Ladder simple logic

[Mel_]

Hello, I´m having a problem trying to program in Ladder. An output should be trigged by two possible contacts. Take a look on the printscreen below:

If BOT_EST_ALIM is true, all three outputs showed should be activated. On the other hand, MOT_EST_BALAN & MOT_EST_MIST should only be activated when their respective contacts be true. However only the last two are working as expected. MOT_EST_BALAN & MOT_EST_MIST (in network #1) works as expected only if the last two networks are deleted.

 

[jaden]

A basic rule for PLC's is that you specify an output only once. The conditions on your lower rungs are in effect overruling the upper ones. Combine the logic so that those outputs are only used once.

 

[padees]

I don't know what you are using here, but the 1st rung fires all the coils you want to use on subsequent rungs.

Not sure how the software works for this, but generally you would limit each coil to a specific rung.

 

[SCADA_Joe]

Agreed try this:

 

[drbitboy]

Do what @SCADA_Joe showed.

The -( )- coil instruction ALWAYS writes either a 1 or a 0 to its operand WHEN its rung is evaluated by the PLC scan.

PLC programming is primarily about time, and the scan cycle is the clock. When something happens (or is evaluated) is more important thant what happens.

To understand the PLC scan cycle, watch this video series.

 

[drbitboy]

However only the last two are working as expected.

No. All of those rungs are acting exactly as they should; they are not working as you expected because your expectations are based on a misunderstanding of how PLCs work.