# 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. A single read command can access up to 40 registers. 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** ![](https://3404778090-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fi4ECt41Kyy9211OWazj4%2Fuploads%2FRDPIYYi5jti5cuB5GScu%2Fmodbus-03-TX-diagram-en.png?alt=media\&token=9118d9b0-8657-44cc-9cde-baad5c2218cf) ![](https://3404778090-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fi4ECt41Kyy9211OWazj4%2Fuploads%2FlDwdjwUgzzWLAWyfIKc6%2Fmodbus-03-RX-diagram-en.png?alt=media\&token=9751a213-080f-48eb-bef2-e85f3a875e5a) #### 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 %}