13
Logix
®
420 Digital Positioner FCD LGENIM0106-07-AQ – 10/15
flowserve.com
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
In order to calculate the maximum cable length, use the following
formula:
Equation 3
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.
Barrier
(If Present)
Wire
R
R
Compliance 10
Voltage VDC
Controller
Voltage
4-20 mA
Current
Source
+
-
Logix
420
Current
Figure 7: Compliance Voltage
Figure 8: Conduit and Grounding
Grounding
Terminal
Conduit
Connection
