Simple Ladder Logic Problem

[Lancie1]

NO that is absolutely NOT the case, the alarm is current, has not been acknowledged, so there is no guarantee that the operator has seen it, to act on it accordingly.

What happens when the plant is a specialty chemical plant that only runs for two 24-hours days once a week? I once was an electrician for a project where the programmer used latched alarms. When the project was shut down, when many of the pumps and conveyors went off, that caused alarms. These alarms were latched on and held until the plant was started the following week. The operators complained loudly about getting bogus alarms during start-up when there were no alarm conditions.

What was causing these phantom bogus alarms? After I had to do much checking and testing of the wiring and the sensors, we decided the program was holding the alarms over from the previous shutdown the week before. The programmer was called in to fix the problem.

In some cases for some plants (nuclear plants ??), maybe the alarms should be held over. But for most plants, it will cause more problems than it solves.

 

[Lancie1]

I believe because of the last part of the question I need it in there to keep on the light if someone goes and holds down the reset button.

But you only need one case of HP. Having 2 contacts where 1 will do the job shows logic inefficiency.

Locate your reset next to the "red" contact in the first parallel branch under HP. At that location, the Normally Closed reset contact can be held all day long and red will still be ON (when HP is still ON).

 

[osmanmom]

The OP need to observe the word ''depressed'' for PB Silence.

 

[Lancie1]

Osman, I believe it is a PLC program, not relay circuit wiring. However MrSHME's drawing does look like a relay circuit because no PLC addresses are being used.

 

[osmanmom]

while the user pressing the PB Silence the horn will active until depressed it will lock the horn

base on the OP LD the horn will deactive during pressing

 

[Lancie1]

In MrSHME's drawing, pressing the Silence pushbutton would immediately deactivate the horn. The horn lock-OUT will stay active until the High Pressure switch goes OFF. That seems to be the desired actions.

 

[osmanmom]

check this

 

[Lancie1]

Osman, you could be technically correct, but that logic is unnecessary.

Shown below is the case where the pressure switch is high, and the Silence pushbutton has been pressed and released. It is not necessary to keep the alarm horn ON while the Silence button is depressed. The entire reason for pressing the Silence button is to stop the loud blaring noise of the horn. The sooner it is stopped, the better everyone will like it.

 

[Lancie1]

Here is the same case and the high pressure is still there, but the reset button is being held down by a dumb operator. The red light stays on, does not even blink, until the pressure drops below the danger point.

 

[osmanmom]

little joke

 

[Lancie1]

Ha-ha! Nice joke.

The last time I tried to use my truck horn was when a dog ran in front. My horn did not even work, but the dog dodged out of the way. I had not used the horn in the past 3 or 4 years. I have moved to a small Southern Dixie town where the people are so polite, that they sit at stop signs waiting for everyone else to go first. Sometimes it takes a minute or two of everyone waving for the others to go ahead, before someone will finally drive out first.

 

[baldone]

IMHO, you should not be using Latches and Unlatches in this logic. You should be using seal-in circuits. Using Seal-in circuits will help you better understand the logic and how it works. I would also do this in Relay logic in lieu of PLC Logic. Call me old school but I prefer knowing that my logic is both electrically and logically sound. P.S. Seal in circuit requires the output of a circuit to hold itself in until it is broken by a NC contact.

+1 on not using OTL and OTU unless the condition has to survive a power cycle.

 

[El Matador6667]

Seal-in vs. Latching bits

+1 on not using OTL and OTU unless the condition has to survive a power cycle.

When learning to program LL I was taught to be VERY careful using OTL, however, not forbidding its use.

The rule of thumb I was taught (and later backed up by my Rockwell instructor when my boss eventually sent me to a proper training) use seal-in for real world things like starters, horns and things that could injure or annoy operators, OTL should (not must) be used for non-annoying indicators, lights HMIs or the like that mustn't be lost after a power cycle
Hard-wired mirror image of the ladder logic for life/safety logic, your typical belt and suspenders system.
I'm no superman programming expert, but this has worked well for me so far.

 

[Lancie1]

...use seal-in for real world things like starters, horns and things that could injure or annoy operators,

I think you were taught well, Matador.

 

[daba]

... use seal-in for real world things like starters, horns and things that could injure or annoy operators

Even though I said I wouldn't - I'm going to continue to disagree on this...

A horn, or audible alarm, is NEVER provided in any respectable control system to "annoy" operators. It is there for a reason, to "alert" them to a problem that is either affecting production, or more importantly, a potential danger to personnel or equipment.

From a health and safety point of view, it is essential, and probably mandatory in most of the world, that audible alarm devices cannot be silenced by cycling power.

IMHO a control system should power back up in the same state that it went off, although the powering up should instigate placing all controlled equipment into a "safe" state. A horn blaring is not classed as an "unsafe" state, it is simply an indication of the state that existed when the power went off.!! The programmed system is informing personnel that an alarm condition exists which, in all other circumstances, has to be acknowledged, to guarantee that the operators have "seen" that circumstance.

The notion of people running round like headless chickens after a power outage, as suggested earlier, does not happen on systems that have been programmed to correctly recover, or to enter "safe state" from power outages. On many systems I have been involved with, acknowledging or muting alarms is logged, together with "who" and "when". "Power Cycle" is not a logged-on operator, so can't mute the alarms.