Limit Swithces NO or NC????

[teamtexas]

Ok I thought I new what I was doing but now I'm not sure. I have all my limits wired in with 2 wires to a set of NC contacts. How are they suppose to be? I was watching a Mach 3 video and they talked about using a common ground and NO contacts?? I'm confused!!

Dan

[lowelife]

NO- stands for normally open, allowing the circuit to close when tripped, and NC - normally closed will complete circuit when activated. Depending on how you want to machine to react is how you would set those up.

[gar]

071030-1912 EST USA

An unactuated NO contact is an open circuit. When actuated it closes. A limit switch with a lever will be actuated when you move the lever far enough to change the switch's state.

An unactuated NC contact is a closed circuit until actuated.

If you combine a number of limit switches to produce a single ouput, then by connecting:

NO switches in parallel any one or more actuated produces a closed output state. Depending upon what you do with this circuit will determine the resultant operation of your system. Suppose the switch parallel network is connected between a +5 V source and a pull down resistor. Then activation of any one or more switches will produce 5 V across the pull down resistor. This circuit is not as noise immune when you really want that immunity as is the NC type circuit. This is described as positive logic inclusive OR circuit.

NC switches in series can achieve the same logical inclusive OR as the above parallel NO switches. Here the noise immunity is much better because the low impedance of a closed contact circuit until you exceed a limit. Here we would connect one end of the series switches to common (ground) and the other end to a pull up resistor. We still have 0 V output until at least one switch is activated. Activation any one or more switches will open the circuit causing the output to go high.

Use the series connection of the NC switches.

.

[stevespo]

Either method (NC or NO) will do the job for you. One advantage of running your switches NC is that in the event of a damaged wire (or other problem), your circuit will "open", triggering your software that some type of event has occured. You can then troubleshoot and fix it before it becomes a real problem.

This happened to me recently, when I got a spurious eStop in the middle of a job. I tracked it down to a loose connection. Since I had it wired NC, I knew I had a problem and fixed it immediately. If I had wired it NO, I would not have known - until I had actually tried to use my eStop switch - and it wouldn't have worked.

Steve

[teamtexas]

Gar, would the NC switches in series be considered "active low" until they are tripped?

[acondit]

Dan,

It would seem to me that you would want "active high" until tripped. Otherwise, if power connection to the circuit fails, you have the same problem as a NO switch. You can't tell until the switch is tripped that it is not working. You don't know whether it is "low" because the switch is closed or "low" because power to the circuit has failed.

Alan

[JRoque]

Hi. Use NC limit switches. That means that there's normally a voltage present in the circuit and there's no action during that state. When the switch opens, the voltage goes to zero and the limit action is triggered. That's called an active-low switch.

NC switches are preferred because they fair better in electrically noisy environments. If you use NO switch a voltage spike caused by noise can trigger the limit switch action (which would be active-high for NO).

JR

[gar]

071031-1104 EST USA

teamtexas:

http://en.wikipedia.org/wiki/Active_low

My interpretation of this definition is that "active low" means that a logic 1 or TRUE is represented by a lower voltage than a logic 0. So in a 5 V system near 0 volts is a logic 1, and towards +5 is a logic 0. In an RS232 system -10 V is a logic 1 and +10 V is a logic 0. The rest state in RS232 is -10 V.

Whether a NC contact is active low or high is dependent upon the definition of the actuator of the switch and how the switch is wired.

Suppose the switch function made is to be treated as a logical 1 and that when made the contact closes, and the switch is connected between +5 V and a gate input, then the signal at the gate input is "active high". Connect the same switch to 0 V, then its output is "active low". As needed you have a pull up or down resistor.

Change the name "made" to "not made" and you reverse the above meanings.

Logically if I put several NO contacts in series, and closed represents a TRUE input, then this series connection performs the logical AND operation and a TRUE output occurs only when all inputs are TRUE (closed) and a closed circuit is logic TRUE.

Logically if I put several NC contacts in parallel, and open represents a TRUE input, then this parallel connection performs the logical AND operation and a TRUE output occurs only when all inputs are TRUE (open) and an open circuit is logic TRUE. If you redefine open circuit as FALSE, then this is the logical INCLUSIVE OR.

Note we can have multiple input AND and OR circuits, but only two input EXCLUSIVE OR circuits.

Fundamentally in the multiple limit switch application the activation of any one or more limit switches should produce a stop action, and I would classify this as an INCLUSIVE OR function. So no matter how I wire the switches what I want is any one or more switches to perform the desired action.

Clear? NO.

Probably not a good idea to get hung up on whether it is active low or high.

.

[Al_The_Man]

In the realm of PLC inputs etc, the term Active Low or High, usually means what the status has to be to activate the named Function of the input.
Take for example over travel switches used in systems like Fanuc or Mitsubishi etc, these are usually dedicated inputs that have to have 24vdc to disable the OT condition, the OT occurs when the NC switch opens and the input goes to a logic 0, This is then Active Low for the assigned condition to occur.
Al.

[acondit]

Originally Posted by Al_The_Man In the realm of PLC inputs etc, the term Active Low or High, usually means what the status has to be to activate the named Function of the input. Take for example over travel switches used in systems like Fanuc or Mitsubishi etc, these are usually dedicated inputs that have to have 24vdc to disable the OT condition, the OT occurs when the NC switch opens and the input goes to a logic 0, This is then Active Low for the assigned condition to occur. Al.

Al,

Then the issue is what is the signal you are tracking. Given a NC limit switch that is high when closed, one could say that it is "active high" for the condition - "within limits" (i.e., the limit switch is closed and powered) but if you are considering the condition to be "over travel" the signal is "active low" when you trip the limit switch (or power to the switch fails).

Alan

[Al_The_Man]

That is what I meant by my first sentence, maybe not very clearly, What I meant was it is the condition that it is in when it performs the function of its description, as in the example, the name of the switch or input is Over Travel Switch.
So if I describe the machine over travels as being active low, it usually means that the logic is looking for a 0 when the overtravel occurs.
Probabally a case of semantics, but it can be important if you are writing the logic for the function.
Some Manuf. indicate a active low input required by placing an Asterisk in front of the input descriptions and designated input number .
Al.

[teamtexas]

Wow, there's alot more to these then I every would have thought. Maybe I need to ask the question in a simpler way (so I can understand). I want to wire up limits for the x, y & Z axis. I will be purchasing Gecko G203v drivers and a CNC4PC- C1G Parallel port interface card. I plan to control all of this with Mach 3.

1. Should I use NO or NC contacts on the limits?

2. Should I run a pair of wires to each (axis) set of of limits. I assume each (axis) set of limits should be connected in series, however I would like to use one limit on each axis as my home switches also.

3. Should the wires be shielded?

4. Should all the limits share a common GND? If so, should this GND be bonded to the metal frame of the machine?

5. Can I use proximity switches on one of the axis?

Thanks in advance for all your help!
Dan