Electrical Student needing help!

[Guest Guest_Brian]

Hi, I am a 4th year apprentice electrician and I am in my last phase of college in Dublin, my class has started doing plc's using Mitsubishi equipment for the last few weeks. It seems pretty handy so far except for probably the most simplist thing that no seems to be able to grasp and no lecturer seems to be able to teach properly. When do I use the inverse contact on a ladder diagram? Does an open software contact have voltage running through it in the ladder diagram or is it the closed one. Does an open contact change to a closed one when 24v hits it and vice versa!!! Very confusing?!?, we asked every lecturer that knows plc's to explain these things and they all gave different answers, Mr Plc is the only one I can rely on now and my plc exams start in 2 weeks. Is this diagram below that one of our lecturers gave us any use? Thanks to anyone that can explain these things to me, then I can pass it on to my class mates, Brian.

[JouniK 2]

I'm not quite sure what you are asking, but an open contact is one where the electricity can't go through it, and a closed one means the electricity does go through it. Normally open contact means, that you have to, for example, push a button, so the contact closes and the electricity flows through it. Normally closed means that when you, for example, push the button, it cuts the electricity. The picture is weird. I don't understand why there's open and closed contacts in both the physical categories.

[Guest Guest]

sorry if the question may sound a bit simple but our lecturers have messed our heads up telling us conflicting info. The open and close contacts in the physical wiring areas are open=a switch, closed=a stop button, If a stop button in the real world, is closed in a schematic diagram then the software contact will be open! Thats what it means.

[PdL 71]

There is no "rule" on how you should use an digital input in your ladder diagram. It all depends on what you want to do with it in the following ladder. Although a stop / emergency switch is a NC contact in the hardware to ensure a broken wire safety, you can program it to be NC or NO in your ladder to accomplish a desired output. Although it lies within the expectation to program a NC hardware contact as a NC ladder contact and vice versa, it doesn't necessarily mean it's the only way. IMHO there's no wrong or wright on how to use a certain input contact in your ladder. And because this is a matter of one's opinion, you won't find a concrete answer. Edited 6 May 2005 by PdL

[panic mode 246]

just as JouniK stated, open contact is one that doesn't pass the signal and closed does. Now "normally open" or "normally closed" mean the relaxed state of the contact. for example door bell uses normally open contact. bell is normally off. when you press the button, you change the state of the contact and it bell is on. you could off course use normally closed contact as well and everything would work. the only problem is that when nobody is at the door, the bell would ring and drive you nuts. pressing the button would change the state of the contact from closed to open and it would silence the ringing. now where do you use open or closed contact depends on application. when programming, i like to think of open contact as YES and closed as NO. also you can think of all conditions as IF statements and all output instructions as actions to be performed. So for example IF Thirsty AND No_beer THEN Buy_Some in ladder logic this would look like      Thirsty         Have Beer                            Buy_Some ------|  |------------| \ |---------------------------------( ) now if your thirst signal is reversed to : Not_Thirsty previous statement could be written as:   Not_Thirsty         Have Beer                            Buy_Some ------| \ |------------| \ |---------------------------------( ) which would read something like: If you are NOT Not_thirsty and have no beer, get out and buy some. (notice double negation in not-not thirsty) I'm so thirsty now...

[Guest Colin Carpenter]

I recall being puzzled by the same thing many years ago when I started programming. The simplest way to think of, say the LDI instruction, is " Load the inverse of the physical input ". In other words, if the PLC input X0 LED is ON, then, in the software: LD X0 would be made. LDI X0 would be unmade. Your hard wiring leading to the input with all the possibilities of N/O and N/C contacts will confuse you. Just think of the input LED state and work from there. After all, that's all the PLC does.