To apply the Superposition Theorem, we replace all the other sources with their own equivalent
internal resistances (voltage sources become “shorts,” and current sources become “opens”):
The HART communicator is “listening” for those audio tone signals sent by the transmitter’s
AC source, but it “hears” nothing because the DC power supply’s equivalent short-circuit prevents
any significant AC voltage from developing across the two wires. This is what happens when there
is no loop resistance: no HART device is able to receive data sent by any other HART device.
The solution to this dilemma is to install a resistance of at least 250 ohms but not greater than
1100 ohms between the DC power source and all other HART devices, like this:
Loop resistance must be at least 250 ohms to allow the 1 mA P-P AC signal to develop enough
voltage to be reliably detected by the HART modem in the listening device. The upper limit (1100
ohms) is not a function of HART communication so much as it is a function of the DC voltage
drop, and the need to maintain a minimum DC terminal voltage at the transmitter for its own
operation. If there is too much loop resistance, the transmitter will become “starved” of voltage and
act erratically. In fact, even 1100 ohms of loop resistance may be too much if the DC power supply
voltage is insufficient.