13/08/2013 PM 9845 0187 01 Page 18 of 24
11. Modbus Protocol Implementation
11.1. Supported Modbus specification
The Gateway module supports the following Modbus-variant:
• Mode of transmission : RTU
• Coding system : binary
• Mode : RS485
• Number of start bits: 1
• Number of data bits: 8
• Baudrate: 9600, 19200, 38400, 57600, 115200, 230400, 460800
• Parity control: even, odd, none
• Number of stop bits: 1, 2
• Error checking: CRC16
Frame synchronization in the RTU mode is done by simulating a synchronous message. The slave device monitors the
elapsed time between receipts of characters. If three character time elapsed without a new character, then the device
assumes that the message is completed and the next byte will be the address.
The message frame format is as follows:
T1 T2 T3 Address Message Body CRC T1 T2 T3
11.2. Supported Modbus functions
The Elektronikon Mk5 Modbus supports the following message type, depending on the type of data involved (see details
below).
• Function 01 : read coil register
• Function 03 : read holding register
• Function 06 : preset single register
NOTE: for Elektronikon Mk5 controllers, it is not possible to change parameters. Consult the address mapping to check which parameters
can be adjusted.
11.3. Function code - data field
In a normal response, the slave echoes the function code of the original query.
In an exception response, 0x80 is added to the original function code.
At the same time, an exception code is added in the Data Field.
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