B&R Industries X20 System, User Manual

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X20 system modules • Digital signal processing modules • X20DS1928
1440 X20 system User's Manual 3.10
Large segments:
Segments are limited to a maximum of 63 bytes. This means they can be larger than the active MTU. These large
segments are divided among several sequences when transferred. It is possible for sequences to be completely
filled with payload data and not have a control byte.
Information:
It is still possible to subdivide a message into several segments so that the size of a data packet doesn't
also have to be limited to 63 bytes.
Example:
Three autonomous messages (7 bytes, 2 bytes, 9 bytes) are being transferred using an MTU with a width of 7 bytes.
The configuration allows the transfer of large segments.
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
A7
A5 A6 C1
B1 B2 C2
C3 D1 D2 D3 D4 D5 D6
D7 D8 - -
-
-
-
- -
-
- - - - - C0 -
- - - -
-
Large segments
-
D9
Transmit/Receive array
With 7 USINT elements according to
the configurable MTU size
Figure 447: Transmit/Receive array (large segments)
First, the messages must be split into segments. The ability to form large segments means that messages are split
up less frequently, which results in fewer generated control bytes.
Large segments allowed → Max. segment length = 63 bytes
•
Message 1 (7 bytes)
➯ First segment = Control byte + 7 data bytes
•
Message 2 (2 bytes)
➯ First segment = Control byte + 2 data bytes
•
Message 3 (9 bytes)
➯ First segment = Control byte + 9 data bytes
•
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 443: FlatStream determination of the control bytes for large segments example