Basic Program Instructions 2
2-1
2. Basic Program Instructions
2.1 What is a Program?
A pro
ram is a connected series of instructions written in a lan
ua
e that the PLC can
understand. There are three forms of pro
ram format; instruction, ladder and SFC/STL. Not all
pro
rammin
tools can work in all pro
rammin
forms. Generall
hand held pro
rammin
panels onl
work with instruction format while most
raphic pro
rammin
tools will work with
both instruction and ladder format. Specialist pro
rammin
software will also allow SFC st
le
pro
rammin
.
2.2 Outline of Basic Devices Used in Programming
There are six basic pro
rammin
devices. Each device has its own unique use. To enable
quick and eas
identification each device is assi
ned a sin
le reference letter;
- X: This is used to identif
all direct, ph
sical inputs to the PLC.
- Y: This is used to identif
all direct, ph
sical outputs from the PLC.
- T: This is used to identif
a timin
device which is contained within the PLC.
- C: This is used to identif
a countin
device which is contained within the PLC.
- M and S: These are used as internal operation fla
s within the PLC.
All of the devices mentioned above are known as ‘bit devices’. This is a descriptive title tellin
the user that these devices onl
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
