Campbell Scientific CR200/CR200X-series Dataloggers - Manual (Page 152)

Section 15. Alternate Telecoms Resource Library
15.1.2.1 Glossary of Terms
Coils (00001 to 09999)
Originally, "coils" referred to relay coils. In CR200(X)s, 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 registers is ignored, and so are assigned together to a single Dim or
Public variable array (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 registers is ignored, and so are assigned together to a single Dim or
Public variable array (read / write).
Holding Registers 40001 - 49999
Hold values resulting from a programming action. Holding registers in the
Modbus domain are read / write. In the CSI domain, the leading digit in
Modbus registers is ignored, and so are assigned together to a single Dim or
Public variable array (read / write).
RTU / PLC
Remote Telemetry Units (RTUs) and Programmable Logic Controllers
(PLCs) were at one time used in exclusive applications. As technology
increases, however, the distinction between RTUs and PLCs becomes more
blurred. A CR200(X) fits both RTU and PLC definitions.
15.1.3 Programming for Modbus
15.1.3.1 Declarations
TABLE. CRBASIC Ports, Flags, Variables and Modbus Registers (p. 141)
shows the linkage between CR200(X) ports, flags and Boolean variables and
Modbus registers. Modbus does not distinguish between CR200(X) ports, flags,
or Boolean variables. By declaring only ports, or flags, or Boolean variables, the
declared feature is addressed by default. A typical CRBASIC program for a
Modbus application will declare variables and ports, or variables and flags, or
variables and Boolean variables.
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