D-14
D.3 Ladder Diagram
D.3.1 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.)
Y0
X0
X1
Y0
Start
M100 X3
Y1
X10
:
:
X100 M505
Y126
End
Send the result to the output point
Read input state from outside
Execute in cycles
D.3.2 Introduction
Ladder diagram is a diagram language that applies 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 accepted 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.
In conclusion, each internal storage unit occupies fixed storage unit. When using these
equipments, the corresponding content will be read by bit, byte or word.
