logix pro question. 3 spst switches workin as two three ways and one four way

I'm in school for the electrical technology program and we are working with logixpro and i can not figure out how to make three spst switches work as two three ways and a four way. i can make two work which is relatively simple. but for some reason i cant figure this one out

dirtbikedustin said:

I'm in school for the electrical technology program and we are working with logixpro and i can not figure out how to make three spst switches work as two three ways and a four way. i can make two work which is relatively simple. but for some reason i cant figure this one out

Click to expand...

Show us a Screen Shot of what you have so far.

Stu....

here it is for two spst switches as two three ways. minus the two bottom rungs. thats me tryin to make it work.

Another way to think of three-way with intermediate four-way switches is with the Exclusive-OR function (XOR). In ladder an XOR can be made like this (the same as the top two branches of dirtbikedustin's diagram:

   S1      S2                            X

--] [------]\[----+---------------------( )

                  |

   S1      S2     |

--]\[------] [----+

The 'X' is just a temporary holder. But what do you do with the third switch? You do another XOR using the result (X) of the rung above with the third switch.

   S3       X                          LIGHT

--] [------]\[----+---------------------( )

                  |

   S3       X     |

--]\[------] [----+

You can actually expand this out, just as you could in a real electrical circuit by adding four-way switches in the middle, by adding another XOR rung for each added input. The end result will be ON if there are an odd number of ON inputs.

thank you sooo much. i have spent quite a few hours on this. again. thanks

Nice Solution Bernie!

Another Solution that seems to work. Try it and see if it works for you.

Stu....

If the course doesn't offer it it would be wise to Google 'Boolean Algebra' which, in essence, is the basics of what ladder logic does.

A whole chain of XOR rungs can be used to decode a Gray Code encoder. It is a very useful function and can be implemented very easily.

Edit - regarding Stu's offering.

If you analyze the 4 contact groups you will see they represent the 'X' as developed in my first rung. They are then XORed with Switch 3. So nearly the same thing without the intermediate holding contact (though this turns on the light if an even number of contacts (including zero) are on. There are various ways to solve this. Though this might be a bit harder to expand - at least not as obvious.

Bernie:

Agreed on learning basic Boolean Algebra and rules, not to mention Number Systems, it will always serve you well with PLCs.

Please don't think I am saying there was anything wrong with your solution. I was just offering another solution that seemed to fit the requirements of his problem to help him improve his knowledge.

As you stated, there are always lots of ways to implement something, and every Programmer approaches it differently. The most important thing is a logical flow, and good documentation IMHO.

Stu....

To add a second 4-way then takes another 4 rung branches. Each additional simulated 4-way light switch probably doubles the number of rung branches.