Comtrol Modbus TCP Quick Start - Manual (Page 59)

DeviceMaster UP Modbus/TCP Quick Start: 2000477 Rev. G Configuring Read/Write Raw/ASCII Devices - 59
Configuring Read/Write Raw/ASCII Devices
Transmit messages have the following characteristics:
• All data is transferred in 16 bit words.
• If operating in Tx (From PLC) Transfer mode of Master (Poll the PLC): The sequence number must be
incremented when there is new data to transmit.
• If operating in Tx (From PLC) Transfer mode of Slave (PLC Writes): The sequence number must be
incremented when there is new data to transmit only if the Disable Tx Sequence Number Check is not
selected.
• The data length field indicates the number of valid bytes contained in this message.
• The actual length of the message received from a PLC may contain extra, unused data.
• Unused data is ignored.
• A request for transmit data returns the last transmit data message.