Appendix
28
Trebing & Himstedt | SPI 3 | MODBUS RTU
Appendix
The chapters in the appendix always refer to the use of the »MODBUS_
Kanal« module.
Creating function blocks
Data between DP master and SPI 3 are exchanged via data channel in the form of
telegrams or telegram fragments. The size of the data channel between DP master
and SPI 3 is defined as 4 words by the »MODBUS_ Kanal« module.
Note!
Telegram fragmentation is necessary if the number of the bytes to be
trans 2 bytes (for control data and telegram length) is exceeding the
data channel of the module (4 words) for the SPI 3.
A data channel contains reserved space for control data and user data:
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte n
User data:
contain the actual information in the form of MODBUS telegrams or telegram
fragments. User data length is transferred in the first byte.
Control data:
control data exchange between DP master and SPI 3. They are used
– to highlight new data,
– to transfer telegrams exceeding the available data channel,
– to control the flow between DP master and SPI 3.
For control data, one byte is available:
– Command bits (stamp) to write commands to the SPI 3.
The DP master uses this to highlight
· the start of a new telegram,
· the start of user data
· or the end of a command.
– Status bits (mirror) to read status information from the SPI 3.
The SPI 3 uses this to mirror the stamp as a signal that
· it is ready for data transfer,
· user data have been processed,
Control data
(n - 1) User data
