Panelview Maintained Pushbutton

Normally to start and stop an item we use a monentary pushbutton, latch a bit and allow the conponent to operate. To stop, another monentary pushbutton unlatches the bit. If an alarm occurs we shut the item down by unlatching the bit.

We could (with latching bits) turn an item on and off with a single momentary pushbutton but it doesn't have the status of the unit on the button itself ( Whether it is on or off)

A customer would like to use a maintained pushbutton (one button rather than two) to start and stop the component. State 0 = off State 1 = on. On would turn the bit on.

My concern or question, if we have a shutdown or alarm condition would the maintained pushbutton keep its state? (Stay on until pushed again for off).

Is this the best way to turn something on and off with a single button and have the status on the button?

Thanks

The maintained pushbutton will follow the state of the bit in the PLC if the STATE SETTINGS is set to Value Control (not Current State). If you unlatch the bit in the PLC due to an alarm or shutdown, the PB should follow this.

If you use Current State, then after the PLC has unlatched the bit, the pushbutton will have to be pushed twice to latch the bit again from the screen. This is because the STATE of the pushbutton does not match the value of the bit in the PLC.

I hope this helps.

Do a search of the forum for "stuck button" and you'll find several discussions on this subject.


My recommendation is to never use maintained HMI buttons. You can get the functionality the operators want with a momentary, and a flip flop in the PLC.

JMHO
Paul

***WARNING*** Using maintained buttons, especially on older panelviews can be problematic. THe worst problem is that multiple panelviews with the same maintained button will not reflect the current state, and are generally out of sync. It seems that the Panelviews do not continuously scan the 'Current State' and update themselves accordingly.

Solution: Use a simple Momentary PB, with the input going to a toggle bit in the PLC, and the status tag coming from the result of the toggle output.

XIC HMI_PB ONS TOGGLE_OS BST XIO TOGGLE_STATE BST OTL TOGGLE_STATE NXB OTE DUMMY_BIT BND NXB XIO DUMMY_BIT OTU TOGGLE_STATE

The Panelview momentary write tag is HMI_PB, and the state indicator is TOGGLE_STATE.