Student Needs Help!

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 that one of our lecturers gave us any use?
Here it is at http://www.hypermedia8.com/proman2/brian/plc.gif


Thanks to anyone that can explain these things to me, then I can pass it on to my class mates,

Brian.

I hope that you learn more in the next year than you have in the last 3 years.
These answers are in your notes from the first half of the second year.

hmm

Sorry over in Ireland we only learn plc's in our last year cause we are trained in all areas of electric disiplines in the previous years, unlike some other countries!!

http://www.plcs.net/contents.shtml start here and then see how you feel about your situation.

A common mistake is to assume that a contact in a PLC program is the same as a contact in a wiring diagram. A PLC is not a relay circuit emulator.

Look at the attached PDF file.

A simple illustration is the basic two button motor start stop circuit. In a hardwired three wire motor start stop circuit you have a normally open contact on the start button and a normally closed contact on the stop button. If however, you wire the same buttons to a PLC you would not use the exact same logic to program a two button start stop. The input that the stop button is wired to is always on unless the button is pushed, so in your program you would use a -] [- instructions and not a -]/[- instruction. You should get in the habit of not thinking of the instructions as normally closed or normally open contacts, but as IF False -]/[- and IF True -] [- instructions. One PLC manufacturer actually calls the instructions Examine If On and Examine IF Off.

This is pretty basic stuff, your instruction manual should have covered it. However, the statement at the end of the image you posted which states

If the physical wiring eg switch is open and the schematic diagram shows it open well then the ladder diagram is open.

Click to expand...

is an incorrect statement as it is not always the case. You must understand that it is erroneous thinking to think of a PLC as a realy circuit emulator.

The sheet you show simply correlates the symbol for "contacts" in ladder logic programming with those used for "real" relay contact symbols in European practice for electrical schematics. The chart wouldn't be required in the US, because our standard relay symbols match the ladder logic programming symbols.

An example of use might be preventing a machine start-up if the lube oil temperature were high. Let's assume your machine has a temperature switch with a contact that closes if the lube oil temperature is high.

Your electrical schematic and your logic might look like this:

  

|

|      Start PB      High Lube Temp

|           X2         X0           Run Machine

|----------] [---------]\[-----------(Y0)

|

|

|

 

If the lube temp switch is in the normal state the PLC input X0 will NOT be on. The "contacts" that correspond to that input will be in their normal state, so the normally closed "contact" for X0 will be closed and when the start push button is pressed both X2 and X0 will conduct, turning on output "coil" Y0.

If the temperature is high, the temperature switch contact will close, turning on the PLC input X0. The "contacts" will change status, and the Normally Closed "contact" will open and not conduct. When the Start pushbutton is pressed X2 will conduct, but X0 won't. Coil Y0 will NOT turn on.

I always found it easier to look at --| |-- and --|/|-- as comparisons.

The first (--| |--) is looking for a "1" in the appropriate data table, the second (--|/|--) is looking for a "0" in the appropriate data table.

Marc