Why use cross contacts in old programs!

[blima]

Hi!

I came across, in several old PLC's programs, some contacts from memory flags, where they use a NC and NO from the same flag, and the rung will never be active by the way the are used!

Why they do that? To avoid some functions of the machine?

Regards,
Bruno

 

[JesperMP]

That would be a plausible reason.
Specifically the programmer wanted to turn something off.
If the programmer wanted to deactivate some code completely he would (or should) have used a jump past the code to be bypassed.

 

[jmsmoreira]

I do that on some of my programs, just not to erase old functions that i don't use anymore (but might use on the future).

Hope you're not reading one of my programs...

 

[LadderLogic]

Some PLCs have built-in system "Always On" and "Always Off" bits and some don't. For those that don't, using this technique allows to disable certain parts of the program, as described above.

I guess some programmers develop a habit and keep doing this even where there are the system bits available.

 

[blima]

Ok!

Thanks for the answers. I was scratching my head to understand the reason why, but it is quite simple as almost always!

 

[bernie_carlton]

I have used this form, with an appropriate rung comment, when it is possible that the change may need to be reverted. I keep the original contact as the first (or upper in case of an OR) one to make it easier to see how to 'undo' the change.