Why do we need to include 250 ohms resistor in
the loop while using HART protocol?
Why we use 250 ohm resistor in series with HART
protocolcommunication?
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
Or in Simple
DCS is given the signal in terms of voltage(1 TO 5 V).
And V=IR,
Now When 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.