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HomeResourcesModbus GlossaryFC23 — Read/Write Multiple Registers
Function CodesModbus Term

FC23 — Read/Write Multiple Registers

Read and write holding registers in a single atomic transaction.

Definition

Function Code 23 combines a read and a write in one request, ensuring both operations occur before any other master can intervene. This is valuable for read-modify-write operations on PLC setpoints where a stale read between separate transactions would cause logic errors.

Quick Facts

FC hex0x17
Max read125 registers
Max write121 registers
AtomicityWrite occurs before read in same transaction

Related Terms

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ModPackQT Modbus Glossary · Updated 2026