It's been a few years since i've been working on itCan busIt's almost impossible to hide. Whether the car code ecu or the industrial plc communication involves the interaction of multiple nodes of data, can is probably the selected option. But many people's understanding of it stopped with the words "difference signals, arbitration mechanisms" and the problem came when they actually designed the application system。
What's the can bus principle
Can's core is one thing:Make several nodes fair on a busI don't know. It was tested by csm/ca plus conflict, not by csm/cd. The node has to listen to the bus before sending the data. In the event of a half-way crash, using digital arbitration of the di, the smaller the priority of the id, the losing party immediately withdrew from the transfer mode and did not lose data or damage the bus. The whole process is fully transparent to the upper levels。
The physical layer is a differential signal, the can h and can l lines, and the visible level (0) covers the hidden level (1), so the visible side has a natural advantage in arbitration. Porter-rate car domain control is often 500 k or 1 m, with industrial sites ranging from 250 k to 1 m. The bitcoding is nrz, which automatically inserts a reverse level bitfill at five consecutive levels to ensure synchronization of all nodes。
How do you design the can bus application
The first step in designing the application system is not to draw circuits, but to define the matrix of communications. What's going on, how many times, who's going to send, what's going on with the wrong frame? I've seen most holesThe signal cycle and nodal processing capability are not alignedFor example, a 10-ms-cycle brake signal with 20 ms of interpolation space in the receiving end. Data is lost in half. In the dbc file, the signal bytes, zoom factors, offsets are determined to die, so that the hardware end and the upper-placer are not misdiagnosed。
On the hardware, the cmu is basically enough with its own can controller, and the externals are attached to mcp 2551 and sja 1000. Do not forget the 120 stopovers at the terminal and put them on both ends of the bus. When the junction is converted, care to isolate--Power separation and signal quarantine separatedThe optical separation of the can transceivers is the most stable option. Solid elements are separated from receiving and sending by os missions or interruptions, with 8 to 16 frames left for queue depths, with downgrading strategies for spilling, low-priority frames and key security signals。
The can agreement does not look complicated, and it is difficult to find the boundary conditions in the project: how to compensate for the delay in carrying the bus to more than 80 per cent of the arbitration, how to restore the error counter at the heat-plugging nodes, and how to plan for multiple bus and the lifting of the can gateway. Without a standard answer, it can only be sharpened by a single version of the test. We've got to get the principle straight, we've got the matrix solid, and we've got to do this over and over again。
