Rede de comunicação – Visão geral

418-00-7 Module Communications Network 418-00-7 DESCRIPTION AND OPERATION Messages in the Controller Area Network (CAN) E71205 A message comprises the following parts: ‡ An identifier which indicates the identity and priority of the message ‡ Information data (value, information etc.) ‡ A check sum which is used to check whether the message is received in full ‡ And end signal which signals the end of the message A complete CAN message is referred to as a signal frame. Priority Conflicts may arise in a network if several control modules try to send messages at the same time as each other. This could for example be the case if the driver applied the brakes while the front passenger adjusted the A/C settings and a rear passenger operated the electric rear window regulators. The messages need to be prioritized so that safe operation can be ensured. In addition, any delays arising as a result of the prioritization process must be kept within limits so that the customer preferably does not notice them at all. In order to avoid conflicts and delays and to ensure perfect operation of the systems, a fixed ranking order applies to the messages. The priority of a message is defined by the number of zeros at the start of the message; the more zeros, the higher the priority. Prioritization takes place as follows: ‡ If the network is available, all modules waiting to send a message deliver the first bit of their message. ‡ All modules register the signals sent out on the network. ‡ If a zero is sent out by a module then the modules which sent out a "1" wait until the network is available again. ‡ The modules with a zero then send the second bit of their message. ‡ If a zero is sent out by a module as its second bit then the modules with a "1" wait until the network is available again. This process is repeated. The message with the highest priority, i.e. the message with the largest number of zeros at the start, is sent first. The end of a message is made up of seven zeros. It shows the modules that the network is free again and that messages can be sent out again according to their priority. Two message types Two types of message are sent in the system: