DeMorgan's Invert Problem

A B D E G
---| |----| / |---+---| / |---| |------+----( )---
| |
| |
C | |
---| |------------+ |
|
|
F |
---| |-----------------------------------+


The problem is that the rung is backwards. G is On when it's supposed to be Off and Off when it's supposed to be On. The rung needs to be modified by inspection to product the correct output. Slashing the coil is not allowed.


A PLC programmer should be able to recognize and do simple DeMorgan's inversions in this head.


As a hint, DeMorgan would have us convert ANDs to ORs, ORs to ANDS, and invert inputs.

I'm not sure how to read this. It seems to me the formatting gets lost when posted as text. Would you be able to improve the formatting with either CODE tags or LADDER tags? If I just copy and paste your puzzle with LADDER tags it gives this:

A        B           D       E               G

---|  |----| / |---+---| / |---|   |------+----(  )---

                   |                      |

                   |                      |

    C              |                      |



---|  |------------+                      |

                                          |

                                          |

   F                                     |

 ---|  |-----------------------------------+

With some minor shifting of the letters between CODE tags it becomes mostly readable, not entirely sure if this is what you had in mind:



Also, convention with puzzles is that we send answers to you by PM, rather than posting here (in other words: no spoilers). Is that your intention?

Thanks for the reformatting.


I'm new to the forum thing. A reply answer is fine.

what does no slashing the coil mean? changing closed to an open or vica versa??? (or examine if ON, examine if OFF AB rubbish )

Where you draw the line at simple depends on the individual, but I go for three black boxes and invert that, and then invert the contents of each of the black boxes until there are no black boxes left.

what does no slashing the coil mean?

Click to expand...

Some PLCs include -(/)- in their instruction set. It is an output that evaluates true when the controlling logic is false and evaluates false when the controlling logic is true.

The answer is correct.


I wouldn't consider this a 'simple' inversion but it's a good exercise.

Steve Bailey said:

Some PLCs include -(/)- in their instruction set. It is an output that evaluates true when the controlling logic is false and evaluates false when the controlling logic is true.

Click to expand...

Codesys for instance allows for that. I find it quite useful.

When combined with the option to assign the output of a rung to two coils, you can do things like simultaneously creating an ALLOWED and a DISALLOWED bit. Or an ENABLED and a DISABLED bit.

Some PLCs have an "invert" instruction you can place ahead of your output "coil", or indeed anywhere you want to.

e.g. --------[INV]---( )-

Produces the same result as -(/)-

Personally I wouldn't modify the rung as it is, except to change the output bit to a temp bit, and then on a new rung just use...



I very often use "inverse logic" turning a bit on when conditions not satisfied, then add the inverting rung to get the result I need.

daba said:

Some PLCs have an "invert" instruction you can place ahead of your output "coil", or indeed anywhere you want to.

e.g. --------[INV]---( )-

Produces the same result as -(/)-

Personally I wouldn't modify the rung as it is, except to change the output bit to a temp bit, and then on a new rung just use...

Code:

   temp     G
---|/|-----( )--

I very often use "inverse logic" turning a bit on when conditions not satisfied, then add the inverting rung to get the result I need.

Click to expand...

Sounds like a voting machine. Get the results you want... LOL