What is the formula for Modbus TCP Frame & MBAP?

Core formula: Frame = MBAP (Transaction ID + Protocol ID + Length + Unit ID) + Function + Data. The tool returns results instantly using this formula and the selected parameters.

What engineering notes should I keep in mind when using Modbus TCP Frame & MBAP?

The MBAP header is always 7 bytes: Transaction ID (2) + Protocol ID (2, always 0) + Length (2) + Unit ID (1). The length field = Unit ID (1) + PDU length (function + data); parsing cross-checks it against the actual frame size. Modbus TCP has no CRC — integrity is handled by TCP; a non-zero protocol ID indicates a private extension. One TCP segment can carry several MBAP/PDU frames (pipelining) and must be split by the length field; multi-byte fields are big-endian.

Modbus TCP Frame & MBAP

Favorite

Pack/unpack Modbus TCP frames (master request / slave response) with MBAP header decoding, protocol-ID and length self-checks — matched against the standard frame format.

Key points

Modbus TCP Frame & MBAP: Pack/unpack Modbus TCP frames (master request / slave response) with MBAP header decoding, protocol-ID and length self-checks — matched against the standard frame format. Formula: Frame = MBAP (Transaction ID + Protocol ID + Length + Unit ID) + Function + Data. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. The MBAP header is always 7 bytes: Transaction ID (2) + Protocol ID (2, always 0) + Length (2) + Unit ID (1). The length field = Unit ID (1) + PDU length (function + data); parsing cross-checks it against the actual frame size.

Inputs

Frame = MBAP (Transaction ID + Protocol ID + Length + Unit ID) + Function + Data

ModeDirectionTransaction IDTransaction identifier: pairs request/response (0–65535)Unit IDUnit identifier (formerly slave address): usually 0 or 1 (0–255)Function codeStart addressRegister/coil start address (0-based; used for writes & echoed responses)QuantityFor read / write-multiple / response data: coils 1–2000, registers 1–125Coil / register valueWrite single (request & response): 05 = ON/OFF, 06 = 0–65535Data (bits / register values)Write-multiple or response: bit string (0F / read-coils response) or register list (10 / read-registers response), comma/space separated

Calculation model (updates with parameters)

Full frame (hex)	00 01 00 00 00 06 01 03 00 00 00 01	Sent by master
MBAP header	00 01 00 00 00 06 01	Transaction ID 1，Protocol ID 0，Length 6
Data field	00 00 00 01	Start address 0, quantity 1
Frame length	12 字节	Unit ID 1，Function code 03 (Read Holding Registers)，request

Engineering notes

The MBAP header is always 7 bytes: Transaction ID (2) + Protocol ID (2, always 0) + Length (2) + Unit ID (1).
The length field = Unit ID (1) + PDU length (function + data); parsing cross-checks it against the actual frame size.
Modbus TCP has no CRC — integrity is handled by TCP; a non-zero protocol ID indicates a private extension.
One TCP segment can carry several MBAP/PDU frames (pipelining) and must be split by the length field; multi-byte fields are big-endian.

FAQ

What is Modbus TCP Frame & MBAP for?: Pack/unpack Modbus TCP frames (master request / slave response) with MBAP header decoding, protocol-ID and length self-checks — matched against the standard frame format.
What is the formula for Modbus TCP Frame & MBAP?: Core formula: Frame = MBAP (Transaction ID + Protocol ID + Length + Unit ID) + Function + Data. The tool returns results instantly using this formula and the selected parameters.
What engineering notes should I keep in mind when using Modbus TCP Frame & MBAP?: The MBAP header is always 7 bytes: Transaction ID (2) + Protocol ID (2, always 0) + Length (2) + Unit ID (1). The length field = Unit ID (1) + PDU length (function + data); parsing cross-checks it against the actual frame size. Modbus TCP has no CRC — integrity is handled by TCP; a non-zero protocol ID indicates a private extension. One TCP segment can carry several MBAP/PDU frames (pipelining) and must be split by the length field; multi-byte fields are big-endian.