Modbus Frame Builder

Build RTU & TCP frames, auto-calculate CRC16, validate messages — all offline.

Frame Parameters

Slave ID / Unit ID

1–247 (RTU) · 255 reserved for broadcast (TCP)

Function Code

Select the Modbus function code for your frame

Start Address

0x0000–0xFFFF

Quantity / Count

Max 125 registers

Live Frame Output

ASCII (terminal view)

:01 03 00 00 00 01 ?? ??

Hex Bytes

01 03 00 00 00 01

Frame Breakdown

SLV01 FC03 ADDR00 00 QTY00 01

CRC16 — RTU (poly 0x8005, init 0xFFFF)

XXXX

CRC16 Validator

Paste a received Modbus RTU frame to verify its CRC integrity.

Paste RTU Frame (hex bytes)

Spaces optional. Leading colon (:) stripped automatically.

CRC16 Reference — Standard Modbus Polynomial

Polynomial

0x8005 (CRC-16/MODBUS)

Initial Value

0xFFFF

Reflection

Reflected (LSB first)

Output XOR

0x0000

Industrial context: Modbus RTU appends CRC16 to every frame for error detection on RS-485 serial networks. Modbus TCP replaces CRC with a 7-byte MBAP (Modbus Application Protocol) header. PLCs, RTUs, and gateways use these frame formats for SCADA and IIoT communication.