Extreme Newbie Question: Registers

I've started the online tutorial and have a question regarding registers and ladder logic. The section regarding "PLC Registers" states that the "true/false" status is stored in the registers. Later on the same page, it states that the "logic" state is stored. In my understanding, the first refers to the "electrical conductivity" of the device or symbol whereas the second refers to the "energized" status of the device. These are identical for "load" and "out" but reversed for "loadbar" and "outbar". What is really stored, conductivity or energized state?

The "Level Application" in the tutorial uses "loadbar" not
"load" for the two inputs; however, the ladder logic diagram uses "load" not "loadbar". This is confusing.

A PLC register in this case stores a value of zero or one. Zero, OFF, and False are synonymous. One, ON, and True are synonymous.

The PLC instruction -] [- asks the question "Is the register value equal to 1 (ON, True)"? If the answer to the question is yes, then the -] [- instruction is True (ON, 1).

The PLC instruction -]/[- asks the question "Is the register value equal to 0 (OFF, False)"? If the answer to the question is yes, then the -]/[- instruction is True (ON, 1).

This takes a little time to get your brain around. Keep plugging away. It took all of us some time before it sank in.

When it comes to control signals, I have a problem with the "register" terminology.

A "register" is a physical device in the processor that holds data for subsequent processing.

When it comes to control signals, I prefer the idea of "bit-status" where the "bit" refers to a particular Input, Output, or Control Relay.

In light of that...

The ---| |--- element is asking... is the particular bit ON?
If it is ON then the answer is TRUE and the logic proceeds through that element.
If it is OFF then the answer is FALSE and the logic does NOT proceed through that element.

The ---|/|--- element is asking... is the particular bit OFF?
If it is OFF then the answer is TRUE and the logic proceeds through that element.
If it is ON then the answer is FALSE and the logic does NOT proceed through that element.

Like Steve says... it takes a while to internalize this concept.

a good thing to do is go back and learn about 8 bits in a byte,,2 bytes in a word,,etc,,then start working with some boolean logic and get that out of the way,,it is a much easier segway into plc logic(errr,,im going to regret saying that),,

once you have the bits and bytes all together in your head,,registers,,bitwise logic,,etc all falls into place easier

Fred Raud

Terry Woods said:

A "register" is a physical device in the processor that holds data for subsequent processing.

Click to expand...

Not only physical, I would say.
The same definition works from the software point of view.
What is important to realize is that a register value may have different meanings. It may be numeric data in one of many formats (decimal, BCD, hex), or symbol code, or simply set of bits.
Also, in integer numeric data some bits have certain meaning.
The bit 0 indicates that the value is odd and the highest bit indicates that the value is negative.
Be aware, most but not all PLC allow data register addressing on bit level. For example, Mitsubishi does not.

Do you not get the sense, even slightly, that there is something oxymoronic about... "not all PLC allow data register addressing on bit level."

If you reference a particular bit in a byte, word, double-word etc., then you are simply referencing a particular "room" in a particular "hotel"... at address such-n-such.

In human-speak, you might reference first the Room Number, then the Name of the Hotel, or you might reference first, the Name of the Hotel, then the Room Number.

As in... "See if someone is in Room 12, at the Hilton on 3rd."
Or... "See if someone is at the Hilton on 3rd, in Room 12."

Now, in this crazy world of computers... if you (the processor) want to access a particular "room" in a particular "hotel", you reference first the "hotel", then the "room". That is, first you specify the address of the Hotel, then you specify the Room Number.

But wait... there's even more craziness...

In computers... the moutain DOES come to Mohammed!

If you want to see if anyone is in Room 12, at the Hilton on 3rd... you do NOT go to the "hotel"!

Instead, you bring the "hotel" to you (the processor)! And you place it in a real, physical place, where you can even touch it (at least you could in the old days before processors were compressed onto single chips). That place is called a "Register".

Once the contents of the "hotel" is loaded into the "register", you can access any room in the register, really, you can.

Now, as Sergei said, some PLCs don't allow you to just walk into any room at the hotel to see if someone is there... however, there are ways.

You can use a Mask (like a crook, maybe?) and extract a particular room from a hotel. Then you force the occupant, if any (or at least a copy of him, if any), to move to the same room number in a different, empty hotel. You can then evaluate the contents of the "empty" hotel to see if anyone is there. If the hotel shows empty, then the original room in the original hotel is unoccupied. If the "empty" hotel is no longer empty, then the original room in the original hotel is occupied!

At that point, that information is available to anyone, as long as that information is not wiped.

Yeah... it might be a cute story, but the point is, strictly speaking, the address of the Hotel is NOT a register. It is simply the address of the hotel. The contents of any hotel can not be examined until the hotel is "brought" to a "register" where the contents can be examined directly, or otherwise operated on.

If you want to call the address of the hotel a register, fine... live in ignorance and bliss... all is happy... whether you really understand what is happening, or not.

To call the address of the hotel a register hides what is really happening... like in the Matrix... things ain't necessarily what they seem!

What's really happening?

When you KNOW what's really happening, you KNOW how to manipulate what happens!

Seems like a small (semantic) difference to some... but the ramifications are significant!