Questions on whether to use Momentary or Maintained bits


Thread Starter


A new project using AB SLC 5/04 & PanelViews will also be interfaced to a DCS system via a Control Logix Gateway for Control Room Graphical interface. My question revolves around the use of momentary or maintained bits for commands to start/stop, enable/disable equipment. Which is more desirable to use though the interface, especially to/from DCS? I know it can be done
both ways, but which one seems to be done more often, and for what reasons?

Thanks in advance for any replies.........

David Lawton Mars

I tend to use momentary push buttons from PV's and do any latching of bits in the PLC. For example, if B3:0/0 is the start push button on the PV I'd then use it to latch a "run" flag in the SLC. The run flag would then get unlatched when the stop push button, say B3:0/1, is pressed. The latched flag can then be used to pass any information to the DCS. If comms update rate to DCS is a concern, latch the push button in PLC logic and then unlatch it from DCS to ensure the handshake takes place i.e. the flag doesn;t get missed in any scan related issues. I'd then maybe add a timeout timer to flag an alarm if the flag stays on for a preset time, say 3 seconds. Be aware though that a latch (OTL) remains in it's last state when power is cycled. If you don't want the bit to maintain it's last state either just use an OTE that latches itself or do a OTU on the first pass.
Hope that helps

Mark Liszewski

OR include an "Initialisation" subroutine which unlatches all set flags on
bootup....executed only on "1st scan".

just a thought.

Mark Liszewski
ZTR Control Systems


Thanks for the info. I'd like to be able to start/stop using only 1 pushbutton. Not a start and stop PB. PV has one START/STOP PB, and DCS has one START/STOP PB. Seems I'll have to create a flip flop that will toggle the commands on/off. I'd like to have the PV be the master, w/ DCS as the slave. The master can override the slave, but not visa-versa.

Any thoughts........

Michael Barb

I think one of the most important criteria for this questions is: "How do you want the system to power up." If the controller powers down but the DCS and Panelview remain powered up. What is the safest way for the PLC only to power up. In general momentary buttons let the PLC make the decision. With maintained the results can be unpredictable.

Note that even if the operator can not selective remove power, maintenace will find a way someday for some reason.

Jeremy Pollard

Easy solution. Use a momentary bit from the PV and toggle a counter in the PLC. Use the first bit of the ACC value as the toggle bit. Press once to set, then press again to reset.

I would STRONGLY suggest NEVER to use a maintained button from the PV since the start up states can cause unexpected operation. Keep control inn one spot

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In all my years of experiance, I have only seen the use of maintained commands on systems inwhich there is only one physical MMI (Man Machine Interface.) weather it is a DCS or PC, and you don't care if the system alters state on a power cycle of the MMI.
My advice is to use momentary signals only from the MMI to your PLC. This allows you to perform functions from any MMI with out fear of another MMI overwiting your request.
Be sure to use a seperate address bit in the PLC to track the state of the PLC Logic. This will allow you to confirm the command you sent was processed by the PLC correctly. It will also allow all of your MMIs to track the status of the logic. (Within reason)
Using these devices will allow you to add MMIs as Necessary, without fear of altering the characteristics of the program control.

John Paley--Graphic Pkg Corp

Use a momentary pb in the screen, and have it trigger a flip flop. The pb write tag triggers the flip flop, and the flip flop output bit is the pb display tag. That way multiple panelviews get and control the same stuff without bumping into each other. You can turn the FF off with the "first scan" bit if you want it to power up off, and you don't care how the screen powers up.

I find it easier to use momentary bits from an operator interface to request a PLC action. It becomes awkward when a maintained bit must reset
from either the OI and PLC logic. One industrial site of which I am aware is adamant that maintained bits be used, although no real rational
is offered other than that being their historic approach. I would be very interested in what other contributors to this list may have to say. This is one of those things that comes up again and again in practice, but is never dealt with in PLC programming 101 type courses.

Ron Sewell

Anthony Kerstens

I have in the past and will continue in the future to prefer maintained HMI bits that are reset by the PLC. My main reason for this is past experience (long ago at a younger, dumber age)
in one particular situation where the moment that the HMI button was released, the network communications failed and the zero never got written to the PLC.

I'm sure there are arguements on both sides of the fence. This is one of those things that my mind cannot be changed on.

Anthony Kerstens P.Eng.


Bouchard, James [CPCCA]

All of us like things to remain constant and are more comfortable with the way we are used to doing things. There is a lot to be said for doing things the same way every where in a plant ( or company ). We are suffering from an influx of second hand equipment that was built at various times with differing standards and practice and the technicians find it more difficult to work with since in some cases each machine is different.

So to get back to your comment about "no real rational is offered other than that being their historic approach." I say that it being their historic approach is a good rational and should be accepted as such.

If there is some technical or cost reason to prefer one approach over the other then present it along with the cost of converting the
existing applications and see what the client decides.

James Bouchard

Ravindra Halkatti

it is better to use maintained bit instaead of momentary bit.
b'coz during PLC scanning,it may miss to read the momenatary info. when PLC is busy in ligic or output scanning & ur purpose will not be solved.
if u used the maintained button,then PLC will read the this bit according its scanning routine & in the program once this readed then u can reset the maintained bit.