Toshiba LF494 - Manual (Page 131)

TOSHIBA
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1L3 Cautionary Notes on Communieatiotis
Observe the following notes and limitations when you use the communications function.
■ Current output loiid
Load resistance: 240 to 1 (including communications line resistance)
Load capacitance:
0
,
22
/ib’ maximum (including commuiiicatiuns line capacitance)
Load inductance: 4mH maximunn (including communiciuions Jiiic inductance)
Cable length; 2 km maximum (approximate value when 1,25 mnr shielded cable
is used nnder standard operating conditions,)
■ Wiring cable
Use a shielded output cable as specified in Table 5.1.
■ Interference on 4-20mA current signal
To communicate with the flowmeter^ a digital signal (amplitude 0,4 to 0,8 V in the case of
500 Q load resistance) with a frequency of 1,2 lo 2,2 kl
i z
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 inierfere with the instrument. I’o prevent this
inierrcrencC;, put a low-pass filter wuth a time constant of about
100
ms into the input circuit of
the receiving instrument,
Current output
I
r —1
4
1
■ ' '1 vvv-------------- ^—1---------
i ^
1
Imd ^^ 1 J
resistance ^ > 1
1
1
1
1
1
]
------------- *—1--------------------- '
1
—[----------'
f
Converter
bllter Receiving instrument
Figure 11,3 Filter connection example
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