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Chapter 16 PLC Function CH2000 Series

16-9

16.4 Ladder Diagram

16.4.1 Program Scan Chart of the PLC Ladder Diagram

Calculate the result by

ladder diagram

algorithm (it doesn’t

sent to the outer output

point but the inner

equipment will output

immediately.)

X0 X1

Start

M100

X10

X100 M505

Y126

End

Send the result to the output point

Read input state from outside

Repeats the

execution in

cycle.

16.4.2 Ladder Diagram

Ladder diagram is a diagram language that applied on the automatic control and it is also a

diagram that made up of the symbols of electric control circuit. PLC procedures are finished after

ladder diagram editor edits the ladder diagram. It is easy to understand the control flow that

indicated with diagram and also accept by technical staff of electric control circuit. Many basic

symbols and motions of ladder diagram are the same as mechanical and electrical equipments of

traditional automatic power panel, such as button, switch, relay, timer, counter and etc.

The kinds and amounts of PLC internal equipment will be different with brands. Although internal

equipment has the name of traditional electric control circuit, such as relay, coil and contact. It

doesn’t have the real components in it. In PLC, it just has a basic unit of internal memory. If this bit

is 1, it means the coil is ON and if this bit is 0, it means the coil is OFF. You should read the

corresponding value of that bit when using contact (Normally Open, NO or contact a). Otherwise,

you should read the opposite sate of corresponding value of that bit when using contact (Normally

Closed, NC or contact b). Many relays will need many bits, such as 8-bits makes up a byte. 2

bytes can make up a word. 2 words make up double word. When using many relays to do

calculation, such as add/subtraction or shift, you could use byte, word or double word.

Furthermore, the two equipments, timer and counter, in PLC not only have coil but also value of

counting time and times.