FBC Ins. w/Logix5000

[JeffB]

Hello Everyone,

I've been tasked with building a panelview screen to display "servo faults". The machine I'm working on has 9 servo axis. The ".servofault" word is a DINT, but only uses bits 0-8. I'm thinking about using a BDT instruction to move these nine bits into a nine word INT array and using a FBC instruction to compare this to a nine word INT array whose bits are set to "0".

Now to my question, is there an advantage to using a FBC as opposed to a DDT?

I would like to use the value in the .POS of the result control structure to indirectly address a value to move into a panelview message display. Is this a possiblity? Would using the value in the .POS compare structure be better suited to this?

One more thing, I'd like to inhibit the FBC once a fault has been detected, and resume comparing once the fault has been cleared, would using the status of the .FD bit of the compare or result structure be appropriate?

BTW, I faulted the processor just experimenting with a FBC, I think I didn't define an array large enough in the result. Do I need a word in the result for every bit I'm comparing?

Thanks for all who reply,

Jeff

 

[Gerry]

First of all, you have to use DINT's - not INT's - for both source and reference arrays in the FBC and DDT instructions. No option.

The 'advantage' in using one over the other (FBC or DDT) depends on which one gives the desired results.

The FBC compares source array bits to reference array bits and flags any differences. The DDT does the same, but then updates the reference to the current value of the source, i.e. it detects change of state.

Depending how you set the inhibit bit (.IN) of the instruction's compare structure, it will find one miscompare at a time or all at once. This determines how big your result array needs to be. If you want one-at-a-time detection, the result array can be dimensioned as 1. The result array (whatever size) contains the numbers (positions) of the bits that mis-compared - no need to use the .POS attribute. In fact, if using the all-at-once mode, the .POS attribute will always point to the end of your source array.

Oh, and your main interest is in the compare structure, not the result structure.

Finally, why move 9 bits into 9 words? The FBC/DDT instructions look at bits, not words.