When set up as a direct connect data logger, The SDL500 submersible data logger can be preconfigured to output environmental data to SDI-12 or Modbus devices. SDI-12 output is typically used for connection to other data loggers, while Modbus output is mainly used for integration into Supervisory Control and Data Acquisition (SCADA) systems.

The SDL500 is used as a dedicated pass-through device and cannot log data when configured with an optional output. When purchased in this way, iChart software is not required for use with the product and the SDL500 USB interface cable does not need to be used during the initial system setup. A UW-plug to flying lead cable is the only required accessory, used for wiring the appropriate signals from the SDL500 communication port into a master device.

NexSens will summarize for the end user the required technical data, whether it is an SDI-12 parameters list, or Modbus communication settings and measurement parameter data addresses (i.e. input registers). This is done on an individual basis to facilitate quick and easy setup and deployment.

To connect additional sensors following the initial factory configuration, user programming with iChart software will be required. Contact NexSens Customer Support for additional information.


SDI-12 (Serial Data Interface at 1200 Baud) is a communications protocol developed for instruments that typically monitor environmental data. When configured for SDI-12 output, an SDL500 data logger is set to run as the slave device in a master-slave configuration. This is beneficial when environmental sensors using communication methods not supported by the master device (data logger) must be integrated into a preexisting network.

With this setup the SDL500 acts solely as an SDI-12 sensor. It does not log data, but rather reports the data that it collects to another data logger. The data is output in a predefined SDI-12 parameter list, which the master device is programmed to interpret.

NexSens Technology provides custom SDI-12 parameter lists based on the sensors connected to the SDL500 and end user specifications.

Modbus RTU

Modbus facilitates the sharing of process data, and is commonly used to integrate NexSens data loggers into SCADA systems where they act as sensor controllers.

Modbus remote terminal unit (RTU) protocol is based on serial communication like RS-232 or RS-485 and is the most common implementation of Modbus. When used as an output for NexSens data loggers, the data logger acts as a remote terminal unit that can be connected to a supervisory computer in a SCADA system.

NexSens Technology matches Modbus RTU settings like baud rate, frame, address, command, timeout, and data type to those used in the existing SCADA network. The followings Modbus RTU settings are supported by the NexSens data logger:

  • Serial communication protocol: RS-232, RS-485
  • Baud rate: 19,200 or 9,600 at even or no parity, 8 data bits, and 1 stop bit
  • Modbus address: 1-247
  • Command: function codes 0x04 (read input registers) and 0x03 (read holding registers)
  • Timeout: user defined, no restrictions
  • Data type: 4-byte (32-bit) IEEE float (Big-endian or Little-endian), or 2-byte (16-bit) integer

Big-endian data is represented with the most significant word on the low input register. Little-endian data is represented with the least significant word on the low input register.

Note that it’s required for the sensors to be used with SDL500 data loggers configured for SDI-12 or Modbus outputs be sent to the factory for the initial setup, testing and programming of the device.


REV: 13G19

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