Negative output voltage from PLC

masterv

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Join Date
Jul 2009
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darthmouth
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I am new to the PLC systems.
I need to use a PLC as a switch. The PLC must be Ethernet controlled.
The peripherals that are connected to the PLC need a voltage between -12Vdc to 0 Vdc. I was planning on using the Allen Bradley controllogix or PLC 5. Most PLC have positive output voltage (e.g 12Vdc, 24 Vdc etc). How can I reverse the voltage to get a negative output voltage from the PLC? Can I connect the common to a negative voltage?
 
That's such a strange request that I'd want to know a whole lot more about the application before I could recommend that...

Will it work? Right now, my answer would be a very definite "maybe" -- In theory, it ~should~ work, but I really want to know a whole lot more about it before I would commit to that answer.
 
The PLC is a switch and it will do the selection between a real-time environment and simulated environment. The types of connection that I use are NTDS type A. The type A is characterized by nominal values of 0VDC and -15Vdc to represent binary one and binary zero. The threshold voltage is at -7.5. Therefore as long as my voltage is below -8 VDC, it should be able to switch to a binary 0. In total with all devices connected, I will have 222 input/ouput connected to the PLC. The output voltage lines of my PLC have to correspond to the voltage of my NTDS device.
 
Still may need a little more detail...

You have an analog signal (-15 to 0) less then -7.5 your digital output will be 0 and greater then 7.6 your digital out will be 1 (on/off or true and false)

And you will have 222 of these?
 
Between -15 to -7.5 Vdc, the digital output is 0.
Between -4.5 to 1.5 Vdc, the digital output is 1.
My digital output is more stable when I get close to -15 VDC(binary 0) or 0 VDC (binary 1)

yes I have 222 of these.
 
Someone else will have ideas as to what PLC to use, but thats a lot of I/O.... 111 analog inputs and 111 digital outputs, not much of a program that part will be easy (greater then or equal to should be just fine, but that is still a lot of analog inputs, you may be better with some kind of micro processor... it should be a lot cheaper
 
Ok, I must be stupid... what is this NTDS device?? I've never heard of such a thing...

From what I can google, it's some kind of communication device. If you're thinking that a PLC is a communication switch, it's not. Think of a PLC more like a really smart programmable light switch.
 
Are there any pull up or pull down resistors on the device inputs? What happens if the input is open, does the device read it as a zero or one? You could always install the appropriate pull up or pull down resistor on the input and use a relay contact output module in the PLC to connect the input to the appropriate voltage.

More details on the inputs would be helpful.
 

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