Hello to everyone, I'm new in Ladder programming. I want to compare if the value of a register is equal to 4. If this is true go to label 1, else go to label 2. How can i check if a register has the value that i want? Thanks!
Every ladder programming environment that I have seen has function blocks for doing things like EQU (equals), LSE (less than or equal), GTE (greater than or equal). Just select the right function block and fill in the parameter spaces.
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In C
if (reg == 4) { /* this is label 1 */ } else { /* this is label 2 */ } /* this comes next */
In Pascal
IF reg = 4 THEN BEGIN (* this is label 1 *) END (* no semi before ELSE *) ELSE BEGIN (* this is label 2 *) END; (* this comes next *)
I never heard of Ladder.
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FWIW, for my masters degree I wrote a ladder diagram editor/compiler that generated C:
The place I had worked always drew their ladder diagrams rotated 90 degrees (the "rungs" were vertical instead of horizontal) so that's how I did my examples.
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Something that did the opposite might be nicer :o) Then you could write all your PLC code in C.
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Except I don't do PLC code.
At the time I was involved in going the other direction: we had piles of ladder diagrams that needed to be implemented in a procedure language. The language wasn't C, but the ladder diagram compiler can generate source code for pretty much any lanuage with infix boolean operators that can be explicitly grouped using parens or brackets of some sort.
The ladder diagrams had originally been implemented using relays and/or diode-matrixes.
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Isn't there some awful Pascal-like thing called "Structured Text" that people use for PLCs and map onto ladder logic?
pete
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I think I remember reading about some ISO or ANSI standardized "structured text" representation that some PLC manufacturers support. IIRC, some of the PLC vendors had editors in which you could even switch back and forth between a ladder diagram view and a structured text view of a program (or something like that).
Since the project I worked on which used ladder diagrams didn't have anything to do with PLCs, I never really looked into it.
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See: IEC 61131-3
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Well, except for the 'map onto'. Structured tesxt is just another language. In prectice it is marginally more portable than the other standard PLC languages. At least cut and paste seems to work between PLC IDEs.
So far I've found one PLC that lets me import from/export to a text file (and thus to a decent editor).
I still haven't figured out a way to get Mitsubishi PLCs to accept functions or variable declarations. Variables must be entered in their database interface and I cannot determine how to enter functions at all. And yes, functions are part of stndard ST. I've had to resort to manually inlining.
Robert
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grouped using parens or brackets of some sort.
I've been out of the control automation world for some time now, but I was impressed with the Step-7 software on Siemens' controllers.
The combined multiple program paradigms and in some cases could switch back and forth views, IIRC.
for a lot of stuff about automation in general, start at
You can probably find a Step7 manual on line if you dig from here
They even have PC-based control (they're not the only ones).
Rufus
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