B&R Industries X20 System, User Manual

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X20 system modules • Analog output modules • X20AO2438
X20 system User's Manual 3.10 445
Large segments and MultiSegmentMTU
Example
Three autonomous messages (7 bytes, 2 bytes and 9 bytes) are being transmitted using an MTU with a width of
7 bytes. The configuration allows transmission of large segments as well as MultiSegmentMTUs.
Message 1:
A1 A2 A3 A4 A5 A6 A7
B1 B2
D1 D2 D3 D4 D5 D6 D7 D8
No more data to transmit
- - ... -
Message 2:
Message 3:
Sequence for bus cycle 1
Sequence for bus cycle 2
Sequence for bus cycle 3
Sequence for bus cycle 4
Sequence for bus cycle 5
Sequence for bus cycle 6
D9
A2 A3 A4 A1 A5 A6 C1
A7 C2 B1 B2 C3 D1 D2
D3 D4 D5 D6 D7 D8
- - C0 - - -
- - C0 - - - -
- - C0 - - - -
Both options
-
D9
Transmit/Receive array
With 7 USINT elements according to
the configurable MTU size
Figure 184: Transmit/receive array (large segments and MultiSegmentMTUs)
First, the messages must be split into segments. If the last segment of a message does not completely fill the
MTU, it can be used for other data in the data stream. The "nextCBPos" bit must always be set if the control byte
belongs to a segment with payload data.
The ability to form large segments means that messages are split up less frequently, which results in fewer control
bytes generated. Control bytes are generated in the same way as with the "Large segments" option.
Large segments allowed → Max. segment length = 63 bytes
•
Message 1 (7 bytes)
➯
First segment = Control byte + 7 bytes of data
•
Message 2 (2 bytes)
➯
First segment = Control byte + 2 bytes of data
•
Message 3 (9 bytes)
➯
First segment = Control byte + 9 bytes of data
•
No more messages
➯
C0 control byte
A unique control byte must be generated for each segment. In addition, the C0 control byte is generated to keep
communication on standby.
C1 (control byte 1) C2 (control byte 2) C3 (control byte 3)
- SegmentLength (7) = 7 - SegmentLength (2) = 2 - SegmentLength (9) = 9
- nextCBPos (0) = 0 - nextCBPos (0) = 0 - nextCBPos (0) = 0
- MessageEndBit (1) = 128 - MessageEndBit (1) = 128 - MessageEndBit (1) = 128
Control byte Σ 135 Control byte Σ 130 Control byte Σ 137
Table 90: FlatStream determination of the control bytes for the large segment and MultiSegmentMTU example