Hello,

I have a gas detector that I need to connect to a PLC that controls a treatment system. The detector has 4-20mA output and our PLC only takes digital inputs. I am looking for something inexpensive to convert the signal. I do not know much about this type of thing so I am looking for something I can buy and not have to tinker around with. Thanks in advance for any information on this.

Sandy

 

Check Non Linerar Systems. They have an 8000 series that will take almost any signal and convert it into another for $400-$500.

 

Try Pheonix, Weidmuller, etc. for A/D converters. They have (or had, I haven't looked recently) small DIN rail mount converters that would take an
analogue signal in and convert it to a digital signal on 8 digital 24VDC output lines. You would connect these outputs to the inputs on a PLC and read the entire byte as a binary integer.

 

Is the Digital input a simple binary input (for a switch) or is it some sort of communications port? If it is the former, then you will need some sort of trip amp that you can set to change state at your required value. RTK/Phoenix/Weidmuller all have trip amps available.

If it is a comms port, you will need to tell us what the protocol, etc. is before someone can suggest what to use.

Steve
www.mtl-inst.com

 

I you don't want to tinker, sounds to me like you need an analog input module for the PLC.

Gerald Beaudoin

 

If you're looking for a serial communications output, then you can use a DGH D1251 module to convert the 4-20mA to RS-232. A Modbus protocol option can also be specified for serial connection to most PLCs by appending an 'M' to the model number.

 

Hi Sandy,

If you understand PLCs then you must be aware that digital impus are contact channel inputs as opposed to analog information that you are handling. Can you confirm this by stating in your post how many digital inputs are available?

In my opinion you need an analog input module.

On the other hand if you just plan to switch some I/Os on or off through the PLC based upon an adjustale set point look of a 4~20mA trip amplifier. Compare the input to the set point & the putput relay contact will now go as input to the PLC.

Hope that helps.
Monti/Vaishakh

 

May be useful for you:

http://www.dataforth.com/catalog/doc_generator.asp?doc_id=17

NB