> For the complete documentation index, see [llms.txt](https://docs.lorric.com/qr/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.lorric.com/qr/fu-tx320-english-v1/communication/modbus-rtu-protocol/read-write-memory-ram/read-write-memory-dual-register-write-command-30.md).

# Read/Write Memory Dual-Register Write Command (30)

{% hint style="info" %}
When large values are processed using different data formats (e.g., *float32* vs. *int32*), minor numerical differences may occur due to internal calculation methods. These differences are typically less than **1/10,000** and are normal.

This command applies **only to 32-bit (2-register) memory items**, and uses a **fixed 10-byte frame** — it does not include a Quantity of Registers or Byte Count field, since the length is always exactly 2 registers.
{% endhint %}

#### Function Code 30

The Dual Register Write command (Function Code 30) writes data to two consecutive registers (32-bit / 4 bytes) using a short, fixed-length frame.\
It is used for parameters that include decimal or signed values.\
Float32 range: \~1.18 × 10⁻³⁸ to 3.4 × 10³⁸\
Examples: Set A18 Low Flow Cutoff to +5.5, or K03 Scale Factor to 1.25.

#### Read/Write Memory – Dual Register Write Command (30) Format

![](https://2757023840-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FfX3l9RzQpCUpspFteU5B%2Fuploads%2FYTqwHG2PBRW4JyatueiI%2Fmodbus-30-TX-diagram-en.png?alt=media\&token=e45a35f9-cb7b-48ff-ad54-50a49cd8ff00)

Function Code 30 uses a mirror-response mechanism. When the write is successful, the **RX response frame** from the slave will exactly match the TX request in both format and content.

***

## Modbus RTU Read/Write Memory – Dual Register Write Command (Function Code 30)

### **Example 1 — A18 Low Flow Cutoff:**

This example shows the master using **Function Code 30** to write **+5.5** to the parameter **“A18 Low Flow Cutoff”**.

According to the Read/Write Memory List:

* **Starting Address:** 00 15
* **Register Length:** 2 (two registers)
* **Data Type:** Float 32

#### 1. Value Conversion and Data Preparation (Hex Data)

Convert the decimal value **+5.5** to **IEEE 754 Float32 (Big Endian)** format.

$$
5.5 ₁₀ = 40B00000 ₁₆
$$

The resulting 4-byte data is:：`40` `B0` `00` `00`。

#### 2. Master Sends Write Command (TX Frame)

The master uses Function Code 30 with a Byte Count of 04, writing 4 data bytes to two registers starting at address 00 15.

```
TX： 01 30 00 15 40 B0 00 00 A8 20
```

* Slave Address: `01`
* Function Code: `30`
* Starting Register Address: `00 15` (A18 Low Flow Cutoff)
* Number of Registers Written: `00 02` (2 registers / Float32)
* Byte Count: `04` (4 data bytes)
* Written Data (Float32): `40 B0 00 00` (+5.5 in Hex)
* CRC: `27 D1` (calculated CRC)

#### 3. **Slave Response (RX Frame)**

When the write command is successful, the master receives a response from the slave as confirmation.

```
RX: 01 30 00 15 40 B0 00 00 A8 20
```

* Slave Address: `01`
* Function Code: `30`
* Starting Register Address: `00 15` (A18 Low Flow Cutoff)
* Registers Written: `00 02` (confirmation of 2 registers written)
* CRC: `D1 CB` (calculated CRC)

Once the master receives the RX frame and verifies that the **Function Code**, **register address**, and **number of registers written** are correct, it confirms that **A18 Low Flow Cutoff has been successfully set to +5.5**.

***

### **Example 2 — A18 Low Flow Cutoff:**

This example shows the master using **Function Code 30** to write **1.25** to the parameter **"K03 Scale Factor"**.

According to the Read/Write Memory List:

* Starting Address: 0A 01
* Register Length: 2 (two registers)
* Data Type: Float 32

#### **1. Value Conversion and Data Preparation (Hex Data)**

Convert the decimal value 1.25 to IEEE 754 Float32 (Big Endian) format.

The resulting 4-byte data is: `3F A0 00 00`.

#### **2. Master Sends Write Command (TX Frame)**

```
TX: 01 30 0A 01 3F A0 00 00 80 98
```

* Slave Address: 01
* Function Code: 30
* Starting Register Address: 0A 01 (K03 Scale Factor)
* Write Data (Float32): 3F A0 00 00 (1.25 in Hex)
* CRC: 80 98 (calculated CRC)

#### **3. Slave Response (RX Frame)**

```
RX: 01 30 0A 01 3F A0 00 00 80 98
```

Mirrors the TX frame exactly, confirming the write succeeded.

**Write Failure — Exception Response**

If the write fails (e.g., the value is outside the parameter's valid range), the slave returns a 5-byte exception frame instead of an echo:

Slave Address | Function Code | 0x80 | Exception Code | CRC (2 bytes)

Example: exception code `0x03` indicates the value is out of range.


---

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