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Basic Program Instructions 2
2-1
2. Basic Program Instructions
2.1 What is a Program?
A program is a connected series of instructions written in a language that the PLC can
understand. There are three forms of program format; instruction, ladder and SFC/STL. Not all
programming tools can work in all programming forms. Generally hand held programming
panels only work with instruction format while most graphic programming tools will work with
both instruction and ladder format. Specialist programming software will also allow SFC style
programming.
2.2 Outline of Basic Devices Used in Programming
There are six basic programming devices. Each device has its own unique use. To enable
quick and easy identification each device is assigned a single reference letter;
- X: This is used to identify all direct, physical inputs to the PLC.
- Y: This is used to identify all direct, physical outputs from the PLC.
- T: This is used to identify a timing device which is contained within the PLC.
- C: This is used to identify a counting device which is contained within the PLC.
- M and S: These are used as internal operation flags within the PLC.
All of the devices mentioned above are known as ‘bit devices’. This is a descriptive title telling
the user that these devices only 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