9
2 Definition of the MODBUS TCP/IP communication
2.1 MODBUS TCP/IP Protocol
The TAC6 controllers use the communication protocol MODBUS TCP/IP in one server, the satellite board, to multiple
clients (maximum 3) architecture. The server is addressed by the client thanks to its IP address and communication port
number which is 502.
Each message has the same structure: the frame is made of the transaction identifier, protocol identifier, length field, unit
identifier, the function code (writing or reading) and the data bytes.
Main specifications:
- MODBUS TCP/IP protocol
- Physical layer:
o Ethernet over twisted pair 10 BASE T for SAT ETHERNET,
o Compatible Wi-Fi 802.11b/g/n for SAT WIFI.
2.2 MODBUS TCP/IP frame
The MODBUS frame is presented in the following way:
Transaction identifier: Used for synchronization between messages of server and client
Protocol identifier: Always 0 for Modbus TCP/IP
Length field: Number of remaining bytes in this frame
Unit identifier: This is the Modbus address of the SAT module device. 1 must be used and only this address will be
accepted since the SAT ETHERNET and WIFI are not Modbus TCP/IP bridges to Modbus RTU.
Function code:
- Supports function code 3 (decimal) for reading multiple holding registers.
- Supports function code 6 (decimal) for writing a single holding registers.
- Supports function code 16 (decimal) for writing multiple holding registers.
Data: All TAC6 data is in 16-bit Modbus holding registers. Modbus coils, discrete inputs or input registers are not used.
The Modbus communication buffers size is sufficient for reading 125 registers at once or writing 123 registers at once.
In the Modbus.org standard documents, holding register addresses are given a prefix of "4" to distinguish them from
other register types. For example, in their documentation a holding register at address 1001 is referred to by "41001".
However, the leading "4" is not really part of the address. Also, the Modbus.org standard documents refer to register
addresses using "one-based" numbering. However, the addresses that are actually sent in a Modbus command
message are "zero-based". For example, to read register 1001, the address that is actually sent in the command
message is 1000.
2.3 MODBUS exception
The TAC6 Modbus protocol may respond with an exception code if the read or write command was invalid. Possible
exceptions are:
- 01 ILLEGAL FUNCTION: The function code received in the query is not an allowable action for the slave.
- 02 ILLEGAL DATA ADDRESS: The data address received in the query is not an allowable address for the
slave. Or the combination of data address and transfer length is not allowable.
- 03 ILLEGAL DATA VALUE: The master attempted to write a non-allowed value into a register. Which values are
allowed may depend on configuration settings of other registers.
When a write of multiple registers is executed, the execution stops at the first invalid data address or invalid value.
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