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Feedback = A001H
The CRC is calculated using the body and header of the message (i.e. whole message excluding CRC
bytes).
8.3
Open Modbus/TCP Communication Format
The basic frame structure for Open Modbus/TCP protocol is as follows. This is the stream data
transported via the TCP socket connection and does not include TCP/IP protocol bytes.
Request
Transaction ID
(msb)
0*
Transaction
ID (lsb)
0*
Unit ID
Function
Code
Protocol Protocol
Length
ID
ID (lsb)
(msb)
(msb)
0
0
0
Function dependent request data
….
Length
(lsb)
*Transaction ID is echoed by the Modbus/TCP server and may be used by a client. The E Series
RTU sets these bytes to 0 in requests.
Protocol ID identifies the message protocol following in the data stream. When both these bytes are
0, it indicates Modbus/TCP protocol.
Length (lsb) indicates the number of bytes following in the rest of the frame. Minimum value is 3,
maximum value is 255.
Unit ID uniquely identifies the Modbus device, and is equivalent to Slave ID of serial Modbus.
CRC error checking is not used for Open Modbus/TCP communications. Instead it relies on TCP/IP
stack layers to provide error free transmission of data.
Function Code and following data is equivalent to serial Modbus RTU protocol.
Response
Transaction
ID (msb)
Transaction
ID (lsb)
Unit ID
Function
Code
Protocol
Protocol
Length
Length
ID (msb)
ID (lsb)
(msb)
(lsb)
0
0
0
Function dependent response data
….
Transaction ID is echoed by the Modbus/TCP server in the response.
Length (lsb) indicates the number of bytes following in the rest of the frame. Minimum value is 3,
maximum value is 255.
Unit ID is equivalent to Slave ID of serial Modbus.
Error Response
Transaction
ID (msb)
Transaction
ID (lsb)
Unit ID
Function
Code
Protocol ID
(msb)
0
Exception
Code
Protocol ID
(lsb)
0
SCADAPack E Series Modbus PLC ISaGRAF Interface Reference
January 19, 2006
Length
(msb)
0
Length
(lsb)
3
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