fault isolation method allows for communication integrity
in the event of a single wiring fault in the LON/SLC
wiring.
The DCU is approved for use with a variety of Detector
Electronics sensors including catalytic combustible gas
sensors, the PointWatch IR gas detector, as well as
electrochemical sensors (hydrogen sulfide, carbon
monoxide, chlorine, sulfur dioxide, and nitrogen
dioxide). It will also accept any sensor with a linear 4 to
20 ma output signal and allows for one person non-
intrusive calibration.
Inputs
The DCU features one 4 to 20 ma non-isolated input,
with an input impedance of 200 ohms in series with a
protection diode. (When used with a Det-Tronics
catalytic combustible gas sensor, a separate transmitter
board converts the millivolt output to a 4 to 20 ma
signal.) The DCU also monitors its 24 vdc supply
voltage and reports to the PC when requested.
Output
The Standard Periodic Report (SPR) is sent to the
gateway at the rate determined at the time of
configuration (from one to ten seconds).
Calibration
A magnetic reed switch, located on the terminal board,
enables calibration of the sensor without opening the
enclosure. The switch is activated by placing a
calibration magnet at a specified location on the side of
the enclosure. Once the calibration mode has been
entered, the DCU steps the user through the process.
For details, refer to the “Calibration” section of this
manual.
Addressability
Device identification is accomplished by setting
switches on an eight position DIP switch (valid address
range is 5 to 250).
Status LEDs
Three LEDs are located at the center of the
communication module circuit board and are visible
through the window on the enclosure cover.
The green LED serves as a power-on indicator and is
illuminated whenever power is applied. During normal
operation, only the green LED is illuminated.
The red LED is used to indicate a calibration, power-up,
fault or alarm condition. The flashing rate of the red
LED indicates the following conditions:
Power-up =
Pulsed at a 0.5 Hz rate
Calibration =
Pulsed at a 1 Hz rate or on steady
Fault =
Pulsed at a 4 Hz rate
Alarm =
Illuminated continuously.
NOTE
If the communication module has not been
configured, the red LED blinks at the 4 Hz rate.
The amber LED is provided for factory diagnostic
purposes and is not used in the system. Illumination of
the amber LED normally indicates a failure in the
communication chip. Replacement of the
communication module circuit board is required.
Calibration Log
The DCU keeps a calibration log in non-volatile memory
that can be used by the operator to evaluate the
remaining life of some sensors. This log includes the
zero, span, date and time for each successful
calibration. An aborted calibration is indicated by zeros
in the zero and span values. The calibration log is
cleared when the sensor replacement switch is pressed
and the calibration is successfully completed.
The initial calibration is logged in position one, where it
remains for the life of the sensor. If more than 8
calibrations are performed without the sensor
replacement switch being pressed, the newest
calibration data will replace the second oldest so that
the initial calibration data can be saved. The old
calibration data will be lost. This feature enables sensor
sensitivity trending to aid in maintenance or
troubleshooting.
The analog value for the sensor is represented in raw
analog to digital counts 0 to 4095, where 0 represents 0
ma and 4095 represents 24 ma.
Alarm Log
A log of low and high alarms is stored in each DCU.
Time, date and alarm type are logged for each alarm
event. A maximum of eight events can be stored. If
more than eight events are logged, the oldest event will
be overwritten.
IV-8