Local HOA status signals to a PLC - Three PLC inputs or only two?

[jmedclay]

What is typical for sending HOA switch position status to a PLC for garden variety process control?
One signal & one PLC input for explicit status of each of the three positions, or two (Hand/Run and Auto) with Off being implied by the state of the other two? I'm thinking false status via wire breaks or switch problems.

Thanks much,
John

 

[djs]

Many system just use one: Remote or Auto. For control via the PLC that is all that is needed.

 

[itsmoked]

Or two. HAND or AUTO both of which are active high (positive logic) so broken wires result in OFF and OFF is the lack of HAND or AUTO.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

I've usually used just one for Auto position. That way, as djs said, the controller will then know it can't control the device if Not in Auto. Also, if Not in Auto but running (as indicated by some other feedback switch, e.g., starter aux contact or process switch), the controller will know the switch is in Hand.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[triden55]

Hand should always go to the starter or device you are controlling, never directly to the PLC. This was if the PLC crashes, you can still run your process manually. All that's needed at the PLC is your Auto signal.

 

[DonUK]

Sounds like that is a hangover from the days when PLCs used to crash. Better to have Hand and Auto signals to the PLC as itsmoked's post above.

 

[itsmoked]

Usually HAND would run only to the device of interest. For instance a motor starter. The other "ON" position, AUTO, would run to the PLC telling it "you control the motor".

The third position of the switch, almost always the center position, is OFF. If OFF nothing is sent to the PLC or the motor starter so nothing should be occurring. You'll notice that broken wires result in NOTHING running or one function disabled which is what you normally want and is easily troubleshot.

Of course the OP has probably moved on by now...

Keith Cress
kcress -

http://www.flaminsystems.com