Section 8. Operation
476
8.10.3 Modbus — Details
The CR3000 supports Modbus master and Modbus slave communications for
inclusion in Modbus SCADA networks. 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 CR3000 communicates with Modbus over RS-232, (with a RS-232 to RS-
485 such as an MD485 adapter), and TCP.
Modbus systems consist of a master (PC), RTU / PLC slaves, field instruments
(sensors), and the communication-network hardware. The communication port,
baud rate, data bits, stop bits, and parity are set in the Modbus driver of the master
and / or the slaves. The CR3000 supports RTU and ASCII communication modes
on RS-232 and RS485 connections. It exclusively uses the TCP mode on IP
connections.
Field instruments can be queried by the CR3000. Because Modbus has a set
command structure, programming the CR3000 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
CR3000 without additional hardware.
A CR3000 goes into sleep mode after 40 seconds of communication inactivity.
Once asleep, two packets are required before the CR3000 will respond. The first
packet awakens the CR3000; the second packet is received as data. This would
make a Modbus master fail to poll the CR3000, if not using retries. The CR3000,
through DevConfig or the Status table (see Info Tables and Settings
(p. 567))
, can be
set to keep communication ports open and awake, but at higher power usage.
8.10.3.1 Modbus Terminology
Table Modbus to Campbell Scientific Equivalents
(p. 476)
lists terminology
equivalents to aid in understanding how CR3000s fit into a SCADA system.
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
CR3000
Master
Usually a computer
Slave
Usually a CR3000
Field instrument
Sensor
Summary of Contents for CR3000 Micrologger
Page 2: ......
Page 3: ......
Page 4: ......
Page 6: ......
Page 30: ......
Page 34: ......
Page 36: ......
Page 96: ......
Page 485: ...Section 8 Operation 485 8 11 2 Data Display FIGURE 110 Keyboard and Display Displaying Data ...
Page 487: ...Section 8 Operation 487 FIGURE 112 CR1000KD Real Time Custom ...
Page 491: ...Section 8 Operation 491 FIGURE 116 Keyboard and Display File Edit ...
Page 496: ......
Page 502: ......
Page 564: ...Section 11 Glossary 564 FIGURE 126 Relationships of Accuracy Precision and Resolution ...
Page 566: ......
Page 594: ......
Page 598: ......
Page 600: ......
Page 602: ......
Page 624: ......
Page 642: ......
Page 643: ......