BS ISO 11783-4 pdf download Tractors and machinery for agriculture and forestry — Serial control and communications data network Part 4: Network layer
6 Network interconnection unit functions
6.1 Forwarding An NIU transfers individual message frames between two or more ports (one port for each network segment). The order of frames received on one port and transmitted on another shall be preserved for a given priority level. An NIU shall forward all queued messages of a higher priority before those of a lower priority. Otherwise, all messages being forwarded to a specific port could be excessively delayed. A simple FIFO queue shall not be used to meet this requirement. When an NIU forwards (7.1 Repeater and 7.2 Bridge) a message to another segment in the same address space, it uses an address identical to that of the originator of the message. Ordinarily, this does not cause arbitration problems, as the unit will not retransmit the message to the segment it originally came from and, moreover, the addresses are unique on a given ISO 11783 network. The sole exception is when an address-claim message is forwarded to a segment in which another CF is simultaneously claiming the same address. In such a low-probability situation, the NIU should stop the automatic retransmission sequence within the CAN protocol chip. Otherwise, the NIU will experience multiple collisions and go “bus off”, thereby preventing other messages from being forwarded until the NIU is able to recover from the bus off condition. An NIU may begin to forward messages from one segment to another before the NIU has claimed an address (i.e. it does not perform address translations) if it is simply acting as a repeater or bridge. NOTE Until an NIU has completed a power up sequence and connected it to the network, the subnetwork and the CFs connected to it cannot receive other messages.
6.2.1 General For the filtering function, messages sent with Transport Protocol, Extended Transport Protocol, Fast Packet, or other message packeting mechanism, shall be handled according to the parameter group number (PGN) of the contained message. If the PGN of the contained message is defined for the filter, the protocol handling messages shall be processed according to the defined filter.
6.2.2 Block mode In block filter mode (0), the NIU shall default to forwarding all messages (7.2). Bus utilization (traffic) can be higher on each bus segment, but if it is within acceptable limits, the message filtering algorithm will be non- existent. The filter database within the NIU can contain identifier entries (PGN values) for messages which shall not be forwarded (blocked). This can be used to reduce the overall bus traffic on a given segment, and is the preferred mode of operation for bridges conforming to ISO 11783. Filter database entries are typically made during assembly or initial configuration and retained in non-volatile memory.
6.2.3 Pass mode In pass filter mode (1), the NIU shall default to not forwarding messages (7.2). Then, in order for a message to be forwarded, an entry shall exist with a specific identifier (PGN value) for that particular message. This mode is best for ports on NIUs that link subnetworks performing specific functions. It requires prior knowledge of the CFs and the functions of the whole network, or that the CFs be able to add entries to the filter database, in which case the NIU can require more memory and processing power if it is to accommodate a large filter database. Moreover, some entries within the database need to be permanent (i.e. configured to be always present), so that corresponding messages are always forwarded across the whole network. Typical applications are network management, diagnostics and global requests.BS ISO 11783-4 pdf download