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Document: LT0273 MX4428 MXP Engineering / Technical Manual
Device Processing
Issue 1.5 24 March 2006 Page 9-9
9.6 IONISATION PROCESSING
The smoke reading of the detector is returned in two parts as inputs AI0 and AI1 of the
device ASIC. In the MXP the average value of AI0 and AI1 is used for all calculations, so
that all returned values to the MX4428 lie within the range of 0 – 255. (Note that TEPG
literature refers to the sum of the two inputs.)
The processing is shown in
Figure 9.6. The tracked value is initialised by setting it to a high
value (100) when the MXP or detector powers up, and allowing the tracked value to
increment / decrement by 1 for each of the first 4 minutes.
Exponential
Filter
Slope
Limiter
Very Slow Slope
Limiter
(
1 step per Trackin
g
Interval
)
TV = Tracked Value
= Assumed Clean Air Value
Raw
Value
+
++
+
-
--
-
Alarm Threshold
Alarm
Subtractor
Subtractor
Comparator
Comparator
CV
PreAlarm Threshold
PreAlarm
SLV
Filter Divisor
Step Limit
Figure 9.6 Ionisation Detector Processing