Message Segmentation
General
Category: Extended
The maximum message size supported by the Anybus CompactCom 40 is normally 1524 bytes. In some applications a maximum message size of 255 bytes is supported, e.g. if an Anybus CompactCom 40 is to replace an Anybus CompactCom 30 without any changes to the application. The maximum socket message size is 1472. To ensure support for socket interface messages larger than 255 bytes a segmentation protocol is used.
Note
The segmentation bits have to be set for all socket interface messages, in the commands where segmentation can be used, whether the messages have to be segmented or not.
The segmentation protocol is implemented in the message layer and must not be confused with the fragmentation protocol used on the serial host interface. Consult the general Anybus CompactCom 40 Software Design Guide for further information.
The module supports 1 (one) segmented message per instance
Command Segmentation
When a command message is segmented, the command initiator sends the same command header multiple times. For each message, the data field is exchanged with the next data segment.
Command segmentation is used for the following commands (Socket Interface Object specific commands):
Send
Send To
When issuing a segmented command, the following rules apply:
When issuing the first segment, FS must be set.
When issuing subsequent segments, both FS and LS must be cleared.
When issuing the last segment, the LF-bit must be set.
For single segment commands (i.e. size less or equal to the message channel size), both FS and LS must be set.
The last response message contains the actual result of the operation.
The command initiator may at any time abort the operation by issuing a message with AB set.
If a segmentation error is detected during transmission, an error message is returned, and the current segmentation message is discarded. Note however that this only applies to the current segment; previously transmitted segments are still valid.
Segmentation Control Bits (Command)
Bit | Contents | Meaning |
|---|---|---|
0 | FS | Set if the current segment is the first segment |
1 | LS | Set if the current segment is the last segment |
2 | AB | Set if the segmentation shall be aborted |
3...7 | (reserved) | Set to 0 (zero) |
Segmentation Control Bits (Response)
Bit | Contents | Meaning |
|---|---|---|
0... 7 | (reserved) | Ignore |
Response Segmentation
When a response is segmented, the command initiator requests the next segment by sending the same command multiple times. For each response, the data field is exchanged with the next data segment.
Response segmentation is used for responses to the following commands (Socket Interface Object specific commands):
Receive
Receive From
When receiving a segmented response, the following rules apply:
In the first segment, FS is set.
In all subsequent segment, both FS and LS are cleared.
In the last segment, LS is set.
For single segment responses (i.e. size less or equal to the message channel size), both FS and LS are set.
The command initiator may at any time abort the operation by issuing a message with AB set.
Segmentation Control bits (Command)
Bit | Contents | Meaning |
|---|---|---|
0 | (reserved) | (set to zero) |
1 | ||
2 | AB | Set if the segmentation shall be aborted |
3...7 | (reserved) | Set to 0 (zero) |
Segmentation Control bits (Response)
Bit | Contents | Meaning |
|---|---|---|
0 | FS | Set if the current segment is the first segment |
1 | LS | Set if the current segment is the last segment |
2...7 | (reserved) | Set to 0 (zero) |