Registers

Having trouble understanding the basic register that is depected in the tutorial. If SW2 as a logic of 1 (true?) and SW1 has a logic of 0 then output 0500 will not operate. I understand the concept that the plc will only energize an output when all conditions on the rung are true, BUT how does SW2 demand a logic of 0 to become true?
SW2 is a NC input and in my mind it must have a logic of 1 to stay true.

Help pound this into my brain. Inverse thinking required?

Think in terms of the signal at the input to the PLC, not in terms of the switch that is connected to the PLC.

For example, a NC pushbutton will feed power to a PLC input (ie, the input is on) until the button is pushed.

-] [- is true when there is power at the assoicated input terminal of the PLC

-]\[- is true only when there is NO POWER at the associated input terminal of the PLC.

Helps if we know you are talking about this:

http://www.plcs.net/chapters/register11.htm

"BUT how does SW2 demand a logic of 0 to become true"

Because the logic is "IF SW2 = 0" NOT "IF SW2 = 1".

If we write the ladder logic as english statement:

COIL = (IF SW1 = 1 AND SW2 = 0)

Therefore, SW2 must equal 0 for the coil to turn on. Better?

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.

One tip that helps me, in additions to Terry's great comments, is to use whatever nicknaming feature is available and to make sure the name is the state when the input is turned ON. This make take some imagination in the case of a NC type device, but then, when nicknames (or whatever they are called) are viewed with the ladder it reads in the same manner as the logic.

Thanks folks for the replies. I should not mention it but I'm dealing with an older version of a brain, but I think I'll figure this stuff out sooner or later. I'll study the replies the next few days and see if it sticks. Worse case for me is being on my death bed and still tryig to comprehend NO and NC contacts with true or false bits on the register. Maybe old electrcians should just short themselves out and thereby become a NC set of contacts in a flash.

WS