HOw to explain the "DIFU" instruction?

I just start learning PLC , so belows the question I hope there will be someone to explain to me:


after reading the One Shot of the PLC on-line learning, I am very confused with
the "DIFU" instruction! I am not very clear about the function of the DIFU
instruction! Could you explain to me by email?

Looking forward to seeing your reply!

Sorry, but I don't think I can do a better job explaining it than Phil has already done. Have you watched the animation?

If you still need help, try to be more specific about what part you don't understand...

-Eric

On second thought....

Hold on a minute! I just looked at Phil's tutorial and I can see where it might be confusing...

The DIFU instruction is an OUTPUT, though it is shown midway across the rung!

For the first rung, try using this (with Phil's explanation):

     0000             +-------+

|----] [--------------|  DIFU |

                      |       |

                      |  1000 |

                      +-------+

Does that help any?

-Eric

it helps me a lot!

can DIFU be treated as a switch?

Think of the DIFU as an output coil. It can operate as many contacts as necessary in your program. The difference is that, unlike the iniating contact, the contacts of the DIFU are only on for the length of one PLC scan. :nodi:

Omron's CX-Programmer offers both a "Differentiate UP", as well as a "Differentiate Down" contact in its instruction list.

Sometimes "terminology" gets in the way of understanding an idea.

The basic concept here is, "Tell me when...

"Bit-X just went ON" -OR- "Bit-X just went OFF"

We are looking for the "transition" from OFF-to-ON -OR- ON-to-OFF.

On any given scan, the bit is either ON or OFF. Now, compare that state to the previous state. If the current and previous states are the same, then there has not been a transition and therefore, no need to indicate that a transition has occurred.

If, on the other hand, the current state is different from the previous state, then a transition has occurred and, therefore, (if you are watching for such things) there is a need to indicate that the transition has occurred.

At that point, it would be nice to know if the transition was from OFF-to-ON or ON-to-OFF.

If there was a transition and the current state is ON, then the transition was from OFF-to-ON.

If there was a transition and the current state is OFF, then the transition was from ON-to-OFF.

On the next scan, the PLC again compares the current state to the previous. If the current state is the same as the previous, then there has been no transition since the last scan and so the "transition-flag-bit" turns OFF. Thus, the particular bit is ON for only 1 scan.

Having this capability allows you to respond when an event happens to a particular bit rather then the state of that particular bit.

Some PLC's use...

--|/\|-- to look for OFF-to-ON

--|\/|-- to look for ON-to-OFF

As usual, Terry has the inate capability to make complex subjects simple. With your permission, I would like to use your description of one shot in my training courses.
Check's in the mail. uragod

Go for it, Jay!

I only look for a dime on the dollar.