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Loop resistance must be at least250 ohms to allow the1 mA P-P AC signal to develop enoughvoltage to be reliably detected by the HART modem in the listening device. The upper limit (1100ohms) is not a function of HART communication so much as it is a function of the DC voltagedrop, and the need to maintain a minimum DC terminal voltage at the transmitter for its ownoperation. If there is too much loop resistance, the transmitter will become “starved” of voltage andact erratically. In fact, even1100 ohms of loop resistance may be too much if the DC power supplyvoltage is insufficient.Loop resistance is also necessary for the HART transmitter to receive data signals transmittedby the HART communicator.
Without the loop resistance in place, the DC power supply would “short out” the communicator’sAC voltage signal just as effectively as it shorted out the transmitter’s AC current signal. Thepresence of a loop resistor in the circuit prevents the DC power supply from “loading” the ACvoltage signal by the communicator. This AC voltage is seen in the diagram as being directly inparallel with the transmitter, where its internal HART modem receives the audio tones and processesthe data packets.
