Modbus function reference
Modbus Function Code 04 — Read Input Registers
Read 16-bit read-only register values
04
Read 16-bit read-only register values
Data model
Input registers (3xxxx reference family)
Access
Read 16-bit read-only register values
Protocol quantity
1–125 registers
Worked example
Read one input register at protocol address 8. The normal response returns value 10.
Typical use: Polling measured or status values that the server exposes as read-only input registers.
Request PDU
04 00 08 00 01Normal response PDU
04 02 00 0APDU examples omit the Modbus TCP MBAP header and the Modbus RTU unit-address/CRC envelope. Multi-byte values are shown in network byte order.
Run the reference
Compose the request publicly, then use Pro for advanced graded exceptions, saved fixtures, complete history and project sharing.