Comtrol Modbus TCP User Guide - Manual (Page 25)

DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I Chapter 2. Programming Interface - 25
Transmit Data Message (Raw Data)
2.2.6. Transmit Data Message (Raw Data)
The following topics are discussed:
• Format on Page 25
• Communication Methodology (Transmit Raw Data Slave Mode) on Page 26
• Communication Methodology (Transmit Data Master Mode) on Page 26
2.2.6.1. Format
The Transmit Data message for raw data contains a simple protocol including a sequence number, length and
serial data fields. The Modbus standard requires a WORD format.
The following table displays the format of the Transmit Data message.
Transmit messages have the following characteristics:
• It transfers all data in WORDs.
• If the Disable Tx Sequence Number Check option is not selected, the sequence number must be incremented
when there is new data to transmit.
• The data length field indicates the number of valid bytes contained in the message.
• The actual length of a message received from the PLC may contain extra, unused data.
• It ignores all unused bytes in a Modbus message.
• The default order of the bytes is Least Significant Byte First. However, you can select the Tx MS Byte First
option in the web page to transmit bytes by Most Significant Byte First. For more information, see Tx MS
Byte First under
3.3.5. Serial Port Packet ID Settings (Raw-Data Only) on Page 39.
• A request for the Transmit data returns the last transmit data message.
Name Data Type Data Value(s) Access Rule
Transmit (PLC to DeviceMaster UP)
message data.
Structure of:
Produced data sequence
Data length (in bytes)
Data array
WORD
WORD
Array of WORD
0-65535 (FFFF hex)
1-236 (Slave Tx Mode)
1-246 (Master Tx Mode)
0-65535
Read/Write