Modbus Communications - Campbell CR300 Series Product Manual

NOTE:
Though COM1 (C1/C2) uses RS-232 logic levels, it is limited to 0 V (logic high) and 5 V (logic
low) output. This may make COM1 incompatible with some serial devices.
COM1 is not capable of TTL logic levels and so is not compatible with TTL-to-RS-232
converters for the purpose of presenting a true RS-232 interface.
COM1 also has a low input resistance that may make it incompatible with some serial devices
with the addition of in-line resistance.

19.2 Modbus communications

The data logger supports Modbus RTU, Modbus ASCII, and Modbus TCP protocols and can be
programmed as a Modbus client (master) or Modbus server (slave). These protocols are often
used in SCADA networks. Data loggers can communicate using Modbus on all available
communications ports. The data logger conducts Modbus over RS-232 using a RS-232-to RS-
485 adapter, or over TCP using an Ethernet or Wireless connection.
CRBasic Modbus instructions include:
ModbusClient()
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ModbusServer()
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MoveBytes()
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See the CRBasic Editor help for detailed instruction information and program examples:
https://help.campbellsci.com/crbasic/cr300/
For additional information on Modbus, see:
About Modbus (p. 111)
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Why Modbus Matters: An Introduction
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How to Access Live Measurement Data Using Modbus
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Using Campbell Scientific Dataloggers as Modbus Slave Devices in a SCADA Network
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Because Modbus has a set command structure, programming the data logger to get data from
field instruments can be much simpler than from some other serial sensors. Because Modbus
uses a common bus and addresses each node, field instruments are effectively multiplexed to a
data logger without additional hardware.
When doing Modbus communications over RS-232, the data logger, through Device
Configuration Utility or the Settings editor, can be set to keep communications ports open and
awake, but at higher power usage. Set RS-232Power to Always on. Otherwise, the data logger
goes into sleep mode after 40 seconds of communications inactivity. Once asleep, two packets
.
19. Communications protocols 110