HART MODEM BASIC CABLING

• Basically, a transducer ( a Flow Transducer for example ), a power supply and a measurement device ( a current-metering instrument or the analogical input of a logical controller for instance ) are linked together in a 4-20 mA current loop.
• The analogical value of the measure modulates the current in the loop.
• A resistor ( which value is between 250 Ω and 1100 Ω ) is inserted in the current loop.
• Then, the resistor generates a voltage from 1 volt ( for 4 mA in the current loop ) to 5 volts ( for 20 mA in the current loop ).
• The two wires of the analogical part of the interface must be linked in parallel to the resistor.
• Then, the modem uses the voltage to extract the signal.

The value of the resistor depends on the global resistivity in the current loop, due to :

• The number of devices ( one or more ).
• The length of the current loop ( up to 3000 meters ).
• The technical features of the wires.
• The internal resistivity of each connected devices.

Be careful, in some special cases the resistor can have a lower value than the minimal value specified in the HART standard.