> 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-lt-english-v1/communication/modbus-rtu-protocol/read-write-memory-ram/read-write-memory-read-command-03.md).
# Read/Write Memory – Read Command (03)
{% hint style="info" %}
When reading large values using different data formats (e.g., Float32 or Int64) and performing conversions, very small differences may occur due to internal calculation methods, typically less than 1/10,000, which is considered normal.
Registers from different parameter categories cannot be read across categories. Read commands must not exceed 20 registers per request. Requests exceeding this length will be ignored.
{% endhint %}
#### **Function Code:** 03
This command can read single or multiple holding registers, used to access configuration parameters defined in the device’s Holding Registers.
#### **Read/Write Memory – Read Command (03) Format**
#### Modbus RTU **Example of Reading Read/Write Memory Holding Registers (03)**
Using Function Code 03 to read the "A08 Pipe Material" register as an example:
According to the Read/Write Memory – Read List, the starting address is 000A, register length is 1 (one register), data type is Int16 (16-bit integer), and the number of registers to read is 0001 (1 register).
Per device specifications, A08 Pipe Material is stored as an Int16 (16-bit integer), occupying 1 register (2 bytes), with Big Endian byte order.
**Master Sends Read Command (TX Frame)**\
**TX Request Command:**
```
01 03 00 0A 00 01 A4 0E
```
* Slave Address = 01
* Function Code = 03
* Starting Register Address: 000A (A08 Pipe Material register)
* Number of Registers to Read: 0001 (1 register)
* CRC Checksum: A40E (Error-checking code calculated using the CRC algorithm)
**Slave Response Data (RX Frame)**
Assume the Pipe Material code is 3 (Hex: 0003).
**RX Response Data:**
```
01 03 02 00 03 B8 44
```
* Slave Address: 01
* Function Code: 03
* Byte Count: 02 (1 register, 2 bytes)
* Data Bytes: 0003 (Pipe Material code – PP material)
* CRC Checksum: B844 (Error-checking code calculated using the CRC algorithm)
#### **Data Parsing and Conversion (to Decimal)**
#### **Step A** – Arrange the Data
* Received Data: 0003
* Data Type: Int16 (16-bit integer)
#### **Step B – Convert to Decimal and Map Material**
* Convert the hexadecimal value 0003 to decimal:\
Decimal Calculation: (0 × 256) + 3 = 3\
Final Value: 3
* Material Mapping: Refer to the A08 Pipe Material list. The value 3 corresponds to PP material.
### **FAQ**
* **Why is there a difference between the “Memory Map” and the “PLC Read Address”?**
Users may see that the memory map shows **40005**, but in the PLC, **you need to enter 4 (or 0004) to read the correct value**.
This is due to Modbus addressing: human-readable addresses (memory map) and actual device read addresses (communication addresses) have a fixed offset.
For **Function Code 03**, **40001** is the base address. Reading starts from this base, so 40001 corresponds to offset **0000** and read address **0000**.
Therefore, when the document indicates reading **40005**, the actual starting address in the Modbus **Function Code 03** packet is **0004**, i.e.:
**Actual communication address=Document address−40001**
{% hint style="info" %}
**Note:** Modbus uses 0-based addressing (communication packets start from 0), So, in general : Actual communication address=Document address−40001.
**BUT, i**f the PLC or HMI is set to 1-based addressing (software adds 1), enter the starting address according to the software’s rules to read the correct register.
{% endhint %}
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