Help with ladder concepts

[scantime]

Hi all, I was recently having a discussion with an associate regarding the function of the "coil" or OTE symbol in ladder. He expressed frustration that he could not use an OTE in multiple places with Schneider PLCs. He indicated that he therefore prefers to use structured text as he claimed that he could turn on a Bool tag in multiple places with ST and that Bool tag was the exact thing as an OTE. I countered with the fact that an OTE can actually have 2 states and it is not the same thing as a Bool tag as in structured text you are most likely only telling that tag to enter one state at a time. I further explained that the issue with using an OTE in multiple places is that as each one can have 2 states, based on the previous instruction (XIC, etc.) there can essentially be a logic race based on the program scan to put either OTE into either desired state. Is my logic (heh) about this concept correct?

 

[Lancie1]

Yes, your logic is correct.

In Schneider PLCs, your associate should use Set and Reset "coils", instead of OTE. It appears that Schneider uses "Active on (Contactor) State" to define their standard output coil, and "S - Latch activation (Set)", and "R - Latch deactivation (Reset)" for the others. He can use those S and R instructions in multiple places to enter only 1 state at a time, and confuse himself and everyone else that has to work with his program, but still it will be legal and will satisfy his desire for complexity.

 

[Steve Bailey]

Too many people render the ladder logic rung:

  X            Y

-] [--------( )-

as
IF X=True Then
Y := True
ENDIF

when in reality it should be

IF X=True THEN
Y := True
ELSE
Y := False
ENDIF

 

[ndzied1]

Too many people render the ladder logic rung:

  X            Y

-] [--------( )-

as
IF X=True Then
Y := True
ENDIF

when in reality it should be

IF X=True THEN
Y := True
ELSE
Y := False
ENDIF

or, you could do it all in one fell swoop as:

Y := X;

But the idea is the same. If X is on, Y turns on AND if X is off, Y turns off.