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.
Industrial communications path
Choose the physical layer, frame the request, inspect the mapped data and diagnose exception evidence in one connected sequence.