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18-09-2021»»[DATE-dddd]



MXLOADING126





LADDER LOGIC PROGRAM

18-09-2021
 |  [RAND-100] - Comments


 1. Ladder Logic Program Examples
 2. Ladder Logic Programming Tool
 3. Ladder Logic Programming Tutorial
 4. Ladder Logic Programming Plc Wiring Diagram
 5. Ladder Logic Programming Consists Primarily Of
 6. Ladder Logic Programming Books
 7. Ladder Logic Program

by Vaidyanath 'Doc' Nanjundaiah

Examine If Closed XIC. We've looked at these instructions at the start of the.
LADDER LOGIC 'Ladder' diagrams Ladder diagrams are specialized schematics
commonly used to document industrial control logic systems. They are called
'ladder' diagrams because they resemble a ladder, with two vertical rails
(supply power) and as many 'rungs' (horizontal lines) as there are control
circuits to represent. Ladder logic is a PLC programming language. It is really
called ladder diagram or just LD, but most people refer to it as ladder logic.
That is also what I will call it in this tutorial. There’s a very simple reason
for its name. Ladder logic is made out of rungs making what looks like a ladder.
It is possible to scale a PLC analog inputfor example, although ladder logic is
mainly for bit logic operations. But even simple bit logic ope. Ladder Logic is
a graphical based industrial programming language used to program and configure
Programmable Logic Controllers, or PLC’s. Ladder programs consist of rungs that
house instructions. Once compiled and downloaded to a PLC the ladder program is
scanned like a book; from top-to-bottom, left-to-right.

A Programmable Logic Controller (PLC) accepts inputs from a variety of devices
such as switches, sensors etc., processes inputs according to user programmed
control logic, and controls a variety of devices (e.g. relay, motors, valves
etc.) connected to the outputs of the PLC. The Relay Ladder Logic is the user
programmed control algorithm.
Relay Ladder Logic is one of the international standards and is the main
programming method used to program PLC’s. It mimics the relay logic (combination
of switches, relays, coils and contacts). The decision to use ladder logic as
the main programming method was very strategic as it did not need much time to
retrain engineers to adapt to this. The first generation PLCs were programmed
with a technique that was based on relay logic wiring schematics. This
eliminated the need to teach electricians, maintenance technicians and engineers
how to program. To this day, it remains the most popular method to program a
PLC.


COMPARISON BETWEEN RELAY LOGIC AND LADDER LOGIC.

Below is a very simple motor control relay logic and it’s corresponding ladder
logic. Relay Logic has a Start switch, Stop switch, Control Relay, Relay Coil
(CR1) and a Motor (Mtr). Ladder logic shares similar look and feel of relay
logic. But the physical switches and coils of relay logic are replaced with
PLC’s memory location which are represented as Inputs (I) and Outputs (O) in
ladder logic.


Typically the PLC CPU scans the ladder from top left to bottom right and reads
and executes the condition of physical I/O’s. The time taken to make one pass
from top to bottom and execute logic is known as scan time.

A PLC system handles many numbers representing different types of information
regarding the process. These process/machine parameters may be anything from the
status of the input or output devices, timers/counters, or other data values.
Each PLC manufacturer has their own conventions for this in their PLCs. These
memory types can be used to store a variety of information and can be used
inside various Relay Ladder Logic instructions.


DISCRETE / BOOLEAN MEMORY TYPE

A Discrete memory type is one bit that can be either a 1 or a 0 (ON or OFF).
Discrete memory area is used for inputs, outputs, control relays, and
timer/counter bits.



WORD / REGISTER MEMORY TYPE

A Word memory type is a 16-bit location that is normally used to store and
manipulate numeric or ASCII data. A word memory location is also called a
Register.



PLC CPU OPERATION SEQUENCE:

Power-up Initialization

At power-up, the CPU initializes the internal electronic hardware. It also
checks if all the memories are intact and the system bus is operational. It sets
up all the communication registers. It checks the status of the backup battery.
If all registers are go, the CPU begins its cyclic scan activity as described
below.

Read Inputs: The CPU reads the status of all inputs, and stores them in an image
table. IMAGE TABLE is PLC’s internal storage location where it stores all the
values of inputs/outputs for ONE scan while it is executing ladder logic. CPU
uses this image table data when it solves the application logic program. After
the CPU has read all the inputs from input modules, it reads any input point
data from the Specialty modules like High Speed Counters.

Execute Logic Program: This segment is also called Ladder Scan. The CPU
evaluates and executes each instruction in the logic program during the ladder
scan cycle. The rungs of a ladder program are made with instructions that define
the relationship between system inputs and outputs. The CPU starts scanning the
first rung of the ladder program, solving the instructions from left to right.
It continues, rung by rung, until it solves the last rung in the main logic. At
this point, a new image table for the outputs is updated.

Write Outputs: After the CPU has solved the entire logic program, it updates the
output image table. The contents of this output image table are written to the
corresponding output points in I/O Modules. After the CPU has updated all
discrete outputs, it scans for the specialty modules. The output point
information is sent to the specialty I/O like High Speed Counters.

Immediate Inputs/Outputs: There is a possibility that an input changes after the
CPU has read or scanned the inputs. If you have an application that cannot wait
until the CPU returns for the next input scan, you can use Immediate
Instructions.

These instructions do not use the status of the input from the image table to
solve the application program. The Immediate instructions immediately read the
input status directly from I/O modules and update the Input table with
appropriate status of input module read. Similarly, Immediate Output
instructions do not wait for the CPU to complete the ladder scan. Immediate
outputs are directly written to the image table and Outputs are updated
accordingly.

Subroutines: The CPU executes subroutines when called for in the ladder program.
These subroutines are useful in performing the same logic operation time and
time again just upon one call so you do not have to repeat the rung logic over
and over again. Subroutines are also useful in executing a logical function, for
example check limits, upon receiving an external interrupt from a PLC I/O
module.


I/O RESPONSE TIME:

I/O response time is typically defined as the time required for the control
system to note a change in an input point and update a corresponding output
point. In majority of the applications, the processor of a PLC responds
practically instantaneously to this task. There are some applications that
require extremely fast I/O scan times. The following four factors affect the I/O
response time of a CPU:

1. The point in the scan period when the field input changes its state.
2. Delay time for Input module to change state.
3. CPU scan time.
4. Delay time for Output module to change state.

See the diagram above. The I/O response time is minimum when the I/O module gets
the input change before the Read Inputs portion of the Ladder execution scan
cycle. In this case the input status is read, the logic program is solved, and
the corresponding output point gets updated. The total I/O response time is
calculated as:

I/O Response = Delay in Input module + CPU Scan Time + Delay in Output module


MAXIMUM I/O RESPONSE TIME:

The I/O response time is maximum when the I/O module notes an input change after
the Read Inputs portion of the Ladder execution scan cycle. In this case the
input status gets noted only in the following Input scan. The diagram shows an
example of I/O response timing for this condition. The total I/O response time
is calculated as:

I/O Response = Delay in Input module + 2 times the CPU Scan Time + delay in
output module


HOW TO GET THE BEST I/O RESPONSE TIME:

Using Interrupt subroutines and Immediate I/O instructions is the best way to
optimize the I/O Response time of your PLC system. The immediate instructions
update the I/O points during the ladder logic program execution.

The diagram shows how immediate input and output instructions affect the I/O
response timing.

The total I/O response time is simply calculated as: I/O Response = Delay in
Input module + Instruction Execution Time + Delay in Output module + Instruction
Execution Time = Immediate Input Instruction Execution + Immediate Output
Instruction + Time for Execution of all Instructions in between.

The total I/O response time for an external interrupt and a subroutine is
calculated as:

Delay in Input Module + execution of subroutine + delay in output module.

As an example, upon an interrupt you can read the status of an input bit,
perform a logical operation on it based upon the value of some other registers,
and turn on an output in less than 50μs.

CPU Scan Time Considerations:

The scan time includes all the tasks that are performed by the operating system
in a cyclic manner. As discussed previously, each scan cycle is made up of
several segments. Each of these segments takes a certain amount of time to
execute. Among all the segments, the amount of time it takes to execute the
application program is the only one that has maximum influence on total scan
time. This also happens to be the one segment you can control as a user. If your
application needs a smaller scan time, then you should try to choose
instructions with as fast execution time as possible. This is because different
instructions take different amounts of time to execute. Your choice of I/O
modules and system configuration can also affect the scan time.

Conclusion: If you are new to Relay Ladder Logic Programming, here is a sequence
you should follow to develop Relay Ladder Logic program.

 * Define your machine automation or automated process.
 * Determine hardware requirements for the control design.
 * Define control algorithm.
 * Assign input and output parameters of the process to the control algorithm.
 * Develop relay ladder logic project.
 * Match I/O addresses of the controller to the correct input/output devices.
 * Transfer the project to the PLC.
 * Validate the project.
 * Run the PLC / project.

I've posted an article about Arduino based PLC and I've an unexpected response.
Many Arduino enthusiasts and students and also many other automation specialists
showed interest to the subject.
This simply shows how there are so many people wanting to know more about open
source and freely developed tools that can be used in automation and in industry
as a whole.


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I remember a friend of mine who had a dream since fourteen years ago. That dream
he had was to replace old PLCs with modern Microcontrollers like PIC, Atmel and
lately Arduino.
I guess now his dream came true.
I received a lot of questions on Arduino based PLC products and how they can be
programmed.
So I’ve searched further and found that there are already some tools that can be
used to program Arduino using Ladder language.


LADDER LOGIC PROGRAM EXAMPLES

PLCs are often programmed in Ladder Logic. This is because PLCs originally
replaced relay control systems, and after all those years, we still haven't
quite let go. A PLC, like any microprocessor, executes a list of instructions in
sequence.


LADDER LOGIC PROGRAMMING TOOL



Today I’m putting these tools in front of you so you can start learning them and
hopefully using them efficiently.






SoapBox Snap is a free and open source PC-based automation platform.
The ladder editor includes standard instructions like contacts, coils, timers,
counters, rising edge and falling edge, and set/reset instructions.


LADDER LOGIC PROGRAMMING TUTORIAL


SoapBox Snap also comes with an Arduino Runtime, which means you can download
your ladder logic programs to an Arduino (UNO, Nano or Mega board) and even do
online debugging and forcing.
http://soapboxautomation.com/products/soapbox-snap/




LADDER LOGIC PROGRAMMING PLC WIRING DIAGRAM

LDmicro



It’s a compiler that starts with a ladder diagram and generates native PIC16 or
AVR code.
Features include:

-timers (TON, TOF, RTO)
-counters (CTU, CTD, `circular counters' for use like a sequencer)
-analog inputs, analog (PWM) outputs integer variables and arithmetic
instructions
-easy-to-use serial communications, to a PC, LCD, or other device


LADDER LOGIC PROGRAMMING CONSISTS PRIMARILY OF

-EEPROM variables, whose values are not forgotten when you lose power
-simulator, to test your program before you generate PIC/AVR code
http://cq.cx/ladder.pl



LADDER LOGIC PROGRAMMING BOOKS

I hope this article could shed some light on the subject.



LADDER LOGIC PROGRAM


Thank you for reading.


This post first appeared onEmbedded Systems In Egypt, please read the originial
post:here







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