Campbell Scientific AM25T 25-Channel Solid State Multiplexer - Manual (Page 17)

AM25T Solid State Multiplexer
'Only the Reference Temperature is measured and stored in RTemp_C.
'To do this, enter 0 for the REP value
AM25T(RTemp_C,0,mV2_5C,1,1,TypeT,RTemp_C,2,1,1,True,0,250,1,0)
'Ten Type T (copper-constantan) are measured on the AM25T.
'Thermocouple measurements are stored in the Temp_C() variable
AM25T(Temp_C(1),10,mV2_5C,1,1,TypeT,RTemp_C,2,1,0,True,0,_60Hz,1,0)
'Call Data Tables and Store Data
CallTable(Table1)
NextScan
EndProg
6.2 Edlog Programming
The CR23X uses Instruction 134 for measuring the AM25T. Other Edlog
dataloggers use a program that contains a loop in which the multiplexer is
advanced to the next channel and the datalogger makes a measurement.
An outline of the looping datalogger program appears below. Steps two
through seven are used to measure the reference temperature for
thermocouples and could be skipped for voltage measurements.
1. Turn on the AM25T
2. Measure the output of the RTD full bridge
3. Find the resistance of the RTD
4. Calculate the reference temperature
5. Pulse the CLK line once (long lead length compensation only)
6. Measure the excitation at the full bridge (long lead length compensation
only)
7. Reset the AM25T by pulsing the RES line (long lead length compensation
only)
8. Loop through the measurements
a) Pulse the CLK line twice
b) Measure the sensor
c) Convert the engineering units
d) Go to a) until all the sensors have been measured
9. Turn off the AM25T
Earlier versions of the AM25T had a zener diode for transient protection on the
RTD excitation. This diode limited the negative excitation to -400 mV.
Instruction 6 used to measure the RTD uses both a positive and negative
excitation voltage. Table 6-3 lists recommended excitation voltages and
measurement ranges to use with these early multiplexers. The newer
multiplexers will work with the excitation voltages in Table 6-3, but the higher
excitations used in the example programs provide more immunity to noise.
AM25T multiplexers starting with serial number 1839 allow an excitation
voltage up to
± 5000 mV.
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