High and Low

Real easy one. After 4 years, I finally got the XIO,XIC, True and False thing down. Now, everyone is using the terms High and Low. Some one want to explain in real easy terms the difference? See, real easy.

Thanks, Olias

Origin of High Low

High and Low come from Digital Electronics / early tube driven Computers. I might be wrong, but I think this is correct.

When you want to state that 5 Volts is present to the input of a device (gate). You would say that there is a “High” at the input to the device. People who speak the secret language of Electonese, know that when you say “High”, it means 5 volts, and when you say “Low” it means 0 volts, with regards to older digital electronic systems.

0 Volts for "Low" and 5VDC for High was used for TTL Logic (Transistor to Transistor Logic). In reality anything above 4.3 VDC was considered High for TTL Logic.

In CMOS component based circuits (Capacitive Metal Oxide on Silicon), running around 3.3 VDC. A high would be > 2.9 VDC.

In a 24 Volt System, a high may be > 21V.

Here is a link to some resource for digital electronics.

http://www.play-hookey.com/digital/

I hope this was easy enough.

Even though we all speak English, we don't speak the same language

There are many terms that mean the same thing, or almost the same thing. And so, different people will use them interchangably.

Terms that mean that a discrete/digital/bit/boolean/binary register/address/tag/memory location has a "one" in it:
True
Set
Closed
High
Made


Terms that mean it's "zero"
False
Reset
Open
Low
Unmade

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This of course, has no bearing AT ALL to the meaning of the -| |- and -|/|- symbols.

In relays and other input devices, -| |- means Normally Open (or N.O.). When connected to a PLC, this will make that register/address/etc. into a "one" (or any other similar word) when the relay coil (or transistor equivalent) has some minimum threshold energy at it.

In ladder logic, -| |- (which in Allen-Bradley, and only Allen-Bradley, is abbreviated XIC) is a logical question, asking if there's a "one" (or equivalent) in that register. If the answer is "Yes"/"True"/"It is", then the instruction "solves as true", and so "passes power". The fact that this instruction is asking a question is exactly why Allen Bradley calls it XIC - "eXamine If Closed" (or "'Check' if 'True'", or "'See' if 'it equals one'", or however you want to phrase it).

Again, the relay -| |- and the ladder logic -| |- have nothing to do with eachother, other than a shared symbol and a shared address. The relay -| |- puts the energy there; the ladder -| |- reads to see if the energy is there.

Clear as Mudd?

But it's contect sensitive...

It depends on what you are talking about. You can have a high voltage level that is a false logical condition and a low voltage level that is a true logic level. If the input device is a sourcing style input this will be the case. the same is true of sinking style outputs.
So a statement like 'Low is logic level 1, high is logic level zero' is perfectly valid.Unfortunately you need to know the context that high and low are used in to know what is meant.

Keith

I worked on an old Westinghouse TTL process control system where a logic "1" was 0 VDC and a logic "0" was 5 VDC. We ripped them out and replaced with GE Series 6. Just when you thought it was safe to assume....

Thanks to all. Allen read though my mush. Yes, I did mean in Allen Bradley PLCs. Seems thats the term the reps like to use now. Just when I got used to using XIC and XIO, they start using High and Low.

Thanks for the clearer then mud replys. ; )

Olias

I worked on an old Westinghouse TTL process control system where a logic "1" was 0 VDC and a logic "0" was 5 VDC. We ripped them out and replaced with GE Series 6. Just when you thought it was safe to assume....

this is due to current savings maintaining a digital high 4.3 volts to 5 volts requires less current than digital lows 0 to 0.8 volts. in TTL logic. I know this sounds backwards however it was the theorem that was going around in 1992. which is why you will notice that the normal state in PCs usually maintain a high at all gates rather than a digital low

Olias,

"After 4 years, I finally got the XIO,XIC, True and False thing down."

If you did, you are doing better than most of us. I did not realize how confusing this is until I recently taught a PLC class at the local community college. No, I am not a teacher and this is the first and last time!

What I finally told the class that they could remember:

XIC: What you "C" (at the Input) is what you get!

XIO: What you see (at the Input) is Opposite of what you get!

Re: Origin of High Low

Hester said:

Here is a link to some resource for digital electronics.

http://www.play-hookey.com/digital/

I hope this was easy enough.

Click to expand...

Very nice link, thanks!


John

Its only "open" and "closed" that confuses.

To my opinion, the following ought to be readibly understandable:

ON = TRUE = HI = SET = 1 etc..
OFF = FALSE = LO = RESET = 0 etc..

The confusion comes from the terms "open" and "closed".

Electrical persons are used to refer to these in the meaning CLOSED = ON. This because we are imagining an electrical contact where the "connection is made when the contact is closed".

Non-electrical people (poor them) will refer to CLOSED as OFF. Imagine a watervalve, an airvalve, or just a door.

I agree. Electricians who know ladder logic, the hardware version, have the hardest time with the software version. I know, i is one of dem. It took me a long time to understand the open / close thing. I realized that the normally open symbol in the logic means true or one. Forget about the field. Some controllers even have hardware inverters on the input card just to add to the confusion. Of course, I discovered that if the field device is inverted the software can actually makes sense. For example the bit tagged "Lube Oil Pressure" will be a one anytime there is pressure. I no longer need to know the state the field device until I determine in the logic that there should be lube oil pressure and the Lube Oil Pressure bit is false. Likewise, The bit tagged "No Lube Oil Pressure" would be true when there is no lube oil pressure. So for me it's the logic, and that is handled separately. To understand the logic all one needs to know is that this symbol means "true" and that symbol means "false". I write my logic and tags to this convention. I condition the inputs at the front of the program or in a subroutine so that the logic can be read without knowing any field contact states. (Mainly so I can figure out what I have written). And once I know I have a field contact problem I know where to go to look at them.

Now to understand that a hardware ladder is a giant parallel circuit and the software isn't....

Andy

Regarding the -] [- symbol and the -]\[- symbol and their relation to the input status, my experience with real-world relays has HELPED my understanding in PLCs. I simply imagine the input card as containing the COILS of gigantic relays with an unlimited supply of normally open and normally closed contacts. Therefore the program is referencing those contacts just as in a standard wiring diagram with no confusion (except of course for the whole scanning thing).

I understand that the external device is either turning on or not turning on that mythical coil. Once that is resolved then I'm just looking at the contacts. For help I name the input by the state when the 'coil' is ON. By that point all considerations of the external 'Normally Open/Held Closed' or whatever other complications have been resolved by the simple on/off state of the 'coil' of the input.

I like that analogy! I never thought of the input as a coil, only as a contact. That makes good sense.

Andy

bernie, andy,

I see that some of you still do not understand the whole issue of why the XIC and XIO are confusing to most programmers. You are still missing the point: The internal contact instructions CANNOT be thought of as being the same as the external input contacts...at least, one of them can't but the other one can. I leave it to the students to figure out which is which.....

Here is a question: If a schematic wiring diagram shows a Stop pushbutton contact as N.C. (normally closed), then what PLC instruction will be necessary to stop a motor starter using the pushbutton as an Input?

According to bernie's thinking

(Quote: "Therefore the program is referencing those contacts just as in a standard wiring diagram with no confusion..."),

the answer to the above question would be the instruction that APPEARS to be a NC contact, the Examine If Open instruction. Of course that is totally backwards; it is really the Examine If Closed (that APPEARS to be a NO contact) that would be used in the PLC program to stop the motor starter. XIC: What you "C" is what you get, meaning that if the physical Input is ON, then the PLC instruction will also be ON, and vice versa.

High & Low Disorder

If you device goes HI then LOW then HI, altering state often, then it may be BIPOLAR. For more information on BIPOLAR Devices click here .