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Why is a 250-ohm resistor needed in series with the 475 handheld controller in order to measure instruments?
It’s not 475 that is required; rather, Hart communication needs a resistor with a value of at least 250. There should be many explanations available on this topic. Probably, the Hart signal is superimposed on a 4-20 milliampere current signal; to detect this signal, there needs to be a voltage signal in the power supply circuit, and resistors are used to prevent this voltage signal from being attenuated. At present, most on-site power supply circuits have this circuit resistance; it is not necessary to add it manually during debugging