3.Modbus RTU Protocol

3.1Introduction to Modbus

Model 6180 inclinometers use the industry standard Modbus Remote Terminal Unit (RTU) protocol to communicate with the chosen readout method. As the name suggests, Modbus was designed to work on what is known as a bus network, meaning that every device receives every message that passes across the network. Model 6180 inclinometers use the RS-485 electrical interface because of its prevalence, simplicity, and success as a robust, industrial physical layer.

More information about Modbus can be found at the following website:

http://www.modbus.org/specs.php

3.2Modbus RTU Overview

The Modbus RTU protocol uses packets (messages made up of multiple sections) to communicate and transfer data between devices on the network. The general format of these packets is as follows:

1.Modbus Address (one byte) – The address of the specific device on the bus. (Labeled on the sensors as #1, #2, #3, etc.)

2.Function Code (one byte) – The action to be carried out by the slave device.

3.Data (multi-byte) – The payload of the function code being sent.

4.Cyclic Redundancy Check or CRC (two bytes): A 16-bit data integrity check calculated over the other bytes in the packet.

3.3Modbus Tables

The most recent sensor readings are stored in memory registers, read using a Modbus command. Angle and temperature readings are available in processed or precursor formats. Register addresses and formats are described in Table 2.

Note: geokon stores the gauge factor and offsets in the sensor during the factory calibration process.  Therefore, the outputs of the A- and B- axes are both corrected values.

Table 3 shows device control addresses. Any non-zero value written to the trigger address initiates a measurement cycle, updating the angle and temperature measurement registers. Any anomalies detected during the most recent measurement cycle produce a non-zero error code. Refer to Appendix C for an explanation of these codes.

The flash password prevents unintended writes to the nonvolatile memory in Table 4. Contact geokon for instructions.

table 2: Register Addresses and Formats

table 3: Device Control Addresses

table 4: Non-Volatile Memory