Unlatch a regular bit

[kurt.brinker]

Can i do this? I want to keep B3:6/3 false as long as B3:2/4 is true.

 

[Mark Buskell]

It looks like it will work in those 2 rungs.

You need to look at the latch to and make sure that you have a xio there of B3:2/4 to prevent it from turning on and possibly messing up other rungs in your program.

 

[jimbo3123]

Short answer: Yes.

Long Answer: It depends on what else you are trying to do with that bit and where else it is used. Is there an OTE (regular coil) for B3:6/3?

If so, that coil will reset that bit on or off, depending on the state of the logic on its rung. The bit will continue to be on (or off) through the PLC scan until it reaches your rung zero above, where it may be set off or may be left alone, depending on the state of B3:2/4.

This is generally considered bad practice because it makes troubleshooting a pain (depending on where in the scan one is looking, that bit may be on or off and it won't be easy to see by someone watching the program running online).

 

[kurt.brinker]

b3:2/4 is coming from the HMI and toggles the unit between local and remote mode. I am setting it so that if the unit i placed in auto, you cannot place certain motors in manual control.


B3:5/0 (the affected bit) is only used 6 times in the program. rungs 0,1,1,2,2,4 in ladder 6

 

[OkiePC]

The way I read it, by forcing the CHARGE_AUTO_MANUAL bit to off, you will be forcing it into auto mode.

The first branch of your rung 0001 would then be able to start Pump P200 as soon as any one of the three conditions in series with the XIO was true.

Are you sure this is what you want? Would it be safer to put XIO B3:2/4 in series with the XIC CHARGE_AUTO_MANUAL bits on the 2nd branch and any other rungs you are concerned with inhibiting?

 

[kurt.brinker]

The way I read it, by forcing the CHARGE_AUTO_MANUAL bit to off, you will be forcing it into auto mode.

The first branch of your rung 0001 would then be able to start Pump P200 as soon as any one of the three conditions in series with the XIO was true.

Are you sure this is what you want? Would it be safer to put XIO B3:2/4 in series with the XIC CHARGE_AUTO_MANUAL bits on the 2nd branch and any other rungs you are concerned with inhibiting?

that is correct and that is what i want to happen. P200 only runs in auto if either p400 or p500 is running or if the timer is timing. the reason i didn't do it the way you described is because the maintained button on the hmi refernces B3:6/3 for its display. (0 is auto and 1 is manual) If i put it in series (which would certainly stop it from coming on) it would show that it was in manual but nothing would come on or off.

Does that make sense?

 

[OkiePC]

Yes, if the address you are unlatching is used as both a write and indicator tag, then I understand.

Bottom line is if it will do what you want, then it will work.

One possible side effect: If the HMI is a Panelview Standard, you may have to hit the button twice after programmatically unlatching it's write/indicator address in order to get it turned back on.

The Panelview Standard internally keeps track of what value it last wrote to the address, and will alternate bewteen writing one and zero regardless of what the address actually contains when you press it. The display tag should update your graphic just fine though, and it will not try to change the address until someone actually hits the button, or on power up (if you are set up to write to controller on power-up).

EDIT: If having to press the button twice is a detail that you want to correct, I would suggest changing it to a momentary with a separate display tag, then program it's behavior as a maintained by using flip-flop logic in the PLC.
Paul

 

[kurt.brinker]

its going to be a panelview plus 600. i see what you are saying about possibly needing to press the button twice because i do believe it is set to just alternate value.

 

[OkiePC]

If it is a panelview plus, you may also want to test it and inspect the button properties to ensure that it will display correctly. I am a novice with RSViewME but have had that exact issue with one of the hadnful of changes I have made to an existing system. I still get confused by the choices "current state" and "value control"...

Paul

 

[kurt.brinker]

the button is set for next state based on current state so i think it will be ok. i'll post back with any problems. thanks