Wiring one signal to an input on 2 plcs. 2

[Larf03]

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

 

[Ashereng]

Larf03,

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
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Larf03]

No, I'm using a level sensing device which has dry contacts and I'm sending a signal from those dry contact to the inputs of 2 machines.

 

[OperaHouse]

The dumb solution would be to have a relay at each machine that disconnects the input when not in use. A little more information would help for a more eloquent solution. Is the level sensor sourcing or sinkin? Placing a resistor as an extra load on the input may help. You should be able to do what you want to do, but cant provide a solution with so little information.

 

[Larf03]

A resistor might help to fall below the actuation voltage of the input and ignoring the current being drained by the other machine...

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

 

[logbook]

This is very difficult without a drawing. It seems your sending end is either 0V or 24V. There are two machine inputs wired in parallel. When one of the machines is shut down the signal, which should be 24V drops to 13V. In this case the problem is simply that the input signal is exceeding the power rail of the off-state input.

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.

 

[nbucska]

I suggest connecting the GND-s of the PLC-s, Diode OR-ing
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/>

 

[itsmoked]

Logbook is correct the powered down unit becomes a low resistance path to ground pulling down the signal. This can be very bad for the input circuitry to boot! You could eventually fail something.

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

 

[nbucska]

"the common is the 24V" - i.e. the sensor is sourcing .

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/>

 

[Larf03]

Thanks everyone! I have tried with a simple resistor in series to the input and it works! I installed a potentiometre and ramped it until the effect I described disapeared. I measure the potentiometre setting and I need a 3.6K resistor.

Thanks!

 

[itsmoked]

Where's my red star?




Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Diodes connected to each of the inputs should have worked in this situation. The diodes on the common may have been a problem if it was common to a couple additional inputs. I don't like just using the resistor. This leaves the entire system vulnerable to an electrical fault in either of the PLCs. I often add a resistor to increase the current with dry contacts to reduce oxidation problems. I've found too many "open" contacts after a couple of years with relays that aren't really rated for low current.

 

[Larf03]

Thanks for all of your inputs... I will conduct some more tests today and decide on a final arrangement!

 

[Larf03]

Turns out the best option for my equipment and for functionality is going with diodes on each inputs. Indeed, as already stated by someone else, The problem was that my diode was originally placed on the common pin of the PLC.

Thanks everyone!!

 

[Ashereng]

Keith, bad form old chap. Solicitation for a star? You are better than that!

"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?

 

[MacGyverS2000]

We all just want to feel loved

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

hahahah!

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

 

[OperaHouse]

Each input to the plc has an internal pull up to +24V. The switched sensor sinks the input to common, "0" activates the PLC. A diode from each input to the sensor prevents the PLC which is NOT POWERED from acting like a current sink. It only has to act like a 1-2K load to activate the other PLC.

 

[nbucska]

Keith:
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/>

 

[Ashereng]

Here is a dumb question.

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?