Why we use 250 ohm resistor in series with HART protocol
communication?
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Answer Posted / jen
Ohms law (V = IR) dictates that a resistor must be present
with current to receive a voltage. Most field devices
transmit HART as +/- 0.5mA imposed on the 4-20mA current
loop. The 4-20mA is the centerline of the 1mA signal. The
resistance in the loop converts the current to a voltage
which is used as a digital voltage signal. The carrier
detect is 100mV nominally. If there is no voltage, there is
no carrier, and therefore no communications.
Frequency Shift Key modulation is used to transmit the
digital HART data via the current loop. FSK allows two
separate frequencies to represent digital data. 2,200 Hz for
binary 0 and 1,200 Hz for binary 1
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