X20 system modules • Digital signal processing modules • X20DS1928
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X20 system User's Manual 3.10
FlatStream adjustment
If the way messages are structured is changed, then the way data in the transmit/receive array is arranged is also
different. The following changes apply to the example given earlier on.
MultiSegmentMTU:
If MultiSegmentMTUs are allowed, then "open positions" in an MTU can be used. These "open positions" occur if
the last segment in a message doesn't fully use the entire MTU. MultiSegmentMTUs allow these bits to be used
to transfer the following control bytes and segments. In the program sequence, the "nextCBPos" bit in the control
byte is set so that the receiver can correctly identify the next control byte.
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 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
C2
A1
A7
A5
A6
C1
B1
B2
C3
C4
D1
D2
D3
D4
D5
D6
C5
D7
D8
-
-
C0
-
-
-
-
-
C0
-
-
-
-
-
-
C0
-
C6
MultiSegmentMTU
-
D9
Transmit/Receive array
With 7 USINT elements according to
the configurable MTU size
Figure 446: Transmit/Receive array (MultiSegmentMTU)
First, the messages must be split into segments. As in the default configuration, it's important for each sequence
to begin with a control byte. The free bits in the MTU at the end of a message are filled with data from the following
message, however. With this option, the "nextCBPos" bit is always set if payload data is transferred after the control
byte.
MTU = 7 bytes → Max. segment length = 6 bytes
•
Message 1 (7 bytes)
➯
First segment = Control byte + 6 data bytes (MTU full)
➯
Second segment = Control byte + 1 data byte (MTU still has 5 open bytes)
•
Message 2 (2 bytes)
➯
First segment = Control byte + 2 data bytes (MTU still has 2 open bytes)
•
Message 3 (9 bytes)
➯
First segment = Control byte + 1 data byte (MTU full)
➯
Second segment = Control byte + 6 data bytes (MTU full)
➯
Third segment = Control byte + 2 data bytes (MTU still has 4 open bytes)
•
No more messages
➯
C0 control byte
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