Modbus Communication Basics for Arc Intelligent Sensors

What is Modbus?

Hamilton ARC and Visiferm ECS dissolved oxygen sensors provide the ability for digital sensor communication via Modbus RTU digital protocol. The Modbus RTU communication protocol corresponds to the Modbus-IDA standard. ARC and Visiferm ECS sensors use an open set of registers, with detailed information for each sensor registers found in individual Programmers Manuals for each sensor family.

Modbus communication enables the user the ability to

  • Configure sensor settings
  • Calibrate the sensor
  • Measure process variables
  • Output secondary measurement parameters *specific to certain sensors

Modbus Connectivity

Hamilton ARC and Visiferm sensors are always connected to digital controlling devices such as PLCs as Modbus slaves. Each Modbus slave has a unique address, with up to 31 addresses able to be added on a single serial line. In this configuration only one query-response cycle may be performed at any given time.

In the connection scheme shown above, each digital sensor must have a unique Modbus device address for proper communication. In order to avoid signal noise, the use of line termination resistors (120 Ohm) is recommended. The effect of signal noise becomes more relevant with long cable length and/or high baud rates.

The serial Modbus connection uses an RS485 port for connection between the master port and the corresponding interfaces of the sensors according to the EIA/TIA RS485 standard.

Basic configuration of Modbus sensor settings should pair all devices with identical port configuration parameters except device address.

  • Baud rate – how fast bits are transmitted along a communication line (Default 19200)
  • Number of data bits – number of bits sent per frame (Default 8)
  • Number of stop bits – bit size for the end of the frame (Default 2)
  • Parity – error checking for serial communication (Default None)

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