Can Tx-Rx Data Line

Question

I am trying to see some data on the CANBUS line , I show the CANH and CANL data on the osciloscope line. And detect Rx line, When I show the Transmit message on the CANBUS line, I show the this data on RX line at the same time. Why this occurs ? Is that possible, or I have error ? I am using stm32f4 microcontroller and canbus tranceiver.

The message we see on the Canbus line CANH-CANL messages are the data I send from the processor to the sensor. Therefore, why is the data I want to send coming back to the Rx line, ACK is added to the end, but only ACK is not required?

And if there is normal situation how does MCU decide to which is Real Rx data and the returned of the sent data with ack. I am so confused about that.

Answer 1

The receiver part of the CAN transceiver is active even when you are transmitting.

So you will always receive the transmitted message (as long as there is no error on the bus). The CAN controller uses this to determine that the message was sent correctly.

If the CAN controller receives exactly what is transmits, it knows that all is good. Then of course, it looks for the ACK after transmitting the entire message.

Answer 2

This is normal. While transmitting, a CAN controller monitors the bus at the same time, and compares it with the transmitted one for possible mismatches.

• If a mismatch occurs in the bus arbitration phase (transmission of CAN-ID), it means that the arbitration is lost to a higher priority message which is being transmitted by another node. In this case, node stops transmitting the message to try again later, when the bus is idle.

• If a mismatch occurs in later stages (after the CAN-ID), it means an error in the bus. I can't remember the details of this case.

ACK is received if at least one other node receives the transmitted message. However, it doesn't mean that the node that acknowledged the message is the intended receiver.

Answer 3

CAN is designed that any node can transmit at any time.

During the arbitration phase of the message each transmitting node will observe if the transmitted bits match the received bits. 0-bits are dominant and 1-bits are recessive. This means a 0-bit will win the arbitration phase. So if two or more nodes transmit at the same time, the node that sees a 0-bit while transmitting a 1-bit will know that it lost the arbitration, and it will stop transmission and delay the message for a bit.

Source: https://en.wikipedia.org/wiki/CAN_bus

This mechanism only works if COB-IDs are unique. This is required to guarantee that colliding messages can always be detected during the arbitration phase.

Is that possible, or I have error ?

This is normal behaviour (by design).

Therefore, why is the data I want to send coming back to the Rx line, ACK is added to the end, but only ACK is not required?

ACK is added by any other node that received your transmitted message correctly.

And if there is normal situation how does MCU decide to which is Real Rx data and the returned of the sent data with ack. I am so confused about that.

If the transmitting node wins the arbitration phase, ACK is the first bit that differs from the transmitted sequence of bits.