ICP DAS USA tGw-735

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Tiny Modbus/TCP to RTU/ASCII Gateway

Leading 6 bytes of Modbus/TCP protocol:

Byte 00

Byte 01

Byte 02

Byte 03

Byte 04

Byte 05

Transaction identifier

Protocol identifier

Length field

(upper byte )

Length field

(lower byte)

Transaction identifier: Assigned by Modbus/TCP master (client)

Protocol identifier: 0

Length field (upper byte): 0 (since all messages are smaller than 256)

Length field (lower byte): Number of following RTU data bytes

RTU Data Structure

Byte 06

Byte 07

Byte 08-09

Byte 10-11

Net ID

(Station number)

Function Code

Data Field

Reference number

(Address Mapping)

Number of points

 Net ID (Station Number): specifies the address of the receiver (Modbus/TCP slave).

The first byte in the message structure of Modbus is the receiver’s address. The valid

addresses are in the range of 0 to 247. Addresses 0 is used for broadcast, while addresses 1

to 247 are given to individual Modbus devices.

 Function Code: specifies the message type.

The second byte in the frame structure of the Modbus RTU is the function code. The

function code describes what the slave is required to do. Valid function codes are between 1

and 255. The slave uses the same function code as the request to answer it. Only when an

error occurs in the system will the highest bit of the function code is set to ‘1’. Hence the

master will know whether the message has been transmitted correctly or not.

ICP DAS USA tGw-735 - Page 90