Rules of LD language

[Didro 0]

Hello all, I'm beginner in PLC programming (but have some experience in PC programming, what might be not good in that case). I can't understand why the LD diagram on the picture below is forbidden ? (I use Russian version of GX, and the message box on the picture says, that "the LD diagram could not be transformed to IL code"). Might be it is GX Developer error ? I've been trying to find the rules of writing well formed LD diagrams, which I could transform to IL code and backwards. Thank you.

[Tech Fred 3]

I am pretty sure it so the vertical rung connectors from rungs with outputs. I seem to remember taht you cannot do that. You can still use that same logic on one rung wihtoutthe output coils y002 and y003 and then have t rungs by them selves for settng those coils.

[Goody 5]

Its probably because some of the logic flows backwards If X1 and X3 are true then Y3 would be energised by the logic flowing through X3 backwards Its just not allowed n ladder

[Sergei Troizky 26]

Such rung does not violate ladder rules in principle, for instance OMRON software accepts it with no problems (and executes with no "backwards flow"), see the attached image. I tried it for Mitsubishi (though in Medoc software) and it is indeed not being accepted in ladder editor. However, I succeed with writing it in Instructions List, after which the ladder editor also displays it properly: LD X1 LD X2 OR X4 MPS AND X3 ORB OUT Y1 OUT Y2 MPP OUT Y3 The trick is a proper use of MPS/MPP instructions. However, the rung still cannot be re-edited in the ladder editor (will indicate the same error). Looks like a compiler defficiency. Tried to download such program into FX1S. The PLC does not start upon download and indicates error 6602 (circuit). Most probably, this is applicable to entire FX-series, or even all Mitsubishi models. Edited 15 Jul 2008 by Sergei Troizky

[FNMdeJong 7]

What logic do you wish to create for Y3 by this ladder??? The compiler warns you for the reverse logic. The compiler cannot handle this, it does not know where to place the memory points. So rewrite the logic that there can only be one explanation for the logic. Writing the program in instruction list in GX Developer and switch to ladder, it cannot display the ladder and fails any conversion and download. The program will not work. Edited 28 Jul 2008 by panic mode