Geokon 3810A, Instruction Manual

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MODEL 3810A ADDRESSABLE THERMISTOR STRING | MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS | 5
4. MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS
4.1 DESCRIPTION
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 software is typically used when
programming in CRBasic.
4.2 WIRING
TABLE 6: Dataloggers with RS-485 Conversion
TABLE 7: Dataloggers without RS-485 Conversion
TABLE 8: 3810A Thermistor String to RS-485 to TTL Converter Wiring
4.3 SAMPLE CR6 PROGRAM
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
'Define Data Tables
DataTable (Test, 1,-1)
Sample (5,Celsius(),IEEE4)
'Sample Celsius for 5 sensors in string
EndTable
Campbell Dataloggers with RS-485 3810A String Description
12V Red Thermistor String Power
Digital I/O RS-485 Port TX Green Communication RS-485-
Digital I/O RS-485 Port RX White Communication RS-485+
G Black Ground
AG Shield Analog Ground
Campbell Dataloggers w/o RS
-
485 RS
-
485 to TTL Converter
Description
12V V IN (12V) Converter Power
Digital I/O RS-232 Port TX TX Transmit
Digital I/O RS-232 Port RX RX Receive
G GND Ground
AG Shield Analog Ground
3810A String
RS
-
485 to TTL Converter
Description
Red V OUT Thermistor String Power
White 485+ Communication RS-485+
Green 485- Communication RS-485-
Black GND Ground
Shield Shield Analog Ground
Const A = 1.128706256E
-3
Const B = 2.342327483E
-4
Const C = 0.8707279757E
-7
Public ErrorCode
'Result of ModBusMaster communications attempt
Public Res As Float
'Resistance of thermistor must be stored as Type Float
Public Celsius(5)
'Calculated Celsius for 5 sensors in string
Public Count
'Counter to increment through temperature sensors