B&R Industries X20 System, User Manual

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X20 system modules • Digital signal processing modules • X20DS1928
1428 X20 system User's Manual 3.10
Sending and receiving
If a channel is synchronized, then the opposite station is ready to receive messages from the transmitter. Before
the transmitter can send data, it needs to first create a transmit array in order to meet FlatStream requirements.
The transmitting station must also generate a control byte for each segment created. This control byte contains
information about how the subsequent part of the data being transferred should be processed. The position of the
next control byte in the data stream can vary. For this reason, it must be absolutely clear at all time when a new
control byte is being transferred. The first control byte is always in the first byte of the first sequence. All subsequent
positions are conveyed recursively.
FlatStream formula for calculating the position of the next control byte:
Position (of the next control byte) = Current po 1 + Segment length
Example:
Three autonomous messages (7 bytes, 2 bytes, 9 bytes) are being transferred using an MTU with a width of 7
bytes. The rest of the configuration corresponds to the default settings.
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
B1 B2
A2 A3 A4
C2
A1
A7
A5 A6
C3
D1 D2 D3 D4 D5 D6
D7 D8
-
- -
-
C4
-
-
C5
-
C1
- -
- -
-
-
- - C0 -
-
Default
-
D9
D9
Transmit/Receive array
With 7 USINT elements according to
the configurable MTU size
Figure 437: Transmit/Receive array (default)
First, the messages must be split into segments. In the default configuration, it is important to ensure that each
sequence can hold an entire segment, including the associated control byte. The sequence is limited to the size of
the activated MTU. In other words, a segment must be at least 1 byte smaller than the MTU.
MTU = 7 bytes → Max. segment length = 6 bytes
•
Message 1 (7 bytes)
➯
First segment = Control byte + 6 data bytes
➯
Second segment = Control byte + 1 data byte
•
Message 2 (2 bytes)
➯
First segment = Control byte + 2 data bytes
•
Message 3 (9 bytes)
➯
First segment = Control byte + 6 data bytes
➯
Second segment = Control byte + 3 data bytes
•
No more messages
➯
C0 control byte