# Read-Only Memory – Read Command (04)
{% 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. These differences are typically **less than 1/10,000** and are considered **normal**.
A single read command can access up to **40 registers**. Requests exceeding this length will be ignored.
{% endhint %}
### Function Code 04
**Purpose:** This memory area is specifically used to store the device's **real-time status**, **measurement results**, and **flow-related memory**.
**Characteristics:** The data is **read-only** and can only be accessed via the **Read command**. Values **cannot be modified** using write commands.
### Read-Only Memory – Read Command (04) Format
### Modbus RTU – Example of Read-Only Memory Read Command (04)
Using the **Read-Only Memory Read Command (04)** as an example, this section explains how to send a command and parse the returned data into a decimal value when the **totalized flow** is **20,000.5 liters**.
**Master Sends Read Command (TX Frame)**
According to device specifications, the accumulated flow is stored in **Float32 format**, occupying **2 registers (4 bytes)**; therefore, the quantity to read should be set to **00 02**. The master must send Modbus function code 04 (Read Input Registers) to read 2 registers starting from address 0000.
```
TX: 01 04 00 00 00 02 71 CB
```
* Slave Address = 01
* Function Code = 04
* Starting Register Address = 00 00
* Quantity to Read = 00 02 (2 registers)
* CRC Checksum = 71 CB
**Slave Response Data (RX Frame)**
After receiving the read request, the slave returns the internally stored **20,000.5 liters (Float32)** data, packaged as **4 bytes** according to **Modbus Register Big Endian** (high byte first) and **IEEE 754 Float32** format.
```
RX: 01 04 04 45 9C 40 00 E2 56
```
* Slave Address = 01
* Function Code = 04
* Byte Count (Number of Data Bytes) = 04 (4 Bytes)
* Data (Data Field) = 45 9C 40 00
* CRC Checksum = E2 56
**Data Parsing and Conversion (Back to Decimal)**
**Step A: Arrange the Data**\
Since the device uses **Big Endian** format, the most significant byte comes first. Therefore, the 4 bytes are kept in the received order and combined as:\
`459C4000₁₆`
**Step B: Decode the Float32 Structure (SEM: S = Sign, E = Exponent, M = Mantissa)**\
Convert `459C4000₁₆` into a 32-bit binary number for IEEE 754 Float32 interpretation.
**Partial Binary Value Explanation**
* **S (Sign bit)** = 0 (1 bit) → 0 means a positive number
* **E (Exponent bits)** = `10001011` (8 bits) → 139₁₀\
Actual exponent = 139 − 127 = **12**
* **M (Mantissa bits)** = `001110001...` (23 bits) → the significand is **1.M**
**Step C: Calculate the Value**
According to the IEEE 754 Float32 format, the calculation is as follows.\
The exponent bits `10001011₂` equal 139₁₀, so the actual exponent is 139 − 127 = 12.
* The mantissa bits are 0011100100000000000000，, giving a significand of 1.001110001...
After shifting the binary point 12 places to the right and converting to decimal:
Value = 2¹⁴ + 2¹¹ + 2¹⁰ + 2⁹ + 2⁵ + 2⁻¹\
\= 16384 + 2048 + 1024 + 512 + 32 + 0.5\
\= 20000.5
Therefore, the Modbus RX response data `01 04 04 45 9C 40 00 E2 56` represents a cumulative flow value of **20000.5 liters**.
{% embed url="?" %}
A Modbus format converter tool is available at:
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