Specifications

Section 15. Alternate Telecoms Resource Library
15-3
serial sensors. Because Modbus uses a common bus and addresses each node,
serial sensors are essentially multiplexed to a CR1000 datalogger.
By default, a CSI datalogger goes into sleep mode after 40 seconds of
communications inactivity. Once asleep, two packets are required before the
datalogger will respond. The first packet wakes the logger up and the second
packet is received as data. Dataloggers can be set to keep communications
ports open and awake, but at higher power usage.
15.2.2 Terminology
TABLE 15.2-1 lists terminology equivalents to aid in understanding how
Campbell Scientific dataloggers fit into a SCADA system.
TABLE 15.2-1. Modbus to Campbell Scientific Equivalents
Modbus Domain Data Form Campbell Scientific Domain
Coils Single Bit Ports, Flags, Boolean Variables
Digital Registers 16-bit Word Floating Point Variables
Input Registers 16-bit Word Floating Point Variables
Holding Registers 16-bit Word Floating Point Variables
RTU / PLC Datalogger
Master Usually a computer
Slave Usually a datalogger
Field Instrument Sensor
15.2.2.1 Glossary of Terms
Coils (00001 to 09999)
Originally, “coils” referred to relay coils. In CSI dataloggers, coils are
exclusively ports, flags, or a Boolean variable array. Ports
are inferred if
parameter 5 of the ModbusSlave instruction is set to 0. Coils are assigned
to Modbus registers 00001 to 09999.
Digital Registers 10001-19999
Hold values resulting from a digital measurement. Digital registers in the
Modbus domain are read only. In the CSI domain, the leading digit in
Modbus digital, input and holding registers is ignored, and so digital,
input, and holding registers are assigned together to a single variable
array. Thus, in the CSI domain, digital registers are declared as Dim or
Public variables and are read / write.
Input Registers 30001 - 39999
Hold values resulting from an analog measurement. Input registers in the
Modbus domain are read only. In the CSI domain, the leading digit in
Modbus digital, input and holding registers is ignored, and so digital,
input, and holding registers are assigned together to a single variable
array. Thus, in the CSI domain, input registers are declared as Dim or
Public variables and are read / write.