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Basic Program Instructions 2
2-1
2. Basic Program Instructions
2.1 What is a Program?
A pro
g
ram is a connected series of instructions written in a lan
g
ua
g
e that the PLC can
understand. There are three forms of pro
g
ram format; instruction, ladder and SFC/STL. Not all
pro
g
rammin
g
tools can work in all pro
g
rammin
g
forms. Generall
y
hand held pro
g
rammin
g
panels onl
y
work with instruction format while most
g
raphic pro
g
rammin
g
tools will work with
both instruction and ladder format. Specialist pro
g
rammin
g
software will also allow SFC st
y
le
pro
g
rammin
g
.
2.2 Outline of Basic Devices Used in Programming
There are six basic pro
g
rammin
g
devices. Each device has its own unique use. To enable
quick and eas
y
identification each device is assi
g
ned a sin
g
le reference letter;
- X: This is used to identif
y
all direct, ph
y
sical inputs to the PLC.
- Y: This is used to identif
y
all direct, ph
y
sical outputs from the PLC.
- T: This is used to identif
y
a timin
g
device which is contained within the PLC.
- C: This is used to identif
y
a countin
g
device which is contained within the PLC.
- M and S: These are used as internal operation fla
g
s within the PLC.
All of the devices mentioned above are known as ‘bit devices’. This is a descriptive title tellin
g
the user that these devices onl
y
have two states; ON or OFF, 1 or 0.
LD
OUT
AND
SET
LD
OUT
X10
Y7
M38
S5
X21
T01
K40
Instruction format Ladder Format SFC Format
Detailed device information:
• Chapter 4 contains this information in detail. However, the above is all that is
required for the rest of this chapter.
FX Series Programmable Controllers