X20 system modules • Analog input modules • X20AI2438
X20 system User's Manual 3.10
239
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 transmit 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 and 9 bytes) are being transmitted using an MTU with a width of
7 bytes. The configuration allows transmission 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 85: 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 sent 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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