Conversion Mode; Conversion Example - 3onedata CAN485 User Manual

6.3.2 Conversion mode

The process of conversion in the lateral side of the serial CAN-Bus converter, will only receive a complete and
correct Modbus RTU frame will be converted, otherwise no action.
As shown below, the address domain Modbus RTU protocol into CAN message frame ID ID4 (extended frame)
ID2 (standard frame), marks the same in the conversion process.
Modbus RTU frame
Address field
Function code
Data field
CRC field
CRC check byte not converted to CAN-Bus message, CAN-Bus message need not check byte with serial frames,
because CAN-Bus itself has better check mechanism.
Conversion is Modbus RTU protocol content -- function code and data domain, when converting them sequentially
conversion in data field can report the frame (from the beginning of the second byte of data, the first data byte is
piecewise protocol), due to the Modbus RTU frame length are different according to different function code.
CAN-Bus message frame can only transmit 7 data, so the converter will be sent messages of Modbus RTU frame
into CAN-Bus long segment with the segmented CAN-Bus protocol. The user receives in the CAN-Bus node take
the function code and data processing domain can be.
For CAN-Bus Modbus protocol data without cyclic redundancy check (CRC16) converter in accordance with sub
protocol to receive end receiver frame analysis automatically cycle redundancy check (CRC16), converted into
Modbus RTU frame is sent to the serial bus. If the received data does not comply with segment protocol, the data
conversion shall be discarded.

6.3.3 Conversion example

In configured to expand the frame, as shown below, in the Modbus RTU frame into the CAN-Bus message, address
0x08 directly to fill the frame ID4, the other frame ID fill 0x00 and in the conversion process of the frame to
remain in this frame ID does not change.
CAN message CAN message 1
Frame information Frame information
Frame ID 1
Frame ID 2
Frame ID 3
Frame ID 4
segmentation protocol
Data Field
22
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CAN message 2
Frame information
00
00
00
Address field Address field
The use of The use of
segmentation protocol
Function code
Data field
Data field
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00
00
00