Comtrol Modbus TCP User Guide - Manual (Page 26)

26 - Chapter 2. Programming Interface DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I
Communication Methodology (Transmit Raw Data Slave Mode)
2.2.6.2. Communication Methodology (Transmit Raw Data Slave Mode)
Raw serial and/or EtherNet
device data is sent in the
Write Multiple Registers
message or, optionally, the
Read/Write Multiple
Register message. The data
is requested by accessing
the corresponding transmit
data address for the desired
port.
The following restrictions
apply to this method:
• The Device Index must be 255 (FF hex) for raw/ASCII serial data and 254 (FE hex) for raw/ASCII
Ethernet device data.
• The variable to transmit the data on the PLC must be:
- In the 40xxxx address range. (For Modicon type PLCs.)
- An array of words.
- Of sufficient size to contain the sequence number, length, and data field associated with the transmit
data structure, typically 128 words. See
2.2.6. Transmit Data Message (Raw Data) on Page 25 for
more information.
• 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 same transmit data message may be sent to the DeviceMaster
UP more than once. However, the data packet will only be transmitted when a new sequence number is
received.
2.2.6.3. Communication Methodology (Transmit Data Master Mode)
Raw serial and/or Ethernet
transmit data is polled from
the PLC at the configured
address and, when the
DeviceMaster UP receives a
transmit message with an
updated sequence number,
the data is transmitted to
the serial or Ethernet
device.
• The following
restrictions apply to this
method:
• The Device Index must be configured for the target PLC.
• The variable to receive the data on the PLC must be:
- In the 40xxxx. (For Modicon type PLCs.)
- An array of 16 bit words.
- Of sufficient size to contain the sequence number, length, and data field associated with the transmit
data structure.
• The PLC will indicate new data to transmit with an incremented sequence number. (The Disable Tx
Sequence Number Check option does not apply to transmit data master mode.)
• The length will indicate the number of bytes to transmit.
• The DeviceMaster UP will expect the length parameter and data to transmit to be updated before the
transmit sequence number is incremented. Therefore, as soon as the DeviceMaster UP receives an
incremented transmit number, it will transmit the data to the serial or Ethernet device.