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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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Q1: Consider part of a control loop, which excludes the transmitter, consisting of a process, a controller and a control valve which may be represented by two dead times of 0.5 min each and three exponential lags of 0.8 min., 1.0 min. and 1.5 min. respectively. We wish to express this system as an overall first order plus dead time (FOPLD) model ie gain, time constant and process dead time. (We will see later that this is often done, to simplify controller tuning). For this exercise, gain is considered to be 1.0. (A) If the transmitter is a flow transmitter whose behaviour can be described by a dead time of 0.2 min. and an exponential lag of 0.5 min. in terms of the overall dead time and overall first order lag how can the system behaviour be approximated ? Overall dead time = Overall time constant = (B) If the transmitter is a temperature transmitter with a temperature sensor in a protecting well whose behaviour can be described by a dead time of 0.7 min. and an exponential lag of 15 min. how can the overall system behaviour be approximated now? Overall dead time = Overall time constant =
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