In Mark V CSP this LZZ has been used in so many places, what does it mean? Why does it keep changing from 1 to 0 and 0 to 1 continuously when viewing dynamic rung display? Could anyone give me a clear idea about this logic signal?

 

It's really kind of a lazy programming practice and very poorly documented at that. The short answer is: It doesn't mean anything, and it's value as seen on Logic Forcing- or Dynamic Rung Displays is unimportant.

LZZ is a "bit bucket". It is used as the "coil" of rungs that aren't intended to provide any function. (These rungs are included as parts of common sequencing elements, sequencing BBLs, that may or may not be completely applicable to a particular application. So, instead of deleting the rung, they just used LZZ as the coil.)

So, there may be two or twenty or thirty-four rungs which have LZZ as a coil; it just depends on the application. If the first rung that has LZZ as its coil is writing a logic "1" to LZZ and the next rung is writing a logic "1 to LZZ and the next rung is writing a logic "0" to LZZ, the value of LZZ on any display will appear to be changing, and part of that has to do with the scan rate of the monitor as well as the value of the last rung writing to LZZ (as the last write "wins").

You should *NEVER* see a LZZ contact in any rung (only LZZ coils). The reason is because LZZ can't reliably predicted to be a logic "1" or a logic "0" under any circumstances. If you ever see LZZ as a contact in a rung, that's a really bad sign.

 

We too learnt about LZZ.

Thanks CSA

take care
G.Rajesh