3.Modbus RTU Protocol

3.1Introduction to Modbus

Model 3810A Addressable In-Place Inclinometers use the industry standard Modbus Remote Terminal Unit (RTU) protocol to communicate with the chosen readout method. 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. The Modbus standard does not specify a physical layer (connection type), but it will work with any interface that can communicate asynchronously with multiple devices (e.g., RS-485, RS-422, optical, radio, etc.). Model 3810A strings use RS-485 as the electrical interface because of its prevalence, simplicity, and success as a robust, industrial physical layer.

3.2Modbus RTU Overview

The Modbus RTU protocol uses packets (multiple sections that together compose a message) 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.CRC (two bytes) – cyclic redundancy check; a 16-bit data integrity check calculated over the other byes in the packet.

3.3Modbus Tables

The most recent sensor reading is stored in a table. The reading is presented in different formats in different sections of the table. The register location and size of these variables are described in the table below.

table 1: RAM Storage

3.4Reading 3810A Sensors

While Modbus RTU supports roughly 20 different function codes, the simple functionality of a bused temperature sensor, such as the geokon 3810A, eliminates the need for all but two of them. Only the Write Holding Register (0x06) function code and Read Holding Registers (0x03) and are needed to read the sensors.

First, use the Write Holding Register function to refresh the temperature measurement. The command initiates multiple analog-to-digital (ADC) conversions that are averaged to optimize precision and resolution. The updated measurements will be ready approximately 0.25 seconds after the request. An example request is shown in the table below.   

table 2: Example Query - Trigger Measurement

Use device address 0 to “broadcast” the trigger command to all drops simultaneously. Measurements between successive address are staggered by 50 milliseconds to prevent excess current draw from simultaneous activation.

The Read Holding Registers function can be used to read one or more 16-bit registers (two bytes each), starting at the Data Address that was sent in the command packet. Temperatures can be retrieved in two alternate formats; one as a raw 16-bit ADC value, the other as a 32-bit floating point (decimal) number that is the computed resistance of the on-board thermistor. For ease of use and program simplicity, the 32-bit floating-point reading is recommended. An example of this query and response is shown in the tables below.

table 3: Example Query - Floating Point Resistance Reading   

table 4: Example Response - Floating Point Resistance Reading

PC Terminal Program Screen Capture

6:    

The above shows the IEEE-754 floating-point response as two parts—each one is composed of two bytes. Because of how this information is stored in memory, the two parts are received in reverse order; the complete floating-point number in HEX is 0x4628C87C (10,802.12 ohms).