instruction form only when converting a program to mnemonic code.
2. Instructions that are used on the right-hand side of the ladder logic diagram are executed according to the conditions on the instruction lines preceding them.
Most of the instructions have at least one or more operands.
Ladder logic diagram
A ladder logic diagram is so-called because the relay logic runs in parallel lines between two power lines and the whole diagram resembles a ladder.
This diagram consists of one vertical line running down the left side, with the horizontal lines branching off to the right. The line on the left is called the bus bar, while horizontal lines are instruction lines or rungs. Along the instruction lines conditions are placed, that lead to other instructions on the right side. Power flow is always from left to right.
Therefore, the logical combination of these conditions from left to right side determines when and how the instructions at the right side are executed.
In a ladder logic diagram, instruction lines can have multiple branches. The vertical pairs of lines are called conditions. Conditions without diagonal lines through them are called NO conditions that correspond to AND, LOAD, or OR instruction.
The conditions with diagonal lines through them are called NC conditions that correspond to AND NOT, LOAD NOT, or OR NOT instruction.
Each condition has a number above/below each condition that indicates the operand bit for the instruction. Operand bit (Input/ Temporary bit) is associated with that condition.
The status of the bit determines the execution condition for the following instructions.
Basic terms used in ladder logic
Normally open and normally closed conditions
Each condition in a ladder logic diagram is either ‘ON’ or ‘OFF’ depending on the status of the operand bit that has been assigned to it. A NO condition is ‘ON’ if the operand bit is ‘ON’ and it is ‘OFF’ when the operand bit is ‘OFF’. On the other hand, a NC condition is ‘ON’ if the operand bit is ‘OFF’ and it is ‘OFF’ when the operand bit is ‘ON’.
In short, an NO condition simply follows the bit status (on => on and off => off) and an NC condition follows inverted bit status (on => off and off => on).
In a ladder logic program, the logical combination of ‘ON’ and ‘OFF’ conditions before an instruction determines the conditions under which the instruction is executed. This condition is called the execution condition for the instruction. Except for the ‘LOAD’ instruction, all other instructions have execution conditions.
The operands designated for any of the ladder logic instructions can be I/O bits, flags, work bits or flags, timers, or counters, etc. In a ladder logic diagram, these conditions can be determined using these operands.
The manner in which the conditions correspond to instructions is determined by the relationship between the conditions, within the instruction lines that connect them. Any group of conditions that go together to create a logic result is called a logic block.
Ladder logic instructions
The ladder logic instructions correspond to the conditions on the ladder logic diagram. Ladder instructions are either independent, or are in combination with the logic block instructions, from the execution conditions, based on which the execution of all other instructions are dependent. The most common ladder logic program instructions and the symbols used are shown in the Figure 2.11.
The last instruction required to complete a ladder logic program is the ‘END’ instruction. When the PLC CPU cycle runs through the program, it executes all the instructions up to the first ‘END’ instruction. After the ‘END’ instruction, it returns to the beginning of the program and begins the execution again. Usually, the ‘END’ instruction is the last instruction in the ladder logic program, but it can be placed at any point in the program, like when program debugging is undertaken. No instruction after the ‘END’ instruction is executed. The ‘END’ instruction requires no operands, and no conditions can be placed with the ‘END’ instruction.
Examples of simple ladder logic instructions
Examples of ladder logic instructions for simple control circuits (AND, OR, AND with OR) are shown in Figure 2.12.
The ladder logic diagram
The ladder logic diagram is one of the methods of programing PLCs and is covered in detail in the IEC standard 61131 Part 3. Ladder diagram is a very convenient way of representing interlocking logics which used to be configured using hard wired devices. Current generation PLCs have other capabilities including that of PID controllers. The IEC standard therefore provides more advanced methods of programing such as Structured Text, Function Block diagram and Sequential Function chart for tasks which cannot be adequately represented using Ladder diagrams alone.
Incoming search terms:
- ladder logic symbols
- ladder diagram symbols
- plc ladder logic symbols
- plc symbols
- plc symbols chart
- ladder logic symbols chart
- transformer ladder
- logic symbols chart
- ladder diagram basic symbols
- ladder mnemonic
- ladder symbols
- electrical ladder diagram symbols chart
- ladder diagram tutorial
- plc ladder diagram symbols
- plc ladder symbols
- plc symbol
- relay logic symbols
- symbols of output devices
- All PLC ladder symbols
- all sysbol in plc ladder diagram
- Different symbols which used ladder diagram
- ELECTRICAL EQUIPMENT Trading in electrical
- electrical ladder diagram
- explain an electical ladder diagram that will display the proper usage of all devices
- ladar diagram in electrical symbols
- ladder dia nd its symbol
- ladder diagram thermostat symbol
- ladder logic diagram builder
- LADDER symbol for relay coil in plc
- operands bits in a relay
- or not contact symbols plc
- plc Components symbols
- plc ladder diagram complete symbols
- plc ladder logic reading
- plc programming symbols
- plc simbols
- plc stl logics reading
- PLC symbol internal relay
- plc symbols pdf
- reading relay logic diagrams
- symbol of plc l
- symbols of relay and ladder diagram
- symbols realy coil in plc
- two main terms used in ladder diagram
- understanding ladder diagrams
- 1 3 describe the electrical circuits that connect input devices to the input sections of plcs and output devices to output sections of plc
- a symbol on a ladder of --(U)-- indicates
- all ladder logic symbol
- audio devices symbols
- basic component and their symbol in ladder diagram of plc
- because logic symbol
- devices that uses ladder logic
- Different ladder symbol
- digital ouput symbol on sama diagrams
- electrical ladder diagrams explained
- games for learning ladder diagrams
- how to convert mnemonic code to ladder diagram
- how to Mitsubishi GX program name symbal
- in a ladder digram Power lines are called
- in ladder diagrams component symbols are placed
- input symbol logic 500
- inut field devices symbo in plc programming
- ladder diagram programming symbol
- ladder diagram programming symbols
- ladder diagram sensor symbol
- ladder diagram symbol explained
- ladder diagram symbols and names chart
- LADDER DIAGRAM SYMBOLS PDF
- ladder language symbols
- Ladder like electrical symbol
- ladder lo
- ladder logic and terminal blocks
- ladder logic diagram symbols name
- ladder logic instructions
- ladder logic program end
- ladder logic symbol chart
- Ladder logic symbol for any actuating device
- ladder logic symbol for photoeye
- ladder logic symbols and meanings
- ladder logic symbols pdf
- ladder logic symbols series circuit
- ladder logic what is the up arrow
- ladder logic with control and timing relays schematic
- ladder logics symbol
- ladder programming symbols
- ladder symbol
- ladder symbol chart
- ladder symbol for NO input contact
- ladder symbol for timer and counter
- ladder symbol of timer counter
- ladder symbol output contact
- ladder symbols for counter
- lader logic diagram symbol
- lather diagram simble
- logic circuit builder from symbols
- logic symbol for programer
- logic symbols
- logic symbols and meanings
- most common ladder logic symbols
- mostly used symbols in plc ladder diagram
- not bit ladder logic
- plc all symbol and meaning of laderlogic
- plc basic symbols
- PLC circuit understanding
- PLC code symbols
- plc control symbol
- plc diagram symbols
- plc diagram symbols and names
- plc diagram symbols name
- plc diagram terms
- plc digram sembol
- plc drawing symbols
- plc instruction symbols ladder
- PLC LaDder DIAGRAM SYMBOLE
- plc ladder diagram tutorial
- plc ladder diagram with name
- plc ladder logic programme symbol and explain all symbol
- plc ladder logic symbol
- PLC Ladder Logic Symbols Visio
- plc ladder logic sysmbols
- plc ladder symbol name
- plc ladder use simbol
- plc lader symbols
- plc logic diagram symbols
- PLC logic output device ladder logic symbol pic
- plc programing all ladder logic symbol
- PLC programing logic symbols
- plc schematic symbols
- plc sy
- PLC symbol Bit/ flag/internal relay
- plc symbols and meanings
- plc symbols means
- plc symbols of the ladder diagram -
- programing symbols
- PROGRAMING SYMBOLS AND THEIR MEANINGS
- read and understand electrical ladders
- reading and understanding ladder logic
- reading PLC logic blocks
- reading relay logic diagram symbols
- relay ladder diagram symbols
- relay ladder logic instructions
- relay ladder logic symbols
- relay ladder symbol
- relay logic symbols chart
- Relay symbol - ladder diagram
- relay symbols in plc
- schematic symbols for electrical output devices
- simbol coil diagram ladder
- simbol program coil nc
- simbol program plc coil nc
- smybols lf ladders
- switch symbol in plc ladder logic
- symbol for plc
- symbol in used lader diagram meaning
- symbol of logic controller
- symbol of plc
- symbol of relay with flag
- symbol used in ladder diagram
- symbol used in ladder logic diagram
- symbol used in plc
- symbol using in PLC ladder logic diagram
- symbols for ladder logic
- symbols for output devices for ladder diagram of plc
- symbols in PLC chart
- symbols of logic controller
- symbols of PLC
- symbols used for input devices in ladder diagram for plc
- symbols used in ladder diagram
- The standard logic symbols that utilize squares with symbols in them are
- The symbol in a ladder logic diagram represents a
- trouble free unless the 318926
- understanding ladder logic
- UNDERSTANDING LADDER LOGIC IN PLC
- understanding plc ladder logic
- understanding plc ladder logic diagrams
- understanding plc logic
- VERTICAL ladder logic diagram symbols
- what are the two methods used to represent a timer instruction within plcs ladder logic program
- what is symbol HC in ladder diagrams