Notes On Communications - Toshiba LF516 Instruction Manual

11.3 Notes on Communications

● Current output load
(1) Load resistance: 240 to 750 Ω (including the communications line resistance)
(2) Load capacitance: 0.25μF maximum (including the communications line capacitance)
(3) Load inductance:4mH maximum (including the communications line inductance)
● Connection method
Use a shielded cable (such as CVV-S) for wiring.
● Interference on 4-20mA current signal
To communicate with the flowmeter, a digital signal with amplitude of 0.4 to 0.8 V (in the
case of 500Ω load resistance) with frequency of 1.2 to 2.2 kHz is superimposed on the
4-20mA current signal. If a high-response receiving instrument is connected to the current
output line, the superimposed communications signal may interfere with the instrument. To
prevent this interference, as shown in Figure 11.3, it is recommended that you put a low-pass
filter with a time constant of about 100 ms into the input circuit of the receiving instrument.
Converter
(The maximum cable length is approx. 2 km when CVV-S 1.25 mm
under standard installation conditions.)
Current output line
＋
Load resistance
−
Figure 11.3 Example of Filter Connection
− 133 −
R
C
Filter Receiving instrument
6 F 8 A 0 9 3 4
2
cable is used