Original question:
Hi Team,
I have one query, if CANH - Shorted to Vbat (Fault 6) occurred, during continuous cyclic message transmission from uC. What was the expected response on CAN TXD and RXD?
Please confirm and share any graphical representations for CAN H, CAN L, CAN TXD, and RXD during the CANH - shorted to Vbat.
Also share what the TXD and RXD responses were when faults 7, 8, and 9 occurred in the same scenario.
Regards,
Jerald
Hi Jerald,
When there is a fault on the CAN bus that shorts a CAN line to it's non-opposate state (CANH to Vcc/Vsup or CANL to ground), it is sometimes still possible to transmit and receive data because the differential signal on the bus can be created by the non-faulted signal line. In the example below, CANH is shorted to Vcc. CANL is still able to pull to ground to create a positive differential and the RXD data is received correctly. This is not always the case as the common mode of the CAN signal is not maintained in this scenario. This will degrade the signal quality and can cause errors to occur in high speed or complex systems.
In other fault cases such as CANH short to ground, CANL short to Vcc/Vsup, or CANH short to CANL, it is not possible to drive a positive differential on the bus and communication may be impossible. In this case, the bus would remain in a recessive state indefinitely and the RXD line would sit at a logic high.
Note that bus conditions do not impact the TXD line as this is controlled by the MCU and is an input to the transceiver.
Let me know if you have any more questions.
Regards,
Eric Schott
