Just being my usual anal-self...
SW-1 SW-2
---| |-------|/|-------( COIL )
.
"If we write the ladder logic as english statement:"
While both, the signal from SW-1 is ON (1) AND
the signal from SW-2 is OFF (0)... then, turn ON the Coil.
"While both..." means "For as long as..." such-n-such condition(s) exist, do this-or-that.
If the required combination of conditions no longer exists... then don't do this-or-that.
The point of that is, in this case, turning on the coil is NOT like turning on the kitchen light!
Turning on a kitchen light switch corresponds to SET or LATCH.
I put stress on "the signal" because it is the state of the signal(s) that determines the state of the coil.
What's the point of that?
First... the switch in the field might, or might not, be labeled correctly. In fact, the switch in the field might not be labeled at all!
In the States, we consider a wall-switch to be on if it is in the up-position. In Ireland, the wall-switch is considered to be on if it is in the down-position.
The supposed (observed) "state of the switch" might not match the state of the signal. That is, if it is supposed that "up is on", and the switch is up... then the assumption is that the signal should be on. That should be true here in the States... not so in Ireland.
Regardless of what the state of the switch appears to be, it is the "state of the signal" that matters.
Then there is the issue of devices with Normally Opened and Normally Closed contacts.
Granted, this might be a lot of confusing information for a novice. However, the sooner the novice recognizes that these issues exist, and recognizes the fact that it is the "state of the signal" that determines the condition being presented by the switch, the sooner he will come to terms with the most common problem that novices must overcome.
That problem is the falsely supposed fixed-relationship between the
--| |-- symbol and the Normally Open contact on a switch, and the
--|/|-- symbol and the Normally Closed contact on a switch.
The sooner the novice can recognize that those issues exist... the sooner he can come to terms with it. The longer this is delayed, the harder it becomes to untangle the misconceptions.
Remember...
--| |-- and
--|/|-- are simply direct QUESTIONS!
--| |-- is simply asking... Is this SIGNAL ON?
--|/|-- is simply asking... Is this SIGNAL OFF?
Inside the PLC, 1 = ON, and 0 = OFF.
You then have to ensure that the device in the field is presenting the proper signal-state for each of the possible switch-conditions ("Acted-On", or "Not-Acted-On").
The next most important thing to learn is how to properly name the signals.
In a real process, names like "SW-1" and "SW-2" simply won't cut it.