80 (104)
Message Segmentation
General
Category
: Extended
The maximum message size supported by the Anybus CompactCom 40 is normally 1524 bytes. In some ap-
plications a maximum message size of 255 bytes is supported, e.g. if an Anybus CompactCom 40 is to re-
place 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.
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 fragmen-
tation 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; previ-
ously 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
Anybus
®
CompactCom
™
40 Common Ethernet Design Guide
HMSI-27–269 2.1
