Comparison of relay control circuit and PLC ladder diagram

Programming using ladder diagrams requires a certain format. Each ladder diagram is composed of multiple rungs. Each output component can
constitute a rung. Each rung can be composed of multiple branches. Usually, each branch can accommodate multiple programming components (different models
have different quantity limits. ), the rightmost component must be the output component.
    When programming, program the steps step by step in order from top to bottom. The vertical lines on both sides of the ladder diagram are called bus bars. Various symbols of the ladder
diagram should be written horizontally one by one from left to right with the left bus as the starting point and the right bus as the end point (the right bus is usually omitted)
. Whether the input is a switch, button, travel switch, transfer switch, or relay or contactor contact,
it is only represented by a normally open contact or a normally closed contact in the ladder diagram, without considering its physical properties.
    It should be pointed out that the left and right buses in the PLC ladder diagram have lost their power supply significance and only
exist to maintain the shape of the ladder diagram. Therefore, the current in the ladder diagram is called "virtual current" and is not the physical current in the relay control circuit
.     Now take the relay control diagram [Figure 1-16 (a)] as an example to draw the PLC ladder diagram [Figure 1-16 (b)].
    In the ladder diagram shown in Figure 1-16 (b), when input contact 00000 is turned on, the current (virtual current) passes through contacts 00000 (closed), 00001 (normally closed), 00002 (normally closed) from the left side
of the ladder diagram ) and coil 01000, so that 01000 is
powered to work, and the contact of 01000 is closed and self-locking. It can be seen that the control process using PLC ladder diagram and using relay are roughly
the same.