> 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-four-register-write-command-31.md).
# Read/Write Memory - Four Register Write Command (31)
{% hint style="info" %}
When large values are processed using different data formats (e.g., **float32** vs. **int64**), minor differences may occur due to internal calculation methods, typically less than **1/10,000**, and are considered normal.
This command applies **only to memory items with a register length of 4**. Refer to the [parameter list](/qr/fu-tx320-english-v1/communication/modbus-rtu-protocol/read-write-memory-ram/read-write-memory-read-listx.md#read-write-memory-list) for details.
{% endhint %}
#### Function Code 31
The Four Register Write command (Function Code 31) writes 4 consecutive registers (64-bit), supporting Int64 or other 64-bit values. It is recommended for writing large parameters such as: D11 Total Target (64-bit), C04 Dosing Target (64-bit)
{% hint style="info" %}
A standard 32-bit floating-point number (Function Code 30) has a precision of about **7 significant digits**. When the value exceeds ten million, using 32-bit floats may cause **accuracy loss and rounding errors**.
In such cases, it is recommended to use **Function Code 31** with **64-bit integers (Int64)** to ensure the data is accurately stored in memory without any rounding errors.
{% endhint %}
#### Read/Write Memory – Four Register Write Command (Function Code 31) Format
Function Code 31 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 – Four Register Write Command (Function Code 31)
### Example — C04 Dosing Target
This example demonstrates how the **master** uses **Function Code 31** to write a target value of **+999,999,999** to the parameter **C04 Dosing Target**.
* **Register Address:** 02 83
* **Register Length:** 4 (4 consecutive registers, 64-bit)
* **Data Type:** Int64
#### 1. Value Conversion and Data Preparation (Hex Data)
Required Scaling: this parameter must be multiplied by ×100,000 before conversion
```
999,999,999 × 100,000 = 99,999,999,900,000
Hex (Int64, 8 bytes): 00 00 5A F3 10 78 B9 60
```
* Target Value: `999,999,999` (decimal)
* Required Scaling: `[VERIFY]` C04 Dosing Target is understood to be stored internally as a fixed-point value with 5 decimal places of precision, so the raw register holds the display value × 10⁵ (×100,000)
* Hex Conversion: 99,999,999,900,000 = `5A F3 10 78`。
* Packed in Int64 Format (8 Bytes):\
Since Function Code 31 requires 8 bytes, prepend zeros
* Complete 64-bit Data: `00 00 00 00 3B 9A C9 FF`
#### **2.** Master Sends Write Command (TX Frame)
```
TX： 01 31 02 83 00 00 5A F3 10 78 B9 60 49 4F
```
* Slave Address: `01` (Flow meter address)
* Function Code: `31` (Write 4 Registers)
* Starting Register Address: `02 83` (C04 Dosing Target 64-bit memory address)
* Data to Write: `00 00 5A F3 10 78 B9 60` (C04 Dosing Target 64-bit value, scaled)
* CRC: `49 4F` (Calculated CRC for error checking)
#### 3. Slave Response (RX Frame)
When the write is successful, the slave echoes the TX frame for verification.
```
RX: 01 31 02 83 00 00 5A F3 10 78 B9 60 49 4F
```
**Write Failure — Exception Response**
As with Function Code 30, a failed write returns a 5-byte exception frame instead of an echo: Slave Address | Function Code | 0x80 | Exception Code | CRC. Example: exception code `0x03` indicates the value is out of rang
---
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