IM 01C25T01-01E
2-1
2. CONDITIONS OF COMMUNICATION LINE
2.
CONDITIONS OF COMMUNICATION
LINE
The HART communication signal is superimposed
onto the 4 to 20 mA DC analog signal. Since the
modulated wave is a communication signal, superim-
posing it on the normal signal will, from basic prin-
ciples, cause no error in the DC component of the
analog signal. Thus, monitoring can be performed via
the 275 HART Communicator while the transmitter is
on-line.
2.1 Interconnection Between
DPharp and the HART Com-
municator
The HART communicator can interface with the
transmitter from the control room, the transmitter site,
or any other wiring termination point in the loop,
provided there is a minimum of 250
Ω
between the
connection and the power supply. To communicate, it
must be connected in parallel with the transmitter; the
connections are non-polarized. Figure 2.1 illustrates the
wiring connections for direct interface at the transmit-
ter site for the DPharp. The HART communicator can
be used for remote access from any terminal strip as
well.
DPharp
Relaying
terminals
Distributor
Control room
Terminal
board
F0201.EPS
HART
communicator
HART communicator
SUPPL
Y
PU
LSE
CHECK
ALARM
Figure 2.1 Connecting the HART Communicator
2.2 Communication Line Re-
quirements
Specifications for communication line:
Supply voltage (general use type): 16.6 to 42 V DC
Load resistance: 250 to 600
Ω
(including cable
resistance)
Minimum cable size: 24 AWG, (0.51 mm diameter)
Cable type: single pair shielded or multiple pair with
overall shield
Maximum twisted-pair length: 10,000 ft (3,048 m)
Maximum multiple twisted-pair length: 5,000 ft
(1,524 m)
Use the following formula to determine cable length
for a specific application:
L =
–
65
×
10
6
(R
×
C)
(C
f
+10,000)
C
Where: L = length in feet or meters
R = resistance in ohms, current sense
resistance plus barrier resistance
C = cable capacitance in pF/ft, or pF/m
C
f
= maximum shunt capacitance of field
devices in pF
2.3 Power Supply Voltage and
Load Resistance
When configuring the loop, make sure that the external
load resistance is within the range in the figure below.
(Note) With an intrinsically safe transmitter, external load resistance
includes safety barrier resistance.
600
250
0
10.5
16.6
25.2
42
External
load
resistance
R (
Ω
)
Power supply voltage E (V DC)
F0202.EPS
Communication
applicable range
R=
E–10.5
0.0244
Figure 2.3 Relationship between Power Supply Voltage
and External Load Resistance
