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A question about ladder

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avr_micro

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make a plc with microcontroller

Hi.I am beginer in plc.Can anybody answer my question?
In a ladder ,How is the chronological execution of elements(inputs , outputs ,...).
for example left to right or up to down or other?
 

Marcel Majoor

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In ladder logic, and other PLC programming 'languages' as well (e.g. STL), the order is basically from top to bottom and from left to right.

But, keep in mind that most (all?) PLC's update the 'real' I/O's at the start/end of a programming block. This means that if on line 1 you change an output to '1', on the next line to '0' and the next line to '1', that the -actual- output only changes one time only to the last state ('1') and does not do 1-0-1. This is because during the programming block an *image* of the inputs/outputs is used, which you will manipulate. At the end of the programming block this image is than used to update the actual outputs, and the image is updated according to the real inputs.
 

njfane

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the ladder language use the sequence logic concept, that means one operation only happens strictly after another, the ladder just implements the real operation of the worker, that is also the reason that ladder originally invented for, easy for industrial engineer to implement who are more familiar with operation than the computers. so it is form top to down and from left to right.
the "image" is one of the key when programming ladder, it likes the variables in the high level language, it stores the medial state in the system, only the real I/O variables could effect the extern operation of the system.
 

sklux

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There s no answer for it, you need to look the manual of the plc you're going to
use. Usually it is read line by line frome left to right but I have already used
plc's that works column by column from up to down (Telemecanique TSX17)

Attention Ladder logic 984 <> Ladder
 

avr_micro

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In fact I want to find out how different sections of plc works and how they communicate with each other.If possible i want to make a plc with microcontroller.
Is this possible?
 

instruite

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PLC performs following operations
1) Input Scan (all input modules are scanned and are stored in corresponding memory registers)
2) Program Scan
- Program is performed From top to bottom
and for each ladder left to right
PLC uses the inputs states stored in its memory and updates the output registers of its memory
3) Output update (depending on the state of the output's memory register all the outputs are updated)
 

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