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6F8A0917
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11.3 Cautionary Notes on Communications
Observe the following limitations when you communicate with flowmeter using HHT.
Current output load
(1) Load resistance: 240Ω to 1kΩ (including communications line resistance)
(2) Load capacitance: 0.25
μF maximum (including communications line capacitance)
(3) Load inductance: 4mH maximum (including communications line inductance)
(For maximum cable length, about 2km is a guideline length when CVV-S
1.25mm
2
is used.)
Wiring cable
Use a shielded cable (CVV-S, etc) for wiring.
Interference on 4 to 20mA current signal
To communicate with the flowmeter, a digital signal (amplitude 0.4 to 0.8 V in the case of 500Ω
load resistance) with a frequency of 1.2 to 2.2 kHz is superimposed on the 4 to 20mA current signal.
If a receiving instrument with fast response time is connected to the analog current output line, the
superimposed communications signal may interfere with the output. To prevent this interference,
put a low-pass filter with a time constant of about 100 ms into the input circuit of the receiving
instrument.
C
R
Current output line
Load resistor
Converter Filter
Receiving instrument
Figure 11.3 Filter Connection Example