Flowserve 420 IOM - Manual (Page 13)

User Instructions - Logix® 420 Series Digital Positioners FCD LGENIM0106-06 12/13
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13
Figure 7: Compliance Voltage
7.2.2 Cable Requirements
The Logix 420 digital positioner utilizes the HART
Communication protocol. This communication signal is
superimposed on the 4-20 mA current signal. The two
frequencies used by the HART protocol are 1200 Hz and
2200 Hz. In order to prevent distortion of the HART
communication signal, cable capacitance and cable length
restrictions must be calculated. The cable length must be
limited if the capacitance is too high. Selecting a cable with
lower capacitance/foot rating will allow longer cable runs. In
addition to the cable capacitance, the network resistance
also affects the allowable cable length.
In order to calculate the maximum network capacitance, use
the following formula:
Equation 2
( )
( )
Example:
(if present)
( )
( )
= 0.08
In order to calculate the maximum cable length, use the
following formula:
Equation 3
Example:
To control cable resistance, 24 AWG cable should be used
for runs less than 5000 feet. For cable runs longer than 5000
feet, 20 AWG cable should be used.
The input loop current signal to the Logix 420 digital
positioner should be in shielded cable. Shields must be tied
to a ground at only one end of the cable to provide a place
for environmental electrical noise to be removed from the
cable. In general, shield wire should be connected at the
source, not at the positioner.
Figure 8: Conduit and Grounding
Barrier
(If Present) Wire
R R
Compliance 10
Voltage VDC Controller
Voltage
4-20 mA
Current
Source
+
-
Logix
420
Current
Grounding
Terminal
Conduit
Connection