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Function CodesModbus Term

FC03 — Read Holding Registers

Read one or more 16-bit holding registers — the most commonly used Modbus function.

Definition

Function Code 03 reads the contents of holding registers (4x range). Each register holds one 16-bit word. FC03 is by far the most used Modbus function — most sensor values, setpoints, counters, and configuration parameters live in holding registers. Up to 125 registers can be read in a single request, and the response returns 2 bytes per register.

Quick Facts

FC hex0x03
Register typeHolding Registers (4x / 40001–49999)
AccessRead-only (use FC06/FC16 to write)
Max per request125 registers
Response bytes2 per register

Example

Request (read 2 registers starting at address 0, slave 1):
01 03 00 00 00 02 C4 0B

Response (registers contain 0x1234 and 0x0001):
01 03 04 12 34 00 01 B8 53
      │  ├──┤  ├──┤
      │  Reg 0  Reg 1
      Byte count (4)

Related Terms

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ModPackQT Modbus Glossary · Updated 2026