# What is a ladder diagram? Ladder diagram programming

Ladder diagram is the most used graphic programming language, it is called the first programming language of  PLC. Ladder diagram and electrical control system circuit diagram is very similar, with intuitive and easy to understand the advantages, it is easy to be mastered by the factory electrical personnel, especially suitable for switching logic control. Ladder diagram is often referred to as the circuit or program, the design of ladder diagram is called programming.
Ladder diagram programming, use the following four basic names:

## 1 ,Soft Relay

Some of PLC ladder diagram programming element follows the name of the relay, such as input relay,output relay, internal auxiliary relay, etc., they are not real physical relays, that are  some storage unit (soft relay), a storage unit for each soft relay and PLC memory image register. The storage unit if the “1” state, the ladder diagram of the corresponding soft relay coil energized, the normally open contact is turned on, normally closed contact is disconnected, this is the soft relay “1” or “on” state. If the memory cell is “0”, the state of the coil and contact of the corresponding soft relay is in contrast to the above, this is  soft relay is “0” or “OFF”. The use of these “soft relay” is often called to as the programming element.

## 2,Flow Power”

Figure, contacts 1 and 2 is switched on, there is a supposed concept of current or Power Flow), flowing from left to right, the direction and execution when a user program is consistent with the order of the logical operations.

Power flow can only flow from left to right. Using the concept of energy flow, can help us to better understand and analyze the ladder diagram.

Figure, maybe two  directions of power flow through the contacts 5 (through contact points 1, 5, 4 or contact points 3, 5, 2), which do not meet the principles of energy current can only flow from left to right

## 3,Bus bar

Ladder on both sides of the vertical line is called public (Bus bar) ,. In the analysis of ladder logic, in order to borrow the relay circuit analysis method, there are conceivable between the left and right side bus (left bus bar and right bus bar) a left and right positive negative DC supply voltage, the “power flow between bus bar from left to right. You can not draw right bus bar.

## 4,The ladder logic solution

According to the state and the logic relation of each contact in the ladder diagram, the state of the programming elements corresponding to each coil in the diagram is obtained,is called  logical solution of ladder diagram.

Ladder logic solutions were considered by from left to right, top to bottom sequence. The results of the resolver, can be immediately utilized by the logic solver behind. – Logic be based on the input image register values, rather than resolver the instantaneous state of the external input contacts carried out.
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