Honeywell SLG 700 User Manual page 89

Consideration is required when selecting intrinsic safety barriers to ensure that they will
supply at least minimum Transmitter voltage (V
250ohms of resistance (typically within the barriers) needed for digital communications.
Transmitter loop parameters are as follows:
R = maximum loop resistance (including barriers and wiring) that will allow
LOOP MAX
proper Transmitter operation and is calculated as
Where:
V
XMTR MIN
If the total loop resistance is too high considering the HART load resistance and intrinsic
safety barrier, consider the following options:
1. Do not add an additional HART load resistance if it is not required. Intrinsic
safety barriers will typically have 250ohms or more, allowing handheld devices
to communicate in the field. Additional HART load resistance is typically only
required for HART connections in the safe area.
2. Using HART multi-drop mode will effectively allow a higher loop resistance.
See section
is required.
3. Shunt the HART load resistor in the safe area with a large inductor so that the
HART load impedance remains high at HART signal frequencies, but much
lower at DC. This solution, while it does work, may be physically inconvenient.
For example, a 0.5 Henry 30 Ohm choke in parallel with a 250ohm HART
resistor will serve this purpose.
4. Use an intrinsic safety isolator instead of a shunt diode barrier. These devices
will provide more voltage to the transmitter since the HART load resistance in
the safe area is isolated from the transmitter loop. These devices are sometimes
called smart transmitter barriers or supplies, or repeater power supplies. Ensure
that the isolator will pass HART signal frequencies in both directions.
For wiring transmitters in hazardous area locations, also refer to Section
information.
Revision 7
R = (V
LOOP MAX
= 14V
1.4.1 for details. This option is not possible if a 4-20mA output signal
Loop wiring is connected to the Transmitter by simply attaching the positive
(+) and negative (–) loop wires to the first two terminals on the left, marked
with LOOP+ and LOOP- on the transmitter body in the Electronic Housing as
shown in Figure 3-37. The third terminal to the right is a TEST terminal and is
only used for loop current diagnostics / measurements.
TIP: A current measurement device with low internal resistance (<10ohms)
can be connected between the TEST terminal "+" and the TEST terminal "-" of
the Transmitter Terminal Block to acquire a rough readout of the loop current.
Loop current wiring should never be connected to the TEST "+" terminal. This
can damage the Transmitter.
Additionally, never connect the current measurement device between the
Loop "+" terminal and the TEST "+"terminal. This also will damage the
Transmitter.
SLG 700 SmartLine Level Transmitter User's Manual
), including the required
XMTR MIN
)  21.8 mA.
– V
SUPPLY MIN XMTR MIN
9 for more
Page 77