4.Modbus & Campbell Scientific Dataloggers
Model 3810A sensor strings can be easily read using Campbell dataloggers that support ModBus RTU commands. Campbell dataloggers such as the CR6 and CR1000X can read half-duplex RS-485 on select RS-485 digital I/O pairs. This allows the 3810A string to be connected directly at the datalogger.
The Model 8020-38 Addressable Bus Converter enables Model 3810A sensor strings to work with Campbell dataloggers that do not support direct RS-485 communications, such as the CR800 and CR1000. The Model 8020-38 is a RS-485 to TTL converter, used with select TTL digital I/O pairs.
CRBasic is the programming language used with Campbell Scientific CRBasic dataloggers. Campbell Scientific's LoggerNet softwae is typically used when programming in CRBasic.
The following sample program reads one 3810A string with five addressable thermistors. The string in this example communicates with the CR6 through the control ports C1 (green) and C2 (white), which are set up as ComC1:
'Constants used in Steinhart-Hart equation to calculate sensor temperature for
'10k thermistor
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Const A = 1.128706256E-3 |
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Const B = 2.342327483E-4 |
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Const C = 0.8707279757E-7 |
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Public ErrorCode |
'Result of ModBusMaster communications attempt |
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Public Res As Float |
'Resistance of thermistor must be stored as Type Float |
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Public Celsius(5) |
'Calculated Celsius for 5 sensors in string |
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Public Count |
'Counter to increment through temperature sensors |
'Define Data Tables
DataTable (Test, 1,-1)
Sample (5,Celsius(),IEEE4) 'Sample Celsius for 5 sensors in string
EndTable
'Main Program
BeginProg
'Open COM port with RS-485 communications at 115200 baud rate
SerialOpen (ComC1,115200,16,0,50,3)
'Read all 5 sensors in string every 4 seconds
Scan (4,Sec,0,0)
'Loop to read each thermistor
For Count = 1 To 5
'Reset temporary storage for next reading
Res = 0
'Flush Serial between readings
SerialFlush (ComC1)
'Write to register 0x118 to trigger thermistor string
'NOTE: ModbusMaster won't send 0x118 unless "&H119" is entered
ModbusMaster (ErrorCode,ComC1,115200,Count,6,1,&H119,1,1,50,0)
'Delay after triggering the measurement
Delay (1,1,Sec)
'Use Modbus command to retrieve resistance from thermistor string
ModbusMaster (ErrorCode,ComC1,115200,Count,3,Res,&H103,1,1,50,0)
'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart equation
Celsius(Count) = 1/(A+B* LN(Res) + C * LN(Res)^3)-273.15
Next
'Call Table to store Data
CallTable Test
NextScan
EndProg
4.3Sample CR1000 Program with Required Model 8020-38 Addressable Bus Converter
The following sample program reads one 3810A string with five addressable thermistor sensors. The string in this example is connected to a RS-485 to TTL converter and communicates with the CR1000 through the control ports C1 and C2, which are setup as COM1:
'Constants used in Steinhart-Hart equation to calculate sensor temperature for
'10k thermistor
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Const A = 1.128706256E-3 |
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Const B = 2.342327483E-4 |
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Const C = 0.8707279757E-7 |
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Public ErrorCode |
'Result of ModBusMaster communications attempt |
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Public Resistance As Float |
'Resistance of thermistor must be stored as Type Float |
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Public Celsius(5) |
'Calculated Celsius for 5 sensors in string |
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Public Count |
'Counter to increment through temperature sensors |
'Define Data Tables
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DataTable(Test,1,-1) |
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Sample (5,Celsius(),IEEE4) |
'Sample Celsius for 5 sensors in string |
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EndTable |
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'Main Program
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BeginProg |
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'Open COM port with TTL communications at 115200 baud rate |
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SerialOpen (Com1,115200,16,0,50) |
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'Read all 5 sensors in string every 4 seconds |
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Scan (4,Sec,0,0) |
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'Loop to read each thermistor |
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For Count = 1 To 5 |
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'Reset temporary storage for next reading |
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Resistance = 0 |
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'Flush Serial between readings |
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SerialFlush (Com1) |
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'Write to register 0x118 to trigger thermistor string 'NOTE: ModbusMaster won't send 0x118 unless "&H119" is entered |
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ModbusMaster (ErrorCode,Com1,115200,Count,6,1,&H119,1,1,50,0) |
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'Delay after triggering the measurement |
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Delay (1,1,Sec) |
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'Use Modbus command to retrieve resistance from thermistor string |
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ModbusMaster (ErrorCode,Com1,115200,Count,3,Resistance,&H103,1,1,50,0) |
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'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart equation |
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Celsius(Count) = 1/(A+B* LN(Resistance) + C * LN(Resistance)^3)-273.15 |
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Next |
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'Call Table to store data |
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CallTable Test |
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NextScan |
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EndProg |
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