Chapter · CAN bus
The message
What a message is made of, how a sender learns somebody heard, the extra bit that keeps the desks in step, and what happens when a frame goes wrong.
- 06What a message looks like4 minA classical CAN message is a strip of fields in a fixed order, from a start bit to an end marker. Its check catches damage by accident and says nothing about meaning.
- 07Somebody heard4 minAfter the check there is one slot in which any receiver that got the message intact shouts. The sender learns that at least one device heard, not which or how many.
- 08The stuffed bit4 minAfter five equal bits the sender adds one of the opposite value, so the rail always has edges to keep the devices in step. The receiver takes the extra bits out again.
- 09When a frame goes wrong5 minFive kinds of error are detected, announced with an error flag and retried. Two counters in every device decide when it must go quiet, and the last stage is its own state called bus off.
Start at 06 and read down — the order is the order things get easier in. Or take the one with your part in it; every article stands on its own, and links back to whatever it needs.