Single pushbutton latching / unlatching (slc5/05)

[PeterW]

I had to re-read this several times before working out that the original poster wanted to use the same push button to start AND stop the machine. This (in my humble opinion!) is bad practise

Not in all cases, when I first used my version, was on an AB HMI, where the button was green and said start, on pressing the application started and the button turned red and said stop.

 

[Outrage]

Hi PeterW,

Touche!!

I must admit, I hadn't really considered the HMI point of view. Although when programming SCADA I have on occasion used this method but I have always used a "toggle" function button, which equates to press it once and it becomes true, press it again and it becomes false. rather like a hardwired light switch - and as you have said it did turn a different colour to indicate which position it was currently in (as per the position of the light switch)so you knew from the colour/position what the next action would be. With a hard wired spring return pushbutton (we havn't been informed as to the type or any other visual aids) you may not be immediately aware of what action the button will enable the next time you press it.

My preference even on HMI / SCADA would still always be seperate buttons using the latch logic in the PLC to enable your actions.

Cheers,

Lee O

 

[Stephen3776]

I believe this is a straight code violation as the start button cannot be red per section 9.2.5.4.1.1 (3) of the NFPA 79 and the stop button cannot be green.

 

[OkiePC]

I believe this is a straight code violation as the start button cannot be red per section 9.2.5.4.1.1 (3) of the NFPA 79 and the stop button cannot be green.

Does this code apply to HMI color codes as well as real pushbuttons?

 

[HJTRBO]

love these thread resurrections

 

[Stephen3776]

I do not remember the section; however, I do remember NFPA 2007 stating operator interface via logic or memory devices adheres to the same standards as push buttons except that start is staret or on and stop is stop or off.