The first programming language, PLC ladder diagram design is key -

PLC ladder diagram is a graphical programming language used most widely, the first programming language is referred to as PLC. Ladder diagram and electric circuit diagram of the control system is very similar, possesses the advantages of intuitive and easy to understand, it is easy to plant electrical staff to grasp, especially suitable for the switch quantity logic control.

ladder diagram is often referred to as circuit or a program, the design of ladder diagram is called programming.

PLC ladder diagram programming, use the following four basic concepts:

soft relay 01 to some of the PLC ladder diagram programming elements used for relay this name, such as input relay, relay output, internal auxiliary relay, etc. , but they are not real physical relay, but some of the storage unit ( Soft relay) Soft relay with PLC controller memory, each image register a corresponding storage unit. The storage unit if as '1' state, has said in the ladder diagram corresponding soft relay coil 'electricity', the normally open contacts connected, normally closed contacts disconnect, says the state is the soft relay '1' or 'ON' state.

if the storage unit as '0' state, the coil and the contact state of the soft relay, contrary to the above, said the soft relay for '0' or 'OFF' status. Use is often called the 'soft relay' programming element.

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can flow is shown in figure 1 contact 1, 2, switched on, there is a hypothetical concept of 'current' or 'energy flow' ( 功率流) Flow from left to right, when the direction of the execution of user programs of logic operation order is the same. Can flow can only flow from left to right. Can flow the concept, can be used to help us better understand and analyze the ladder diagram. There are two possible directions of the figure 1 a can flow through the contact 5 ( Through contact 1, 5, 4, or through contact 3, 5, 2) , can't meet the principle of flow can only flow from left to right, therefore should be changed to ladder diagram as shown in figure 1 b.

03 busbar vertical public line called bus (on either side of the ladder diagram 母线) ，。 On the analysis of the ladder diagram of the logical relationship, in order to borrow the relay circuit diagram analysis method, as you can imagine (left and right sides of the bus Bus left and right busbar) Between a negative dc power supply voltage, left is right between a bus 'energy flow' flowing from left to right. Right bus can not draw.

04 ladder diagram logic decoding

according to the ladder diagram and logical relationship of each contact in the state, the programming of each coil is corresponding with the picture element of state, called a ladder diagram logic decoding.

in ladder diagram logic solution is from left to right, from top to bottom of the order. The result of calculating, immediately can be used for calculating the logic behind the. The values in the logical solution is based on the input image register, rather than according to the calculating the instantaneous state of external input contact.

05 the difference between the PLC ladder diagram and relay control system diagram

with relay control by PLC ladder diagram of the system, you can see the ladder diagram of the graphic symbols and the symbols in the relay circuit diagram is very similar, the structure of the two figure is also very similar, expressed by the logical relationship is consistent, this is because the evolve from relay control diagram and ladder diagram, but in use and has a certain difference, so the ladder diagram and relay control system diagram has both the same and the difference, the difference between the two is as follows.

1) relay control relay used in the figure is the actual physical relay; In the PLC ladder diagram, still continue to use the relay this name, such as: input relay, relay output, intermediate relay and so on, but these are not real physical relay, but the PLC's internal registers, known as 'soft relay'.

(2) relay control diagram connection must be through hard wiring connection between electrical components, to change the control function, must change the actual control circuit wiring; And the PLC wiring is achieved by application of 'soft connection', only need to change the user program, does not need to change the external connection, you can change the control function.

(3) relay control chart in the number of contact is limited, life is limited; And PLC programming each element corresponding to an internal register, its status can be read over and over again in the program, you can think of PLC relay contact number is infinite, there is no limit to the service life of. The composition of 06 ladder diagram

1) bus: ladder diagram to the left of the vertical line known as the starting bus, on the right side of the vertical line called end bus ( Termination of busbar can be omitted) 。 Bus equivalent circuit of the power cord, ladder diagram from the beginning of the bus left, after contact and coil, ending at the right bus.

2. Contact: contact have a normally open contact of ladder diagram and the normally closed contact of two. These contacts can be external contact, also can be internal state of the relay, every contact with a label, contact of the same label can be used repeatedly. Contact placed in the left side of the ladder diagram.

(3) coil, coil in the ladder diagram is similar to the contactor and relay coil, on behalf of the logic of the output as a result, in use by the coil in the same label normally only appear once. Coil placed on the right side of the ladder diagram.

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