BASIC INSTRUCTIONS : PLC TUTORIAL- 8

Now let's examine some of the basic instructions is greater detail to see more about what each one does.

Load
The load (LD) instruction is a normally open contact. It is sometimes also called examine if on.(XIO) (as in examine the input to see if its physically on) The symbol for a load instruction is shown below.

This is used when an input signal is needed to be present for the symbol to turn on. When the physical input is on we can say that the instruction is True. We examine the input for an on signal. If the input is physically on then the symbol is on. An on condition is also referred to as a logic 1 state.

This symbol normally can be used for internal inputs, external inputs and external output contacts. Remember that internal relays don't physically exist. They are simulated (software) relays.

Load Bar
 
The LoaD Bar instruction is a normally closed contact. It is sometimes also called LoaD Not or examine if closed. (XIC) (as in examine the input to see if its physically closed) The symbol for a load bar instruction is shown below.

 
This is used when an input signal does not need to be present for the symbol to turn on. When the physical input is off we can say that the instruction is True. We examine the input for an off signal. If the input is physically off then the symbol is on. An off condition is also referred to as a logic 0 state.
This symbol normally can be used for internal inputs, external inputs and sometimes, external output contacts. Remember again that internal relays don't physically exist. They are simulated (software) relays. It is the exact opposite of the Load instruction.


*NOTE- With most PLCs this instruction (Load or Load bar) MUST be the first symbol on the left of the ladder.

Logic State	Load	LoadBar
0	False	True
1	True	False

Out
The Out instruction is sometimes also called an OutputEnergize instruction. The output instruction is like a relay coil. Its symbol looks as shown below.

When there is a path of True instructions preceding this on the ladder rung, it will also be True. When the instruction is True it is physically On. We can think of this instruction as a normally open output. This instruction can be used for internal coils and external outputs.

Out bar
The Outbar instruction is sometimes also called an OutNot instruction. Some vendors don't have this instruction. The outbar instruction is like a normally closed relay coil. Its symbol looks like that shown below.

When there is a path of False instructions preceding this on the ladder rung, it will be True. When the instruction is True it is physically On. We can think of this instruction as a normally closed output. This instruction can be used for internal coils and external outputs.

It is the exact opposite of the Out instruction.

Logic State	Out	OutBar
0	False	True
1	True	False

A Simple Example

Now let's compare a simple ladder diagram with its real world external physically connected relay circuit and SEE the differences.

In the above circuit, the coil will be energized when there is a closed loop between the + and - terminals of the battery. We can simulate this same circuit with a ladder diagram. A ladder diagram consists of individual rungs just like on a real ladder. Each rung must contain one or more inputs and one or more outputs. The first instruction on a rung must always be an input instruction and the last instruction on a rung should always be an output (or its equivalent).

Notice in this simple one rung ladder diagram we have recreated the external circuit above with a ladder diagram. Here we used the Load and Out instructions. Some manufacturers require that every ladder diagram include an END instruction on the last rung. Some PLCs also require an ENDH instruction on the rung after the END rung.
Next we'll trace the registers.


Registers? Let's see...


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