Practical Industrial Data Networks:Design, Installation and ...

DeviceNet overview 173busRecessiveDominant1bus line212331 losses 3 lossesFigure 12.22DeviceNet arbitrationBecause of this method of arbitration, the node with the lowest number (i.e. the mostsignificant ‘0’s in its identifier field) will win the arbitration.12.10.3 FragmentationAny device that needs more than 8 bytes of data sent in any direction will causefragmentation to occur. This happens since a frame can only contain 8 bytes of data.When fragmentation occurs, only 7 bytes of data can be sent at a time since the first byteis used to facilitate the reassembly of fragments. It is used as follows:First byteSignificance00 First fragment (number 0)41–7F Intermediate fragment(lower 6 bits of the byte is the fragmentnumber)80–FFLast fragment (lower 6 bits of the byte is thefragment number)Example:Data packetDescription00 12 34 56 78 90 12 34 First fragment, number 041 56 78 90 12 34 56 78 Intermediate fragment number 142 90 12 34 56 78 90 12 Intermediate fragment number 283 34 56 Last fragment number 3