Panasonic FPS Series, User Manual

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FP Σ User's Manual
3.5 Thermistor Input Functions
31
3.5 Thermistor Input Functions
CPU types whose part number ends in TM have lead wires that accept thermistor input, in
place of analog volume. You can connect a thermistor to these lead wires and load changes
in the thermistor’s resistance as analog input.
Loading of thermistor input
Changes in the resistance of an externally connected thermistor are read in as changes in
voltage. Then, an AD converter with a microcomputer is used to load these as digital values.
The converted digital values are stored in special data registers DT90040 and DT90041 and
can be read by the user program.
Block diagram:
V REF
V IN
GND
FP
Σ
3.3V 3.3V
2.2kW
1
Lead wire (red)
2
Lead wire (black)
3
Thermistor
The space between the thermistor input and the power supply connector (24V) is not isolated;
the red lead wire is connected to the 3.3V power supply, the black to V IN.
Total precision
Total precision = (total precision of microcomputer in AD converter:
±
5 LSB 1) ) + (precision of
thermistor)
1)
±
5 LSB indicates a margin of error of
±
5 for the post-A/D conversion value (0-1000).
Conversion formula
The following formula is used to convert the thermistor resistance into a digital value.
The range of the digital conversion value is 0-1000.
R [k
Ω
]
=
(x + 12)
1024 x 2.2
- 2.2
R Thermistor resistance
x Digital value