Hardware manual

12 Modbus
2-245
Chap.2
12.6.6.2 Basic Format
The following table lists the basic format of communications. The same format applies to both
requests and responses.
Data is processed as a byte sequences.
Description
Byte 0 Transaction ID. The same value is returned from the server.
The value is normally “0”.
Byte 1 Transaction ID. The same value is returned from the server.
The value is normally “0”.
Byte 2 Protocol ID. The value is always “0”.
Byte 3 Protocol ID. The value is always “0”.
Byte 4 Message length (high byte). The value is always “0”. (Since the
message is 256 bytes at maximum.)
Byte 5 Message length (low byte). The length of the following message.
Byte 6 Unit ID
Byte 7 Function code
Byte 8- Data
Transaction ID
The data included in a request is returned from the server without changes. The client (host
device) sends a different Transaction ID for each request, and identifies the response by
checking the Transaction ID of a response. Enter “0” to not check the Transaction ID.
Protocol ID
The number indicating the MODBUS/TCP protocol, and is always “0”.
Message length
Indicates the length of the following message in units of bytes.
Unit ID
ID used for identifying devices. The ID is not used with the MICRO/I. When the ID is used in a
request, the returned data is unchanged.
Function code
Numbers assigned for functions such as reading and writing.
Data
Data required for each processing.