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Function CodesModbus Term

FC01 — Read Coils

Read the ON/OFF state of one or more output coils.

Definition

Function Code 01 reads the current state of digital output coils (0x register range). Each coil is 1 bit — either ON (1) or OFF (0). Coil states are packed into the response bytes, LSB first. You can request up to 2000 coils in a single request.

Quick Facts

FC hex0x01
Register typeCoils (0x / 00001–09999)
AccessRead-only
Max per request2000 coils
Data size1 bit per coil

Example

Request (read 2 coils from address 0, slave 1):
01 01 00 00 00 02 FD CA

Response (both coils ON):
01 01 01 03 10 27
            │  └─ CRC
            └─ Data: 0x03 = bits 0b00000011 → coil 0=ON, coil 1=ON

Related Terms

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ModPackQT Modbus Glossary · Updated 2026