Campbell Scientific CR200/CR200X-series Dataloggers - Manual (Page 151)

Section 15. Alternate Telecoms
Resource Library
15.1 Modbus
15.1.1 Overview
Modbus is a widely used SCADA communication protocol that facilitates
exchange of information and data between computers / HMI software,
instruments (RTUs) and Modbus compatible sensors. The CR200(X)
communicates via Modbus over RF and RS-232.
Modbus systems consist of a master (PC), RTU / PLC slaves, field instruments
(sensors), and the communications network hardware. The communications
port, baud rate, data bits, stop bits, and parity are set in the Modbus driver of the
master and / or the slaves. The Modbus standard has two communications
modes, RTU and ASCII. However, CR200(X)s communicate in RTU mode
exclusively.
Field instruments can be queried by the CR200(X). Because Modbus has a set
command structure, programming the CR200(X) to get data from field
instruments is much simpler than from serial sensors. Because Modbus uses a
common bus and addresses each node, field instruments are effectively
multiplexed to a CR200(X) without additional hardware.
A CR200(X) goes into sleep mode after 40 seconds of communications
inactivity. Once asleep, two packets are required before the CR200(X) will
respond. The first packet awakens the CR200(X); the second packet is received
as data.
15.1.2 Terminology
TABLE. Modbus to Campbell Scientific Equivalents (p. 139) lists terminology
equivalents to aid in understanding how CR200(X)s fit into a SCADA system.
Table 19. 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 CR200(X)
Master Usually a computer
Slave Usually a CR200(X)
Field Instrument Sensor
139