Read-Only Memory – Read Command (04)

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.

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 5,000.0 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 1F 66

• Slave Address = 01

• Function Code = 04

• Byte Count (Number of Data Bytes) = 04 (4 Bytes)

• Data (Data Field) = 45 9C 40 00

• CRC Checksum = 1F 66

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 00111000100000000000000, giving a significand of 1.001110001...

After shifting the binary point 12 places to the right and converting to decimal:

Value = 2¹²+2⁹+2⁸+2⁷+2³ = 4096+512+256+128+8 = 5000.0

Therefore, the Modbus RX response data 01 04 04 45 9C 40 00 1F 66 represents a cumulative flow value of 5000.0 liters.

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