Data Sheet

2B F7

Cyclic redundancy check (CRC-16-IBM)

The response for this message will be as following:

01 04 04 00 00 12 34 F6 F3

Name

Description

Node address

Function code (Read Input Registers)

Total bytes read (always 4 bytes)

00 00 12 34

Value in big Indian notation (MSB first).

F6 F3

Cyclic redundancy check (CRC-16-IBM)

2.2.2 Modbus TCP (1)

Modbus TCP message is the same as RTU over TCP message by removing CRC and adding MBAP header

(Modbus Application Header) is being added to the start of the message. Also, node address moved from

into MBAP header and named Unit ID.

The MBAP header is consisting from the following:

Name

Description

Transaction ID

2 bytes set by the Client to uniquely identify each request. These bytes

are echoed by the Server since its responses may not be received in the

same order as the requests.

Protocol Identifier

2 bytes set by the Client, must be 0x0000.

Length

2 bytes identifying the number of bytes in the message to follow.

Unit Identifier

Node address.

For examples, to read VAR1, you need to read 2 registers from address 0x20C1 so you need to send the

following TCP message:

00 03 00 00 00 06 01 04 20 C1 00 02