New Figure 3       (05/22/14)

In TTL logic chips, they have "and" gates, "or" gates, and they have inverters.  
An inverter changes a 1 to a 0; a true to a false; a high to a low; an on to an off.   
We can have the inversion function in ladder diagrams by using n.c. contacts of a relay.  You put turn-on current through the relay coil, and it energizes, and you use a set of its n.c. contacts to turn something off.  
But when the coil deenergizes, those n.c. contacts turn that something on.  


A question might be, "If the software I'm using to create the ladder diagram only allows six sets of contacts on a rung to the left of the coil, and I had more than six conditions I wanted to put on that rung, what do I do?"
No problem.  Put six of your conditions on a rung, and then from the coil on that rung, put a set of n.o. contacts on a new rung.  Now on the new rung, add your additional conditions.  
Still not enough room?  Add a third rung in the same way.  


Figure 3 is a ladder diagram for using a PLC to control an elevator.  

K4 and K5 are physical relays for Down and Up, respectively, because their n.o. contacts are connected to output terminals of the PLC to turn on the Down and Up contactors for the three phase motor that will move the elevator down or up.  
Three phase power is normally used for large horsepower motors such as those used for elevators.  Heavy duty contactors (large relays) are used to turn on and off the large currents to those motors.  
At a place where I worked, we had an abrasive cutoff wheel that we didn't use a lot.  So it normally wasn't connected to power until we needed it.  
A three phase motor was used to spin the abrasive cutoff wheel.  
If you want to use the wheel, you turn off the power source that you're going to connect to (be sure to use the right voltage), and you connect the three power wires to the device, plus the green ground wire for safety.  
Now you turn on the cutoff wheel.  If it spins in the wrong direction, you turn off the power source, and swap any two of the three power wires.  
That's how you reverse the direction of spin of a three phase motor.  
Your elevator up and down contactors are doing that for you.  
K1, K2, and K3, however are "soft" relays kept track of by bits in memory in the PLC.  
K1 "ands" together switches S4, S5, and S6, which are the three door closed switches.  Unless all three doors are closed, the door on the Ground floor, the door on the 2nd floor, and the door on the elevator car, you can't move the elevator up or down.  
N.O. contacts of K1 are in series with the coils of K4 and K5 to prevent going up or down unless all three doors are closed.  
An n.c. set of contacts of K5 is in series with the K4 coil, and a n.c. set of contacts of K4 is in series with the K5 coil to help ensure that both K4 and K5 don't energize at the same time, because if both three phase contactors energized at the same time, there would be a horrific short circuit, probably destroying the contactors, and perhaps doing a lot of collateral damage.  
There are commercially available three phase forward/reverse motor contactors that come in pairs to control three phase motors.  These generally have a mechanical interlock that won't allow the armatures of both contactors to go to the energized position at the same time.  
But if some Electrician just wired up two contactors to control the motor, then that mechanical interlock may not be present.  
Switch S7 controls relay K2.  S7 is the limit switch that, when its n.o. contacts close says that the elevator car is present at the Ground floor.  S7 and S8 are mounted on the elevator shaft wall, and are operated by a cam on the elevator car.  
When the S8 n.o. contacts close it says that the elevator car is present at the 2nd floor, and relay K3 energizes.  
S1, S2, and S3 are pushbutton switches with n.o. contacts.  When a finger presses the pushbutton, the n.o. contacts close.  When the finger is removed, a spring pushes the contacts back to the open position.  
S1 is the switch for calling the elevator to the Ground floor.  
S3 is the switch for calling the elevator to the 2nd floor.  
In parallel with S1 and S3 are "holding contacts" of the relay that they're energizing, which have the same effect as keeping the finger on the pushbutton.  After you energize K4 or K5, they latch themselves with their holding contacts until their coils drop out because of K2 or K3 n.c. contacts opening.  
S2 is the switch for the passengers on the elevator to use to go to the other floor.  No holding contacts in parallel with S2 because the holding contacts shown at S1 or S3 will do the job.    
N.O. contacts of relay K2 and K3 will cause S2 to energize either K4 or K5.  Whichever floor you're at, when you press S2, you go to the other floor.
N.C. contacts of K2 or K3 deenergize K4 or K5 when the elevator car arrives at the destination floor.