# Modbus RTU Protocol

### Communication Format

| Name                    |           Value | Description                                                              |
| ----------------------- | --------------: | ------------------------------------------------------------------------ |
| Bit Order               |       lsb first | Data is transmitted starting from the Least Significant Bit (LSB).       |
| Byte Order              |      big endian | Data is transmitted starting from the Most Significant Byte (MSB).       |
| Data bits               |               8 | Each character or byte consists of 8 data bits.                          |
| Parity                  |            None | No parity bit is used for error checking during transmission.            |
| Stop bits               |               1 | 1 stop bit is used at the end of each character.                         |
| Floating-Point Standard |   IEEE 754-1985 | Floating-point numbers (Float32) use the 32-bit single-precision format. |
| Register                | 2 Byte / 16 bit | Floating-point numbers (Float32) use the 32-bit single-precision format. |
| Byte                    |           8 bit | The basic unit length of data transmission.                              |

{% hint style="info" %}
In **Modbus RTU communication**, a **Register** is the basic unit of memory communication, with a length of **2 bytes (16 bits)**. A **Byte** is the basic unit of data transmission, with a length of **8 bits**.
{% endhint %}

***

### **Memory Definition and Functions**

Modbus memory is divided into two categories, **read-only** and **read-write**.

* **Read-only** type is mainly for flow meter status related memory, such as instantaneous flow rate. This is also the most commonly used memory. Refer to [Read-only memory](/qr/fu-es-echosense-instruction-manual/communication/modbus-rtu-protocol/read-only-memory-rom/read-only-memory-read-instruction-04-and-memory-lists.md)
* Read-write memory is mainly to set-up parameters such as pipe diameter, and to reset variables, such as set zero on the cumulative flow. Refer to [Read-Write Memory (RAM)](/qr/fu-es-echosense-instruction-manual/communication/modbus-rtu-protocol/read-write-memory-ram.md)

{% hint style="info" %}
Read-write memory is only readable by using instruction 03. You can only change one parameter at a time with the write operation. If the length of the target register is 1, please use instruction 06. If the length is 2 registers, please use instruction 30. Refer to R
{% endhint %}

<table data-header-hidden><thead><tr><th width="153.65625"></th><th width="281.1875"></th><th></th></tr></thead><tbody><tr><td><strong>Memory Types</strong></td><td><strong>Read-Only Memory</strong></td><td><strong>Read/Write Memory</strong></td></tr><tr><td>Main Functions</td><td>Read the device's real-time status and results.</td><td>Set the device's operating parameters and calibration values.</td></tr><tr><td>Stored Content</td><td>Stores memory related to <strong>flow status</strong>, such as instantaneous flow and totalized flow.</td><td>Stores memory related to <strong>flow configuration parameters</strong>, such as pipe diameter, calibration coefficients, and flow cutoff values.</td></tr><tr><td>Access Rights</td><td><strong>Read-only</strong>: can only be read, not written</td><td><strong>Read/Write</strong>: can be both read and written.</td></tr><tr><td>Function Codes</td><td><strong>Read Command</strong>: 04 <br>(Read Input Registers)</td><td><strong>Read Command</strong>: 03<br><strong>Write Command</strong>: 06 / 30 / 31</td></tr></tbody></table>


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.lorric.com/qr/fu-es-echosense-instruction-manual/communication/modbus-rtu-protocol.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.