AirTest CN9000 Series Manual - Página 2

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AirTest Model CN9000 Series Sensor Controller

THEORY OF OPERATION continued

A TYPICAL SYSTEM WALK THROUGH

The external sensor's power and 4-20mA status signal wires are connected to the input modules wiring

terminals. Processing of the 4-20mA takes the form of a comparator circuit where it's digi t al output si g nal

changes state when the voltage across the 100 ohm current sensing resistor is greater than a reference voltage.

The voltage at which the state change occurs is called the "tri p point".

The trip point voltage can be a fixed value, as in the sensor fail detect (out of operating range) circuit or be

variable through user adjustable trim potentiometers. The base input module incorporates four comparator

outputs, 2 of which have user settable trip points and the remaining 2 are have fixed trip points for high and

low sensor fault conditions. The 2 fault state comparator outputs are tied common to allow one fault

condition signal. The resulting 3 digital signals are placed onto 1 of 16 available signal channels on the

interconnect board through jumpers on the input module.

The signals avail a ble on the interconnect board are used by the output module. Channel selection is

accomplished but setting the appropri a te jumper on the output module. There are 3 channel selection blocks, 1

for each of the 5 amp relays.

Common controller configurations use 3 relays as the final output to external devices. A ventilation system is

acti v ated by the contact closure of a relay when a low gas level is detected, while a second relay activates an

audi b le and/or vi s ual alarm when the gas level reaches a higher level i n dicating a possible problem with the

ventilation system. The third relay is used to signal a fault with the system such a sensor signal loss which may

indi c ate a sensor fault or a break it sensor wiring. In any case it is important to incorporate basic fault detection

into any safety system.

Distributed By:

GLOBAL CONTROLS, INC.

3008-B 16TH AVE W.

SEATTLE, WA 98119

(800) 821-4863 TOLL FREE

(206) 282-4666 TEL

(206) 282-4888 FAX