Campbell Scientific CR200X CR200/CR200X-series Dataloggers - Page 151
Alternate Telecoms, Resource Library
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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