Modbus Function Code 3

Read Multiple Holding Registers on Modbus

It is used for reading contents on a contiguous block of holding registers in a remote device. The Request PDU specifies the starting register address and the number of registers. In the PDU Registers are addressed starting at zero. Therefore registers numbered 1-16 are addressed as 0-15.

Request

Response

The register data in the response message is packed in two byte values per register (Table 2). For each register, the first byte contains the high order bits and the second contains the low order bits.

Error

Example

Request

This command is requesting the content of analog output holding registers # 40112 to 40114 from the slave device with address 11. Each register contains 16 bits.

• 0B: The Slave Address (0B hex = address 11)
• 03: The Function Code 3 (Read Multiple Holding Registers)
• 006F: The Data Address of the first register requested (006F hex = 111, +40001 offset = input #40112).
• 0003: The total number of registers requested (read 3 registers 40112 to 40114).
• 357C: The CRC (Cyclic Redundancy Check) for error checking.

Response

• 0B: The Slave Address (0B hex = address 11)
• 03: The Function Code 3 (Read Multiple Holding Registers)
• 06: The number of data bytes to follow (3 registers x 2 bytes each = 6 bytes)
• AE41: The contents of register 40112
• 5652: The contents of register 40113
• 4340: The contents of register 40114
• FACD: The CRC (Cyclic Redundancy Check)

Workflow Diagram for Modbus FC 3

Contents retrieved from

• http://modbus.org/
• http://www.simplymodbus.ca/