Modbus communications

The data logger supports ClosedModbus Communications protocol published by Modicon in 1979 for use in programmable logic controllers (PLCs). 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 ClosedSCADA Supervisory Control And Data Acquisition networks. Data loggers can communicate using Modbus on all available communications ports. The data logger supports RTU and ASCII communications modes on RS-232 and RS-485 connections.

CRBasic Modbus instructions include:

  • ModbusClient()
  • ModbusServer()
  • MoveBytes()

See the CRBasic Editor help for detailed instruction information and program examples: .

For additional information on Modbus, see:

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 ClosedDevice Configuration Utility Software tool used to set up data loggers and peripherals, and to configure PakBus settings before those devices are deployed in the field and/or added to networks. 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 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: