Why we use 250 ohm resistor in series with HART protocol
communication?
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Answer Posted / 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.
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