Programming instructions

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LESSON 6 – System Devices
To write a program for a PLC, it is necessary to be familiar with the devices that are
used in the instructions. An overview is provided here, with more detailed information to
follow in later lessons.
Lesson Objectives: At the conclusion of this lesson, you will be able to…
9 Name and describe the devices used to make a program.
Materials: FX-Series PLC Training Manual
A common question when discussing system devices is the number of each that
is available to use. This varies depending on the CPU. Make sure to check the
user’s manual for the model of CPU under question.
X – Physical Inputs
X addresses are used to designate physical inputs.
Y – Physical Outputs
Y devices are used to designate physical outputs.
M – Auxiliary Relays
M relays are internal bit devices. They are internal bits that can be used
for any function needed. When an M device coil is energized, the
corresponding M device contact becomes active.
In GX-Developer, it is possible to configure M bits to be battery-backed.
This means the bits will maintain their current state (Off or On) in the event
of power loss.
There is a group of M devices that have dedicated functions. They are
M8000 ~ M8511. The meanings are described in tables in chapter 36 of
the FX3U Programming Manual.
S – State Relays
S relays are internal bit devices. They are used in STL programming to
indicate which step, or section of ladder logic code, is active. If STL
programming is not used, these bits can be utilized in the same manner as
M bits.
In GX-Developer, it is possible to configure M bits to be battery-backed.
This means the bits will maintain their current state (Off or On) in the event
of power loss.