are required before it will respond. The first packet awakens the data logger; the second packet is
received as data. This would make a Modbus client fail to poll the data logger, if not using retries.
More information on Modbus can be found at:
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www.simplyModbus.ca/FAQ.htm
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www.Modbus.org/tech.php
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www.lammertbies.nl/comm/info/modbus.html
19.2.1 About Modbus
Modbus is a communications protocol that enables communications among many devices
connected to the same network. Modbus is often used in supervisory control and data
acquisition (SCADA) systems to connect remote terminal units (RTUs) with a supervisory
computer - allowing them to relay measurement data, device status, control commands, and
configuration information.
The popularity of Modbus has grown because it is freely available and because its messaging
structure is independent of the type of physical interface or connection that is used. Modbus can
coexist with other types of connections on the same physical interface at the same time. You can
operate the protocol over several data links and physical layers.
Modbus is supported by many industrial devices, including those offered by Campbell Scientific.
Not only can intelligent devices such as microcontrollers and programmable logic controllers
(PLCs) communicate using Modbus, but many intelligent sensors have a Modbus interface that
enables them to send their data to host systems. Examples of using Modbus with Campbell
Scientific data loggers include:
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Interfacing data loggers and Modbus-enabled sensors.
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Sending and retrieving data between data loggers and other industrial devices.
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Delivering environmental data to SCADA systems.
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Integrating Modbus data into PakBus networks, or PakBus data into Modbus networks.
19. Communications protocols111
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