PLC Simulator
Modbus function reference

Modbus Function Code 06 — Write Single Register

Write one 16-bit register

06
Load in Modbus sandbox

Data model

Holding registers (4xxxx reference family)

Access

Write one 16-bit register

Protocol quantity

Exactly 1 register

Worked example

Read the PDU field by field.

Write value 1 to protocol address 10. A normal response echoes the address and value.

Typical use: Sending a setpoint, mode or command value to one documented holding register.

Example protocol data units

Request PDU

06 00 0A 00 01

Normal response PDU

06 00 0A 00 01

PDU examples omit the Modbus TCP MBAP header and the Modbus RTU unit-address/CRC envelope. Multi-byte values are shown in network byte order.

Common implementation mistakes

  • Writing an engineering value without applying the documented scale
  • Using signed values without two’s-complement handling
  • Assuming an echoed write means the command was accepted by application logic

Run the reference

Turn a protocol table into evidence.

Compose the request publicly, then use Pro for advanced graded exceptions, saved fixtures, complete history and project sharing.

Open sandbox
01 Read coils03 Read holding02 Illegal address

Technical reference and worked-example guide

Modbus function 06 single-register write guide: field reference

Direct answer

The reader can bound a function 06 request, encode the intended 16-bit value, interpret the echo response and verify the permitted change independently.

Written for pLC programmers writing one documented setpoint or parameter to a Modbus server in a controlled environment.

an industrial communications bench used to inspect Ethernet and isolated serial topology, identity, request-response evidence and mapped device data while studying authorized single holding-register writes, encoding and read-back proof
System map / 02

NODE 01observable

Definition

Authorized client, server identity, function 06, holding-register offset, 16-bit encoding, signedness, scale, permitted range, operating mode and write authority.

NODE 02observable

Signal path

Approved engineering value through encoding and write request to server validation, echo response, read-back and independently observed application state.

NODE 03observable

Worked example

One bounded write changes only the documented register and produces the expected read-back and permitted result.

NODE 04observable

Limits

Minimum and maximum value, signed boundary, illegal address, illegal value, read-only state, duplicate request, timeout, restart and lost response.

NODE 05observable

Common mistake

An authority, identity, offset, encoding, range, mode, response, read-back or application mismatch.

NODE 06observable

Verification

The change reviewed, backed up and verified with current device documentation and a safe target test.

Answer surface / 07

What does Modbus function code 06 do?

It requests that a server write one holding register and normally echoes the accepted address and value in the response.

Does the echoed response prove the equipment changed?

No. Verify the register by reading it back and confirm the permitted physical or application result independently.

Industrial communications path

Follow the transaction from wire to register

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