Comtrol Modbus TCP User Guide - Manual (Page 27)

DeviceMaster UP Modbus/TCP User Guide: 2000447 Rev. I Chapter 2. Programming Interface - 27
Sequence Number Messages (Raw Data)
2.2.7. Sequence Number Messages (Raw Data)
Read Holding Registers and Write Multiple Register messages can read and modify both receive and transmit
produced data sequence numbers. These are the same sequence numbers returned to the PLC in the Receive
Data Message and sent to the DeviceMaster UP in the Transmit Data message. Access to these sequence
numbers are provided primarily for initialization purposes at the start of the PLC program when you may
want to initialize the sequence numbers on the PLC, DeviceMaster UP or both.
2.3. I/O Scanner (Raw Data)
The I/O Scanner is an optional
PLC communications method
that is implemented on some
PLC programming software
such as the Concept
programming package for use
with the Schneider Electric
Modicon PLCs. The I/O
Scanner provides a rather
simple method that requires
minimal programming effort. It
automatically performs the
polling and transmitting of
data at set time intervals and typically utilizes the Read/Write multiple Registers message.
The following restrictions apply to this method:
• The Receive and Transmit mode for the serial and/or Ethernet device must both be set to Slave mode.
• 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 receive 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 received
data structure, typically 128 words. For more information, see the
2.2.5. Receive Data Message (Raw
Data) definition on Page 23.
• New received data will be indicated with an incremented sequence number.
The same data may be returned more than once. However, the same data packet will also return the same
sequence number.
• No receive data will be indicated with a length of zero.
• The variable to transmit the data on the PLC must be:
- In the 40xxxx address range. (For Modicon 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.
• The DeviceMaster UP should be reset before starting a PLC program using the I/O Scanner due to PLC
program execution scheduling. If the DeviceMaster UP is not reset, the sequence numbers may be out of
sync. This may result in receiving outdated serial data as well as an unexpected transmission of serial
data. A Transmit Unexpected Sequence Number error may also occur.