Answer / preyas

Becoz DCS is given the signal in terms of voltage(1 TO 5 V).
And V=IR,
Now Whwn I=4ma
V Should be 1 V
putting in V=IR we get r=250 ohms.

Same in case when I=20 ma
and V=5V WE GET r=250 ohms.

Answer / swami.r

As per international Standards we are using DCS standard
signal 1V to 5V and 4-20mA.according to ohm's law
V=IR,thefore we are using 250ohm.

Answer / 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

Answer / abel002

Becoz DCS is given the signal in terms of voltage(1 TO 5 V).
And V=IR,
Now Whwn I=4ma
V Should be 1 V
putting in V=IR we get r=250 ohms.

Same in case when I=20 ma
and V=5V WE GET r=250 ohms.

Answer / debasish sarkar

For Load Resistance.

Answer / mahalingam

The 250-ohm resistor is not placed in the loop for the idea of converting the current (4-20mA)from transmission to voltage (1-5V)for reception, and also the value 250 is not a special value for a loop resistance needed for HART communication.

Explaining first the true purpose of having a resistor in the loop:

The HART communicator is communicating with the transmitter through FSK protocol. Understanding this protocol, it is a communication in analog platform. Digitalization of the signal happens within the HART devices and not along the transmission line.

The resistor's position in the loop is between the transmitter and the DC power supply. As we connect the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? Because the power supply has a LOW PASS CAPACITOR filter that basically squelches analog signals like ripples. Thus, if there is no resistance in between the PS and the TX that analog signal coming from the HART devices will only be filtered out.

Explaining the value 250:

The resistance value is not necessarily be 250 ohms. Maybe because of the idea that the controlling current of 20mA will give us 5 volt calibration if there is a loop resistance of 250 ohms. Maybe not a good theory.

Actually you can use a number of possible resistance values that will allow communication between HART devices. This value depends on the transmitter minimum operating voltage requirement. Simple OHM's LAW. For example, if the voltage requirement for a certain transmitter is 12-24 volts use a resistor that will not give you a drop of below 12 volts or else you will not deliver power to your transmitter. Most transmitters are supplied with 24 volts and as per computation 250 ohms to 500 ohms gives a drop of almost 18 v which is somewhat in between the operating voltage

Answer / ankush

for load resistance and to follow the international standard
for dcs i.e.1 to 5 volt.

Answer / naseer jarral

this is the hart requirment hart need minimum 250ohmes it
need to communication between hart and transmiter in dcs
and singel loop already have this resistence but bench
calibration need to comunication with hart

Answer / zaldy

the 250 ohm resistor converts the 4-20 mA analog signal from the HART transmitter to a 1-5 volt drop to allow the Analog Input card of the DCS or PLC to read the analog signal. The HART digital FSK (Frequency Shift Keying) signal is also present on this line and supplies the HART digital data to the HART modem chip usually present on the same Analog Input card for all new DCSs and PLCs. The Bell 202 protocol used for this FSK
digital circuit needs no terminating or load resistor. The 250 ohm resistor is only for the analog circuit.

Answer / ahmet

If it had'nt been resistor, 24v had been connection in
parallel to HART device. And a constant current. then there
had not been communication.
That is for isolated 24v and HART com.
for communication with trnsmitter and HART.

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