I've trying to read 1-5v on a 2 wire loop generation of 4-20ma 500 ohm impedance limit. How would i read this? I dont have 250 ohm resistor, but i have used 220 ohm resistor in the past with success. Would reading 220ohm resist on the + and - of the 4-20ma signal read 1-5v? or do you need a signal converter that ive seen on the web. Keep in mind this is a signal LOOP with + and - , do you treat this the same way as a transducer?.... Now that i think about it, does the signal generator act like a transmitter and needs a power supply to give you a current reading? Think that sounds more logical then what i was first thinking.
j4cm:
I've trying to read 1-5v on a 2 wire loop generation of 4-20ma 500 ohm impedance limit. How would i read this? I dont have 250 ohm resistor, but i have used 220 ohm resistor in the past with success. Would reading 220ohm resist on the + and - of the 4-20ma signal read 1-5v? or do you need a signal converter that ive seen on the web. Keep in mind this is a signal LOOP with + and - , do you treat this the same way as a transducer?.... Now that i think about it, does the signal generator act like a transmitter and needs a power supply to give you a current reading? Think that sounds more logical then what i was first thinking.
Yes the transmitter requires a DC voltage source so that it can then translate the measurement value into a constant current output signal where the value of the constant current reflects the measurement value of the sensor, so that 4ma output current represents 0% measurement value and 20ma output current represents a 100% of range of sensor value.
These so called 'two-wire' transmitters usually require a voltage source in the range of 12-48vdc. If all you have is a 220 ohm resistor that will work but just give you a slightly different output voltage range of .880 (@4ma) vdc to 4.40 vdc (@20ma) for 0% and 100% sensor measurement range. You can then in your software sketch scale this to any linear numeric range you wish using say the map() function.
So wiring is as so:
Positive +12 (up to 48vdc) 'loop power supply" wired to sensor positive terminal, sensor negative terminal wired to 220 resistor, other end of resistor wired to loop power supply negative terminal. Negative loop terminal wired to a arduino ground pin. Finally a wire from the non-grounded end of the resistor to a arduino analog input pin should do it.
Lefty
Nice nice, I might be going out of the bounds of the forms, but here is another thing floating in my head. I have a mega, Ethernet shield and i was gonna connect it to plc switching router, im assuming the router acts like normal router and i can communicate with the scada computer, then install xampp on it and run my php mysql data base script that receives post request from the mega that transmits the data i want to store. Do you think installing localhost on a scada computer would interrupt the scada data from the alley bradley plc?
j4cm:
Nice nice, I might be going out of the bounds of the forms, but here is another thing floating in my head. I have a mega, Ethernet shield and i was gonna connect it to plc switching router, im assuming the router acts like normal router and i can communicate with the scada computer, then install xampp on it and run my php mysql data base script that receives post request from the mega that transmits the data i want to store. Do you think installing localhost on a scada computer would interrupt the scada data from the alley bradley plc?
I haven't a clue about that, best to see if someone else can help. 
Lefty