Logix
®
520MD+ and 510+ Digital Positioners FCD LGENIM0105-15-AQ – 05/16
28
7.2.2 Cable Requirements
The Logix 520MD+ digital positioner utilizes the HART Communi-
cation 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 capaci-
tance, the network resistance also affects the allowable cable length.
See Figure 26.
In order to calculate the maximum network capacitance, use the
following formula:
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 520MD+ 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.
7.2.3 Intrinsically Safe Barriers
When selecting an intrinsically safe barrier, make sure the barrier is
HART compatible. Although the barrier will pass the loop current and
allow normal positioner control, if not compatible, it may prevent
HART communication.
7.2.4 Grounding and Conduit
The grounding terminals, located by the electrical conduit ports
should be used to provide the unit with an adequate and reliable earth
ground reference. This ground should be tied to the same ground as
the electrical conduit. Additionally, the electrical conduit should be
earth grounded at both ends of its run.
NOTE:
The grounded screw must not be used to terminate signal
shield wires. Shield wires should be terminated only at the signal
source.
This product has electrical conduit connections in either thread sizes
1/2" NPTF or M20x1.5 which appear identical but are not interchange-
able. The thread size is indicated on the side of the positioner near the
conduit connections. Conduit fittings must match equipment housing
threads before installation. If threads do not match, obtain suitable
adapters or contact a Flowserve representative. See Figure 27: Con-
duit and Grounding.
4-20 mA
Current
Source
Logix
520MD+
Controller
Voltage
Compliance
Voltage
Current
Wire
R
R
Barrier
(If Present)
10
VDC
Figure 26: Compliance Voltage