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Function CodesModbus Term

FC04 — Read Input Registers

Read one or more 16-bit input registers (read-only, typically sensor values).

Definition

Function Code 04 reads input registers (3x range). These are read-only 16-bit registers typically holding live process values from hardware — ADC readings, temperature, flow rate, etc. The protocol is identical to FC03; the only difference is the target register table. Some devices map the same data to both 3x and 4x ranges.

Quick Facts

FC hex0x04
Register typeInput Registers (3x / 30001–39999)
AccessRead-only
Max per request125 registers

Related Terms

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ModPackQT Modbus Glossary · Updated 2026