9 Communication
9.2 Modbus
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SICAM MMU 7KG9663, Device Manual
E50417-H1040-C514-A3, Edition 01.2016
Exception Code 02
ILLEGAL_DATA_ADDRESS
•
An attempt is made to read out or write to a nonexistent Modbus register (see chapter 9.2.7, Modbus
mapping for valid registers).
•
An attempt is made to read out or write to too many registers. A Modbus message enables reading out
125 holding registers and writing to 123 holding registers maximum.
•
The Modbus clients tries to write to a register in the Modbus server for which only read access is allowed
according to the Modbus mapping (see chapter 9.2.7).
Exception Code 03
ILLEGAL_DATA_VALUE
•
The redundancy client addresses a register that does not allow access to partial data because it is part of
a data type with a complex data structure distributed across several registers and can be read or written
only as a whole.
•
The Modbus client attempts to write to the Modbus server for which the access rights are set to "read
only".
Exception Code 04
SERVER_FAILURE
•
Error during the time format conversion in the Modbus server because a faulty date/time format was re-
ceived via Modbus (for example month format > 12).
9.2.3
Modbus TCP
Properties of the Modbus TCP
•
Connection-oriented Ethernet protocol based on TCP/IP
•
Use of IP addresses for addressing individual components connected to the bus (bus nodes)
•
The Modbus TCP protocol has the TCP port number 502 reserved on the server side. It is possible to use
a parameterized port number.
•
All data types in the Modbus TCP messages which are larger than 1 byte, are stored in the Big-endian
format, that is the most significant byte (MSB) is stored at the lowest register address and is transmitted
first.
•
Communication sequence:
−
The client sends a request to the server to start a data transfer from the server to the client.
−
If the requested data are unavailable, the server sends an exception response to the client.
•
The Modbus TCP data packet has a maximum size of 260 bytes:
−
253 bytes max. for data and
−
7 bytes for the Modbus TCP header
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