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Appendix
32
Trebing & Himstedt | SPI 3 | MODBUS RTU
Telegram for data transfer with fragmentation
If you want to transfer telegrams that exceed the available data channel (4 words), the
telegram has to be fragmented and transmitted in several parts (fragments). Bear in
mind that, besides the user data, you need 2 additonal bytes for control data and
telegram length.
Note!
Telegram length is transmitted and received only in the first telegram
fragment.
Send telegram with fragmentation
Bit 7
0
…6 …5 …4 …3 …2 …1 …0
Byte 0
…3
…2
…1
DP Master
SPI 3
OUT
Bit 7
0
…6 …5 …4 …3 …2 …1 …0
Byte 0
…3
…2
…1
IN
Bit 7
1
…6 …5 …4 …3 …2 …1 …0
Byte 0
…3
…2
…1
OUT
Bit 7
1
…6 …5 …4 …3 …2 …1 …0
Byte 0
…3
…2
…1
IN
Send telegram 1
Telegram length
User data
0x17
A
B
Receipt telegram 1
0
Example original configuration
Send telegram (only toggle Bit 0 in Byte 0, specify telegram length in Byte1, specify user data from Byte 2 onward)
1
Request (DP Master)
Response (SPI 3)
Request (DP Master)
Response (SPI 3)
0
0
…n
…n
…n
…n
…
0
0