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If the current loop is not being used, it is safe to connect the DB9 connector directly to a
PC; pins 1&9 will not be dammaged or couse dammage.
The RS-232 levels are generated by the instrument only when an RS-232 level is
detected at the DB9 input. Therefore, when operating from a battery to conserve its
power, disconnect the RS-232 cable unless necessary for the operation.
6.2 Current Loop Output
The current loop signal is provided on the 9 pin sub ‘D’ female connector located at the
rear of the instrument. The current loop output functions only when the instrument is
operated from an external power supply. Pin 1 is the positive current loop output while pin
5 is the ground.
This output is isolated from the sensor and case, however as a sefety precoution, there is
a 1M
Ω
shunt to prevent electrostatic buildup and discharge.
Note that this connector is shared with the RS-232C interface, see section 6.1 for full pin
assignments. Pin 9 controls the range of the current loop, when grounded the output is 4-
20mA, when left open the output is 0-24mA. The current loop is capable of driving loads
from 0
Ω
to 500
Ω.
The output is linearly proportional to the selected engineering units.
The output may be scaled such that it spans only a portion of the full range of the sensor,
this feature may be useful in cases where a higher resolution output is required over a
narrow dewpoint range, or vise versa. To verify or change the current loop scaling follow
the instructions in the set-up mode section 5.3.
The current output is computed by the microprocessor as follows:
If pin 9 is grounded for a 4-20mA range:
Where:
O
=output current in mA.
R
= value of Reading shown on display in selected engineering units.
L
= value of Low Limit of Output Scaling converted to selected engineering units.
H
= value of High Limit of Output Scaling converted to selected engineering units.
See section 5.3 to select values for L & H. Their factory defaults are the upper and lower
range of the sensor e.i. for XTR-100 L= -100°C and H= +20°C. The values for L & H are
set up always in °C, however the microprocessor converts them to the currently selected
units. Note that because R, L and H are all in the same units the output current is linearly
proportional to those units. The current is linearly proportional to dewpoint, if the selected
units are °C or °F, however if the selected units are Lbs, ppm or G/M
3
then the current is
approximately logarithmically proportional to dewpoint see Appendix C.
After hooking up the current loop output, it can be forced to its low, mid and high points
by following the instructions in section 5.3. This procedure may be helpful in testing the
connection and setting-up the termination equipment.
O
R
L
–
(
)
H
L
–
(
)
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16
4
+
×
=
