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Old January 29th, 2003, 11:39 PM   #1
Phil Melore
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Normally open or closed or...

Hi folks,

As discussed a while ago, the normally open/closed feedback newsletter issue has been distributed. (should arrive in the coming hours)

This thread is for further comments as discussed in the newsletter.
(free newsletter subscription available here if you don't yet subscribe)

For reference, the thread from where the credited posts come from is Here

Discuss away...

Enjoy,
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Old January 30th, 2003, 02:22 AM   #2
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It is best to keep it seperated from the actual input real world operation and use ONLY a POSITIVE or NEGITIVE logic concept for all. Sink or source does not matter and in the case of POS / NEG DC inputs maybe mixed.

Myself I use POSITIVE, if the discrete input is ASSERTED ( active, if you like ) the open ladder contact is closed to give power flow left to right through the contact.

In the case of say a standard N.C. Emergancy STOP button, the normally asserted input is thus called "NOT" STOP:
____
STOP or /STOP and the logic scheme is positive.

In most cases this would be used as a N.C. contact in the ladder and would open when the N.C. STOP button is pressed.

In the case of a standard N.O. momentary Start buton, the input is asserted when the button is pushed and thus named:

START and the logic scheme remains positive.

In most cases this would be used as a N.O. contact in the ladder and would close when the START button is pressed.
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Old January 30th, 2003, 04:21 AM   #3
Steve Clanton
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I am kind of biased because I used relay logic before I ever saw ladder logic, but I lean more toward the normally open and normally closed. That is how I was taught and for an electrician talking about true and false is new to their vocabulary.

I think the phrase "held open" or "held closed" is helpful for explaining the limit switch or other input that not in its normal position normally {i.e. when nothing is happening the machine holds it out of position or the dark sensing photoeye example}.

The way I think of it is as a contact that is in that position when power is not applied. Said the other way, it changes when power is applied.
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Old January 30th, 2003, 08:28 AM   #4
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The most important thing in designing a control system is to make sure it is safe!. To make a field device failsafe, it should be wired closed (on) in the "PROCESS" normal condition whis is different from the "shelf state" elecrtical normal condition. Field devices that are closed during process normal conditions will then change state when there is a process problem or if the field wiring becomes damaged. A save control system is designed to take care of all the little what if? possibilities. Motor controls should be a permissive to run, not a command to stop. If someone disconects the wrong wire, a control system should go to the safe condition, if not, thing go boom in the process.
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Old January 30th, 2003, 09:19 AM   #5
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--| |-- : This is NO contact. Similar to a Pushbutton which has a normally open contact. Use this when an output operation has to get operated when you press the pushbutton.

There is one more case. If the pushbutton has a NC contact then use this symbol so that output operation does not get operated when you press the push button ie circuit is open.

--|\|-- : This is NC contact. Similar to a pushbutton which has a normally close contact. use this when an output operation has to get operated when you do not press the button.

There is one more case. If the pushbutton has a NO contact then use this symbol so that output operation does not get operated when you press the push button ie circuit is open.
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Old January 30th, 2003, 11:06 AM   #6
rytko
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Sometimes electricians used to thinking in terms of relay ladder logic have trouble dealing with this whole issue because they try to program the logic the same way they would hard wire the system, but they have to keep in mind how the presence/absence of the signals on the inputs is affected by the field device wired to the input.

I separate the action of the field devices from the action of the PLC bits. I tell the techs I train to first pay attention to whether or not a signal is present on the input. A signal present makes the NO bit true (or allows power through it); a signal absent makes the NO bit false (or blocks power flow). Vice versa for the NC bit.

At that point, they should pay attention to the type of switch in the field. If a NC switch, a signal will be present on the input assigned to that device until the field device is activated. If a NO switch, a signal will be absent on the input assigned to that device until the field device is activated.

The NO/NC status of a switch often confuses people new to the field until they figure out that the state of the device determines the switch position. The "Normal" position is the position the switch is in when the device is deenergized/not active (i.e., the shelf state). It is not the position the switch is in "when the system is operating normally."
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Old January 30th, 2003, 01:43 PM   #7
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Cool NO/NC Contacts

This subject can be confusing because there are three simultaneous functions going on in regards to use of NO/NC contacts.

In the first NO/NC function, there is identification of the state of the field devices, which by de Facto standard, is the shelf state(the state of the switch before it has been installed is a way to look at this)or you can look at it as the non-powered state. For a switch, NO means that the switch does not have electrical continuity in the shelf state while NC means that the switch does have electrical continuity in the shelf state.

In the second NO/NC function, there is the state of the "virtual" switch contact in the PLC logic when the switch is connected to a PLC input card. The PLC does not really have a "self state", however, the use of the non-powered state definition of NO/NC applies. If the power is off to the PLC input, the NO PLC contact does not pass power in the PLC logic while the NC PLC contact will pass power. This description is easily understood by looking at the PLC input as a virtual relay with virual contacts in the PLC where the virtual relay's power off state or "shelf state" is when there is no power from the field switch. Another different perspective is that the NO contact tracks the field input(power on for the field input means power on for the NO PLC contact) while the NC contact is the opposite to the field input(power on for the field input means power off for the PLC NC contact). For failsafe designs(see below), the general practice(and a good engineering practice) is that the PLC contacts will track the field contacts because the logic will be designed similar to the way the logic would be done with descrete relays, i.e. serial or AND logic and it is generally how people think. There may be auxillary logic where the PLC contacts will be either NO or NC.

In the third NO/NC function, there is the state required for proper operation of the system logic which consists of the field input devices, field output devices, and the logic solver(PLC). This involves choosing which contact(NO or NC) to use for the field device, what contacts to use in the PLC to achieve the logic function desired, and what the output contacts should be. In general, there are two types logic used, failsafe(normal state is energized or de-energize trip) and non-failsafe(normal state is de-energized or energize to trip). By far and away, failsafe is the most common. For failsafe operation, the field switch position(NO or NC) is choosen based on the switch being energized ie. the ON state(electrical continuity) under normal operating conditions. For example, for a low flow switch, a NO field switch position is selected because the NO switch is closed or energized in the normal flow condition and is open or de-energized in the low flow condition which represents the abnormal or alarm state. For a high flow switch, a NC field switch position is selected because the NC switch is closed or energized in the normal flow condition and is open or de-energized in the high flow condition which represents the abnormal or alarm state. This logic applies whether it is flow, level, temperature, pressure, etc. In the PLC, the PLC selection of NO/NC is the same field position (when the field and PLC switch/contacts track), ie. for a low flow switch a NO contact is used. In another example, for a start button which is in the field normally a NO would be a NO contact in the PLC and a stop button which is a NC contact would be a NC contact in the PLC. There are people who presist in using inverse logic, i.e. the opposite to the field logic which in my opinion is a bad practice.

There are some other uses of NO/NC terminolgy that can be confusing in terms of selection of NO/NC. For example, position switches have NO/NC terminlogy but do not have a clear definition of what the normal process state is. For example a valve with two position switches(one open and one closed) has two switches to decide whether to use NO/NC. The common practice in this case, the normal state is considered the confirmed state(open or closed) and the NO contact is used. For our example, the open switch will indicate power when it is in the open state and the close switch will indicate power when it is in the closed state.

For outputs, internal output coils in a PLC reflect the energized or de-energized state of the PLC outputs. However, NO/NC contacts can be generated from the logic or by selection/configuration of the output cards bacause these card have contacts in them. For example, a relay output card can be configured or specified with NO or NC contacts. Whether a NO/NC is generated is a function of the needs of the output field devices and any logic in the output field logic. Some output field devices such as solenoid valves and motorized valves also have their version of NO/NC.

NO/NC for solenoid valves(and motorized valves) can be confusing. While the de Facto standard of shelf state is still used, the terminology for close and open is the opposite to the electrical terminolgy, i.e. NO means fluids(pressure) can flow through it in the shelf state and NC means fluids(pressure)cannot flow through it shelf state. For a 2-Way solenoid, NO means that the solenoid valve is open in the shelf state while NC means that the solenoid valave is closed in the shelf state. For 3-Way solenoids, the terminology gets a bit confusing and means the shelf state of the flow paths in regards to the supply, output, and exhaust ports. For example, for a 3-Way NC solenoid, in the shelf state a NC solenoid blocks the supply port and exhausts pressure from its output port while a 3-Way NO solenoid, in the shelf state, supplies pressure from supply port to its output port and blocks its exhaust port. If you use a NO/NC solenoid, alway hook it up the way the manufacturer recommends. Higher "Way" solenoid do not typically have NO/NC configurations. For a universal solenoid, you can hook the ports up in any manner and you only need to worry that you have the right paths when you are energized or de-energized.

Hope this clears up some of the confusion.
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Old January 30th, 2003, 10:31 PM   #8
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Lightbulb

PLC programming has evolved into something more than simple relay logic. I know this is obvious to some, but it is a point of view not often considered by people who deal with it everyday. I personally do not think of ---||--- as a relay contact anymore, to me it translates as an IF ... THEN ... type of statement. That way the rung:

|----||-----------( )--------|
|**00.00******00.01*****|

Translates as: (Omron Addressing)
IF Bit 00.00 is ON THEN Change Bit 00.01 to ON.
Rung :
|----|/|----------( )--------|
|**00.00******00.01*****|

Translates as: (Omron Addressing)
IF Bit 00.00 is OFF THEN Change Bit 00.01 to ON.
Rung :
|----||----||-----( )--------|
|**00.00******00.01*****|
|*******00.02**********|
Translates as: (Omron Addressing)
IF Bit 00.00 is ON AND Bit 00.02 is ON THEN Change Bit 00.01 to ON.

I quit thinking of them as real world contacts. Some of the people that I have taught will argue this until we start talking about work bits. Ladder Logic has progressed from a conversion of hard wired relay logic to a intermediate level programming language with much easier access to the real world than a desktop computer running DOS or WINDOWS. How the real world input is converted to the state of that bit is a function of the hardware being used and not the logic of the program. I can wire dozens different devices to that input, "Normally Open" "Normally Closed" "Sinking" "Sourcing" "Dark On" "Dark Off", that have no effect on the way the progam logic is executed - BUT have a definite effect on what happens when then machine is running... Therefore it is easier for me to seperate the way the rung is executed from the way the hardware is wired.
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Old January 30th, 2003, 11:34 PM   #9
rsdoran
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Quote:
PLC programming has evolved into something more than simple relay logic. I know this is obvious to some, but it is a point of view not often considered by people who deal with it everyday. I personally do not think of ---||--- as a relay contact anymore, to me it translates as an IF ... THEN ... type of statement.
Personal opinion: That is the problem with those that program and create the programs for plc's.

For many years ladder logic has been effective, plc's have expounded on that concept...I will admit with more features and possibilities.

The problem with the aforementioned way of thinking is that it doesnt follow electrical training/thinking. Ladder diagrams were a product of POWER and control diagrams. Electricians to date are still taught power and ladder diagrams.

WHO has to deal with troubleshooting problesm with machines with relays or plc's? ELECTRICAL Maintenance! These are guys that usually go thru an apprentice program to run conduit and wire to follow a specific layout.

IF THEN OR ELSE is a product of working/studying in a programming environment and not specifically related to anything relay/electrical.

You can argue this concept till the moon turns black and I will not change my mind. Ladder logic is understood easily by many and can be enhanced (if the effort was made) to be more graphical ( who can't understand pictures?) so the many features offered using flowcharts, IL, FBD, etc could be readily understood by many electrical personnel.

WHY do you think it is so hard to understand the difference between hardwired and software applications?

BECAUSE there is a difference. In Relay logic there wasn't.

Hitler was a fool even though he controlled an empire. He created an environment that created a World War. He also cared about the working people and the Volkswagen was created from it, a machine that many could own and was easily understood.

I DO NOT care about the price of the machine/object, but do want it to be easily understood. TO ask that electricians/maintenance people understand programming concepts like IF, THEN< ELSE is ridicoulous.

Its kind of strange though, many electricians/maintenance people can understand the "BLACK BOX" theory..ie an item that is designed to perform a specific thing. Thats why plc's like Siemens S7 intrigue us, they offer this "black box" format in a way we can understand. Example being they offer a "box" that is titled "toggle". What does this do? Allows you to use one PB/input to on/off a bit/output.

I have no problem with YOUR way of thinking, PLEASE do not apply that to your WAY OF PROGRAMMING.

BTW I dont care how you program it, if a NO contact is used this should not be powered till it is closed.

A NC contact should reflect that power is there until it is OPENED.

HOW you program/document this does not matter. DOCUMENTATION is the KEY
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Last edited by rsdoran; January 30th, 2003 at 11:45 PM.
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Old January 31st, 2003, 06:06 AM   #10
David
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G'DAY
I will offer a very simplistic method of getting around NO NC instructions.
I tell my trainees that they have three things that they can do with an input,
1/ They have an input that must be on, then do this ---] [---
2/ They have an input that must be off, then do this ---]/[---
3/ Or they can ignore it.
The coin usually drops after the first time they use a stop button as an input in the program and find that to make the program work the stop button input has to be entered as ---] [---
i.e. It must be on it is wired to a normally closed contact on the stop button.
It may not be the most technical explanation, but it does get the new folks up and running.

Regards David
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Old January 31st, 2003, 10:15 AM   #11
steve ryan
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NO / NC contacts

I don't use the terms "normally open" or "normally closed" with my students...I tell them to regard NO as a "copycat" contact, and NC as a "contrary" contact.

Then they only have to answer the question: "Do I want my ladder contact to behave the SAME way as the associated real-world input (use "copycat") or do I want it to have the OPPOSITE behavior (use "contrary")?
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Old January 31st, 2003, 10:50 AM   #12
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Dog

Finnish view (not finish):
Discrete variables in PLC programming includes 3 properties:
- Absolute Address as I 0.0 in Simatic World *)
- Symbolic Name as EMS-PB1 (for ex. el-position)
- Signal Name as 'Not Emergency Stop'
Where Signal name means when variable (Input Bit etc.) is 'True'
I'm not indrested what are the Equipment properties (as contact type or colour) that is connected to variable (Input).

*) not nessary with ControlLogix memory
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Old February 1st, 2003, 04:20 PM   #13
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To be, or not to be, that's a question?

We owe all the consternation about NC and NO stuff to the old timers that were practicing our trade when PLC's were introduced. Come to think of it, I am almost one of those old timers. Hmmm, oh well that has little to do with things except I remember my first exposure to the Examine On and the Examine Off instructions for the AB plc that I cut my teeth on. I had been used to working with the hardware of the day and the concept of NO and NC was not a problem. A colleague of mine told me that there were switches that were in a "box" or "shelf" state due to the spring holding them closed or open and the need for another force to make them go to the opposite state. So, I believed him and also saw that it was true in actual practice. In electrical control drawings of the day, ladder logic back then, I was also told to assume that the drawing was put together as if all switches, relays and other things that had to be turned on or mechanically made were shown in their shelf state. This also made sense and proved to be accurate in all the applications I have run in to since. So, now to plc's, they were, are and always will be computers. Can you imagine telling the electrician in the plant in the mid 70's or so that he was going to have to learn to program a computer? Well, one out of maybe 100 would have welcomed the challenge and the other 99 would have revolted. Had this been the case, the plc would have met stronger resistance than it did. The AB reps used to tell all of us that the plc was just a relay replacer and that was about it. Indeed it was and still is just such a machine. But, it took me a few tries to figure out how to interpret the ------] [------- and just what it meant to my program operation and also, what it meant as to what I connected to an input. I first started out thinking that the instruction was to be used literally. If you had a NO contact wired to your input, then you used the -------] [------- symbol in your program. I soon learned that did not work very well. So, now all these many years later, I am teaching plc's twice a year at the community college where I work. I still have the same trouble explaining this to new students as I always have. Except those who never learned about NO and NC hardware and associated the XIC and XIO instructions with the terminology. My introduction lectures include the history above plus, when talking about the plc, I call it what it is. Computers are not foreign to the electrical workers of today. I tell them that the instructions for this computer ask two questions about the state of an input. XIC asks is there a voltage present? XIO asks if there is no voltage present. If the answer to either question is yes, then the instruction is true and when used in a logical rung, true instructions from left to right constitute an output or action in most cases. Obviously if the answer is no to either question then the instruction evaluates false and no logic continuity will exist. I treat the outside world of hardware and hard wiring, and the inside world of the computer as they need to be treated. Separate, but compatible when accepted for what they both are and how they came to be. Sorry for the length of this treatise but you know how teachers are, long winded!
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Old February 3rd, 2003, 12:14 AM   #14
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Hi Guys,
Most cases NO / NC contacts will be defined depends upon the healthy
field contact and what action you need to perform, if it opened or closed. Some applications use always closed contacts NC, open will break the circuit.
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Old February 3rd, 2003, 12:30 AM   #15
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Lightbulb NO / NC Contact

Generally it specify the the type of contact used to perform required operation. If it represent a direct field contact, then the initial or healthy state could be closed or open, depends upon the type of field configuration. If it represent a coil inside logic, then it will be an another type. Basically I believe configuring NC / NO contacts in the ladder soley depnds upon the programmer and the type of operation / requiremnet to be performed........
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