Analog Signal Selector

[ampal]

Does anyone know of a product or maybe a manufacturer name that would make a singal selector that would choose between two (2) 4-20mA input signals and output only one of those signals based on a digital input.

Thank you,

 

[eric9822]

Can't say I have seen anything like that as that would typically be taken care of in the control system now days. You may want to check out http://www.miinet.com/ to see if they have anything and call them if necessary. I would also recommend you post some more detail about what you are trying to accomplish to see if anyone has any other suggestions.

 

[Smart $]

Does anyone know of a product or maybe a manufacturer name that would make a singal selector that would choose between two (2) 4-20mA input signals and output only one of those signals based on a digital input.

Thank you,

You didn't state the digital input's protocol???

Perhaps http://www.bb-elec.com/product.asp?sku=232DRIO

 

[G._S._Ohm]

Does anyone know of a product or maybe a manufacturer name that would make a singal selector that would choose between two (2) 4-20mA input signals and output only one of those signals based on a digital input.

Thank you,

If you can't find it and you're handy with electronic circuitry, this thing is about $20 in parts and a weekend of assembly and test/debug.

 

[ampal]

I have two different 4-20mA signals, one from a PLC and another from a magmeter. Either one of these signals can be used to control a valve controller. My PLC signal will be my main control signal but I am trying to use the magmeter signal as a backup in case the PLC fails. Since the controller that they are using only accepts one input I was looking for a selector that will choose between the PLC or the magmeter based on a on-off digital signal coming from the PLC.

 

[GeorgeB]

I have two different 4-20mA signals, one from a PLC and another from a magmeter. Either one of these signals can be used to control a valve controller. My PLC signal will be my main control signal but I am trying to use the magmeter signal as a backup in case the PLC fails. Since the controller that they are using only accepts one input I was looking for a selector that will choose between the PLC or the magmeter based on a on-off digital signal coming from the PLC.

You can switch 4-20 with a conventional low-current relay with no problems. I recommend one with "crossbar" contacts as we use with high impedance analog voltage signals, but darn near any will do ok. You can either hold it in with a PLC output which drops, or pull it in with a PLC output which energized on your fault condition.

 

[eric9822]

I have two different 4-20mA signals, one from a PLC and another from a magmeter. Either one of these signals can be used to control a valve controller. My PLC signal will be my main control signal but I am trying to use the magmeter signal as a backup in case the PLC fails. Since the controller that they are using only accepts one input I was looking for a selector that will choose between the PLC or the magmeter based on a on-off digital signal coming from the PLC.

If the PLC fails how is it going to drive the digital output? PLC's are pretty reliable and I wouldn't think a standalone switching mechanism would provide any more reliability. If you are just trying to provide a means to manually switch the signal source then I would use a manual selector switch. If you need an automatic switch then a relay will work as George suggests but you will get a bump in the process and you would want something other then the PLC to drive it. If I had to do this I would swap out the existing PID controller and install a programmable single loop controller that could handle the switching internally.

 

[Smart $]

...a on-off digital signal coming from the PLC.

An on-off signal is referred to as a discrete signal, not a digital signal.

As stated, a simple relay configuration will do the job.

Another option is if your PLC has a NO/NC contact block (module, if you prefer)... you can just run the PLC output signal through an NO contact, which the PLC holds closed under nominal operating conditions, and the mag signal to a paralleled (Form C configuration?) NC contact, which the PLC holds open under nominal operating conditions. On a full PLC fault the contacts change state and your mag signal is sent to the controller.

 

[G._S._Ohm]

". . . this thing is about $20 in parts and a weekend of assembly and test/debug" and I forgot to add about $5000 for UL certification.

 

[eric9822]

An on-off signal is referred to as a discrete signal, not a digital signal.

Some PLC manufacturers call them discrete, some call them digital.

 

[Smart $]

Some PLC manufacturers call them discrete, some call them digital.

So we should refer to this type of signal by whatever the PLC manufacturer says it is???

Technically, in the context of electronic process control, an on-off signal is not a digital signal unless it is a part of a byte worth of on-off signals. To state this on-off signal is a discrete signal separates it from all digital communications protocols, and in this case, as in many, clarifies the hardware necessary. If you needed an RS-232 serial communications interface, would simply saying a digital input is required make a clear distinction of the type?

 

[eric9822]

So we should refer to this type of signal by whatever the PLC manufacturer says it is???

Technically, in the context of electronic process control, an on-off signal is not a digital signal unless it is a part of a byte worth of on-off signals. To state this on-off signal is a discrete signal separates it from all digital communications protocols, and in this case, as in many, clarifies the hardware necessary. If you needed an RS-232 serial communications interface, would simply saying a digital input is required make a clear distinction of the type?

If I called the vendor that sells me PLC components and asked for a digital input card he would ask me for the voltage, not the protocol. If I need a communications card I specify the protocol and don't even mention digital communications. I am not saying it's right or wrong, just thats the way it is.

 

[Smart $]

If I called the vendor that sells me PLC components and asked for a digital input card he would ask me for the voltage, not the protocol. If I need a communications card I specify the protocol and don't even mention digital communications. I am not saying it's right or wrong, just thats the way it is.

That's the way it is for you in your world... and I don't doubt you even the slgihtest bit. I'm just passing on some knowledge that can be either assimilated or ignored... your choice.

Suffice it to say a discrete signal is a unique subset of all possible digital signals.