Wiring one signal to an input on 2 plcs.
Wiring one signal to an input on 2 plcs.
(OP)
I have a common sytem using dry contacts for level monitoring and the dry contacts send signals to two machines. I have wired the input on the same contact i.e: Input 13 of machine 1 is wired to the same contact as Input 13 of machine 2. When both machines are powered up, everything works fine but when I shut down power feed to one machine, the inputs are activated even if the contacts are not closed.
There is obviously a leak from input card of machine 1 to input card of machine 2 up to the power supply of machine two (wich acts as a resistive circuit when power is down). I work with 24 volts supply and when the problem occure, I read 13 volts at the contact wich is supposed to feed the input)
I have tried putting a diode on the input card's commom pin but it doesn't work. I really need a solution because the whole goal of this common system was to have one unit running while the other one was serviced and still be able to monitor the level application
There is obviously a leak from input card of machine 1 to input card of machine 2 up to the power supply of machine two (wich acts as a resistive circuit when power is down). I work with 24 volts supply and when the problem occure, I read 13 volts at the contact wich is supposed to feed the input)
I have tried putting a diode on the input card's commom pin but it doesn't work. I really need a solution because the whole goal of this common system was to have one unit running while the other one was serviced and still be able to monitor the level application





RE: Wiring one signal to an input on 2 plcs.
I am a bit confused. Are you sending a 24 VDC output signal from a single PLC output channel to 2 different machines? If yes, I would suggest you use 2 separate channels - one output to each machine.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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RE: Wiring one signal to an input on 2 plcs.
RE: Wiring one signal to an input on 2 plcs.
RE: Wiring one signal to an input on 2 plcs.
I'm sourcing I think (I'm always confused between the two. My common pin is 24 volt positive (on the PLC card) and the signal I'm giving it is the 0V (negative) to activate the input.
I will try other voltage droping devices such as diodes and resistors Thanks
RE: Wiring one signal to an input on 2 plcs.
Take the case of a simple logic gate. It has a very high input resistance when it is on. If you turn the power off there is an internal diode from the input to the positive power rail. This diode provides a path for current into the input.
This could be solved by wiring a diode in series with the input to each machine. Put a pull-up resistor at the machine end of the diode. The diode is connected such that it conducts when the sending end is at 0V.
RE: Wiring one signal to an input on 2 plcs.
the 24V supplies so the higher will supply the nominally
24 V to the sensor.
Use a relay with two normally open poles or two optocouplers to drive both PLC-s.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Wiring one signal to an input on 2 plcs.
One solution might be just placing a resistor in series with each input. If the inputs are not big loads than the resistor won't effect the signal to the input. But when the input turns into a near ground(on power down) the resistor will limit the drain current allowing the voltage to stay high enough to remain high to the other machine.
I do not see how a diode will help.
This is all assuming you are feeding a 24V signal to the inputs.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Wiring one signal to an input on 2 plcs.
Unless you want to use a separate supply for the sensor, the 24V must come from either PLC --diode-ORing the +24V-s assures the sensor supply if either PLC is on.
To make the two inputs independent, I suggest a DPST relay
or two optocouplers.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Wiring one signal to an input on 2 plcs.
Thanks!
RE: Wiring one signal to an input on 2 plcs.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Wiring one signal to an input on 2 plcs.
RE: Wiring one signal to an input on 2 plcs.
RE: Wiring one signal to an input on 2 plcs.
Thanks everyone!!
RE: Wiring one signal to an input on 2 plcs.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Wiring one signal to an input on 2 plcs.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Wiring one signal to an input on 2 plcs.
I guess I am not getting this diode thing. I don't see it. Someone please explain.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Wiring one signal to an input on 2 plcs.
RE: Wiring one signal to an input on 2 plcs.
The two PLC-s have separate +24V-s and GND-s and so must
have the sensor, too. You can connect either the GND-s -- preferably -- or the 24V-s.
In this case you have two separatelly powered PLCs.
The sensor must have either a separate +24V supply
or must get the 24V from one PLD.
If you connect it to one PLD and you switch it off,
the sensor loses the power. I suggest diode-OR-ing the two
24V supplies -- so until at least one PLD is on, the sensor is powered.
In the case the 24V is common, you can OR the GND-s.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Wiring one signal to an input on 2 plcs.
A diode is like a one way check valve right? That is, current only pass in one direction?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Wiring one signal to an input on 2 plcs.
Sure, diodes are the way to go in this case!
Ashereng; Answer: Yes!
Unless you cause the diode to avalanche by increasing the reverse voltage to the point where the diode can no longer block the voltage. This is usually destructive unless the current is limited.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Wiring one signal to an input on 2 plcs.
I'll have to remember this one.
Sometimes, the sensor has additional contacts that can be used. I wonder if this problem will still exist if a second set of contacts was used?
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Wiring one signal to an input on 2 plcs.
The standard Microswitch is a "form C" with a common terminal, a normally open and a normally closed terminal. So.. There normally would NOT be two separate normally open contacts available or that would be the best solution!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Wiring one signal to an input on 2 plcs.
One example that readily comes to mind, since I am working on it, is my limit switch on a valve.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Wiring one signal to an input on 2 plcs.
See wikipedia.com for a pretty good definition of the various types of diodes. Just do a search on the word diode and look at the links. All diodes do NOT function the same way.
Hope this helps,
Scott
In a hundred years, it isn't going to matter anyway.