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Figure 2. Modbus TCP Message

Name

Length

(bytes)

Function

Transaction ID 2 For synchronization between messages of

server and client

Protocol Identifier

0 for Modbus/TCP

Length Field

Number of remaining bytes in this frame

Unit Identifier

Slave Address (default value 0)

Function Code

Function codes as in other variants

Data Bytes

Data as response or commands

Table 1. Modbus TCP frame format

The Modbus RTU serial protocol’s message data is in hexadecimal format (i.e. raw

unconverted binary). The frame for the Modbus-TCP message is 12 bytes long for read

requests, write requests, and write replies. Messages can be longer than 12 bytes, but not

exceeding 260 bytes for read replies if more than one register is requested. Table 2 is an

example of a read request, and Table 3 is an example of a write request.

Field

Transaction ID

Protocol

Identifier

Length

Field

Unit

Identifier

Function

Code

Data Bytes

Mem. Location

No. Read

Byte#

Byte Value

(hex)

0x00

0x0D

0x00

0x06

0x00

0x04

0x00

0x66

0x00

0x01

Byte Value

(Decimal)

13 0 6 0 4 102 01

Byte Value

(Binary)

0000

1101

0000

0110

0000

0100

0000

0110

0000

0001

Table 2. Example Modbus TCP read request for Genesis Controller circuit #2 heater

current

Brand	Thermon
Model	GENESIS
Category	Controller
Language	English

Thermon GENESIS User Manual

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Thermon GENESIS Specifications