Problem Description

Ladder logic and wiring
The signal connections and programming standards vary somewhat between different models of PLC, but the concepts are the same, 
so both power wiring and programming are somewhat generic 
The following illustration shows a simple PLC, as might appear from a front 
view. Two screw terminals, L1 and L2, provide a 120 volts AC connection to supply the internal circuitry of the PLC. Six screw terminals
on the left side allow you to connect input devices, each terminal representing a different input channel with its own "X" label. The lower
left screw terminal is a "common" connection, which is usually linked to L2 terminal (neutral) of 120 VAC power supply.

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Within the PLC, connected between the input terminals and the common terminal, is an optocoupler device that provides a 
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high
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signal to the PLC's internal circuitry when a 120 VAC signal is applied between the corresponding input terminal and the common 
terminal. A LED indicator on the PLC front panel gives visual indication of an energized input:
The output signals are generated by the CPU circuit of the PLC that activates a switching device (transistor, TRIAC, or even an 
electromechanical relay), connecting the "source" to any of the terminal outputs "Y". The terminal "Source", therefore, is usually 
associated with L1 from the 120 VAC power supply. As with each entry, a LED indicator on the PLC front panel gives a visual indication
of an energized output:
The actual logic of the control system is set in the PLC by means of a software. This software determines which output is energized 
under what conditions of entry. Although the program itself seems to be a ladder logic diagram, with the symbols of switches and 
relays, there is no actual switch contacts or relay coils in the PLC to create the logical relationships between input and output. These 
are imaginary contacts and coils. The program is loaded into the PLC and is seen through a personal computer connected to the PLC 
programming port.
To clarify how the ladder logic is related with the PLC wiring consider the following circuit and PLC program:

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When the switch button is not pressed (off), there is no current in the PLC X1 input. The software shows a normally-open X1 contact in 
series with a Y1 coil. While the X1 input signal is not "high" is not sent current to the Y1 coil since the contact is normally open. 
Therefore, the associated output to Y1 remains de-energized and the lamp remains turned off.
If the power button is pressed the current flows through contact, which now changes state to closed, and sent a signal "high" to the PLC
input X1. Each and every one of the X1 contacts appearing in the program will assume the drive (not normal), as if it were relay 
contacts actuated by the excitation of a relay coil named "X1". In this case, the activation of input X1 X1 will normally open contact is 
closed and thus permit the passage of current to the coil Y1. When the Y1 coil program "energizes" the real Y1 output energized, and 
thus the lamp has energy to light.
The true power and versatility of a PLC is revealed when we want to change the behavior of a control system. A PLC is a 
programmable device that can alter its behavior by changing its internal logical instructions without having to reconfigure the electrical 
components connected to it. For example, suppose that what we wanted to do with the lamp was an inverted switching: by pressing the
button to turn off the lamp, and releasing it to turn it on. The "hardware" solution would require that a normally closed switch is replaced 
by a normally open switch. The "software" solution is much easier: simply change the program so that X1 contact is normally closed 
instead of normally open. Besides, since each PLC output is nothing more than a bit in its memory, we can assign contacts in the PLC 
program "commanded" by an output status (Y). Let's Take, for example, a control circuit for the start-stop of a motor:

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The switch button connected to X1 input serves as the "start" switch, while the switch connected to X2 input serves as the "stop". 
Another contact in the program, named Y1, uses the output coil state as a contact seal so that the motor contactor will remain 
energized after the "Start" button is released. In the initial state (sequence 1) we can see the normally closed contact X2 in a color 
block, showing that it is in a closed state ("conducting electricity")..
Pressing the "Start" button (sequence 2) PLC input X1 is energized, so that contact X1 is closed in the program, and thus current to the 
Y1 coil is sent. In this way the Y1 output also is energized and the 120 volts AC are applied to the motor contactor coil. The parallel Y1 
contact is also "closed", which latches the "circuit", ie, if the start button is released, the normally open X1 contact will return to "open", 
but the motor will continue running due to Y1 contact continues providing "continuity" to the Y1 coil current, keeping the Y1 output 
energized (Sequence 3).
To stop the motor, is necessary to press the "Stop" button, which activates the X2 input and open the normally closed contact, breaking
the current continuity to Y1 coil. When the "Stop" button is released the X2 input is disabled, and X2 contact is back to its normal state, 
closed. The motor, however, will not resume until the "Start" button is activated, because the contact that was holding it on, is de-
energized with the circuit continuity break, when is pressed the Stop button. 

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