Logix 500 - TON never sets DN bit?

TimD · Mar 6, 2008

Hi, I have a timer (TON) It counts up 10 seconds on rung true. If it reaches 10 seconds, I want the DN bit to be set so that I can fail (valve open until pressure is up to 10psi within 10 seconds).

I was trying to use the DN bit to indicate failure.

any help appreciated, thanks!

OkiePC · Mar 6, 2008

Is the timer in a subroutine that is not being scanned?

Can you see the ACC value changing?

Is there any other instruction acting on the same timer address or its DN bit?
Can you post the code?

TimD · Mar 6, 2008

OK, I got it now, but perhaps I am doing it 'loosely'. I don't like latch/unlatch, at least direct to output coils. I'm using a L/U bit to capture DN. It works, but I'm sure there is a cleaner way?

I attached the code as a jpg, related timer stuff highlighted in red boxes.

Thanks!

brucechase · Mar 6, 2008

I assume the reason you have a branch with the T4:15.TT bit is because I:4/1226 is only on momentarily for you timer to start. Since you have the TT bit jumped around the input, as soon as the timer reaches the preset, it is no longer timing and drops out the rungs. The DN bit does go on. You could change the TT bit to the EN bit, but then the reset I:4/1226 would not reset the timer. You could then put a XIO of the I:4/1226 after that seal in if you want.

Personally, I find nothing wrong with the Latch/Unlatch. I use it all the time. Once you get used to programming with it, it is a very powerful tool. There have been many arguments for/against using these, but there is nothing wrong with them. I would suggest that you get comfortable with them and use them. If you document what you are doing, then the techs (if they aren't comfortable with L/U) should have no problem.

Ron Beaufort · Mar 6, 2008

I completely agree that Latches and Unlatches are basically "good" little instructions - and that they definitely DO have a place in many situations ... but ... just make sure that you understand that Latches are "retentive" - while "seal-in" type rungs are not ...

in some cases the difference can cause a safety situation - particularly when a PLC-controlled system is shut down and then later restarted ... this can happen in many "power cycle" situations ... as long as you're aware of the differences in operation, party on ... if you're not sure about those differences, you might want to read this post and some of the following discussion in that same thread ...

preview: sometimes using Latches is UNsafe ... but sometimes NOT using them is UNsafe ...

TimD · Mar 6, 2008

Thanks guys,

I did notice the latch held when the PLC was restarted. I have set FirstScan to unlatch and all works well.

I usually comment most lines, I just haven't done this section until it's done and working.

Thanks again!

Tim

Steve Bailey · Mar 6, 2008

just make sure that you understand that Latches are "retentive" - while "seal-in" type rungs are not ...

We're talking about Allen Bradley in this thread, but you should also be aware that different PLC manufacturers treat the "retentiveness" of latch instructions differently. In some brands, certain memory addresses are reserved for retentive functions. In others, there are retentive and non-retentive latch instructions.

I think that's one reason why people caution against the use of latch instructions, because the techniques you learn for Brand X may not be appropriate when you're programming brand Y.

robertmee · Mar 6, 2008

I don't get the purpose of CV-100 Reset in series with your timer. It seems you are using that for a reset of the fault condition, but it would be necessary to do one reset to even start the timer the next time. If I understand the purpose of your fault, you are wanting to always determine a position fault of the valve (output vs Position Input).

If that's the case, your rung should really just be the logic you have without the TT latch and the reset. Replace those with XIO of the fault bit. Then the reset will unlatch the fault and arm your timer again, plus your timer will be armed any other time (like after a successful opening -> close -> re-open of the valve).

Since it is via SDN, I'm assuming the reset is a PB on an OI for resetting the fault on that valve.

TimD · Mar 8, 2008

Hi robertmee,

The reset button is sort of mis-labelled by my steam engineer. The reset button actually never resets anything unless you place the HOA in OFF first, like a 2-hand operation. The code has changed significantly since I posted, and we worked out all possible failure situations.

The reset button acts like a START permissive, if all other permissives are met and the HOA is in Auto, then 'reset' allows the timers to start and the valve to open etc. If failure occurs, an operator MUST turn the HOA OFF, then AUTO then 'reset'.

I asked the latch question because even if timer rung conditions remained true, my TON never 'showed' it's DN bit long enough for other rungs to sense there was a problem, so I latched it, now everything works perfectly.

I figured if my timer elapsed the time setpoint, regardless if the rung was still true or not, the DN bit would be on (1) until the timer was reset. Perhaps this is true for other processors, I am using a MicroLogix 1500.

Thanks to all for your help!

robertmee · Mar 8, 2008

TimD said:
Hi robertmee,

The reset button is sort of mis-labelled by my steam engineer. The reset button actually never resets anything unless you place the HOA in OFF first, like a 2-hand operation. The code has changed significantly since I posted, and we worked out all possible failure situations.

The reset button acts like a START permissive, if all other permissives are met and the HOA is in Auto, then 'reset' allows the timers to start and the valve to open etc. If failure occurs, an operator MUST turn the HOA OFF, then AUTO then 'reset'.

I asked the latch question because even if timer rung conditions remained true, my TON never 'showed' it's DN bit long enough for other rungs to sense there was a problem, so I latched it, now everything works perfectly.

I figured if my timer elapsed the time setpoint, regardless if the rung was still true or not, the DN bit would be on (1) until the timer was reset. Perhaps this is true for other processors, I am using a MicroLogix 1500.

Thanks to all for your help!

Ah okay....I still think then the reset is not needed if I understand your post above. The reason being, if the reset is also turning on the valve output, then the valve output is your permissive and you don't need the reset or TT at all. In otherwords, you would have a latching circuit somewhere that turns on the output. That output starts your timer. The fault bit would unlatch your output and reset your timer by default because the output would go low.

Logix 500 - TON never sets DN bit?

TimD

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