Comtrol Modbus TCP User Guide - Manual (Page 23)

DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I Chapter 2. Programming Interface - 23
Receive Data Message (Raw Data)
2.2.5. Receive Data Message (Raw Data)
The following topics are discussed:
• Format on Page 23
• Communication Methodology (Receive Raw Data in Slave Mode) on Page 24
• Communication Methodology (Receive Data Master Mode) on Page 24
2.2.5.1. Format
The Receive 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 Receive Data message.
Receive messages have the following characteristics:
• It returns all data in WORDs.
• The DeviceMaster UP increments the sequence number when it returns new data.
• The message received from the PLC determines the actual length of the Modbus message returned to the
PLC. (This is often greater than the length of the actual number of valid bytes in the Receive Data
Message.)
• All unused bytes in a Modbus message returned to the PLC are filled with zeroes.
• The default order of the bytes is Least Significant Byte First. However, you can select the Rx MS Byte
First option in the web page to return bytes by Most Significant Byte First. For more information, see Rx
MS Byte First under
3.3.5. Serial Port Packet ID Settings (Raw-Data Only) on Page 39.
Name Data Type Data Value(s) Access Rule
Receive (to PLC) message data.
Structure of:
Produced data sequence
Data length (in bytes)
Data array
WORD
WORD
Array of WORD
0-65535 (FFFF hex)
0-246 (slave Rx mode)
1-1024 (serial Rx master)
1-2048 (Ethernet Rx master)
0-65535
Read only