Could I Test CAN Tranceiver with External Loopback Mode? - can-bus

Question

Hello I am using STM32H753 over a custom PCB.
I have no any CAN Analyzer except oscilloscope, multimeter and logic analyzer.
We have a problem with CAN BUS communication.
I am suspecting of CAN Tranceiver but we couldn't find the root cause yet.
There are external and internal loopback modes in STM32H7 to test CAN, However these modes also allow us to see the messages what we sent on CAN TX.
I have two different product which has different STM32H753 variant. One is working and the other one is not. Their tranceivers are different but almost same pinout and behavior.
My assumption was if I see my messages on CAN TX then I have to see them CAN H and CAN L.
I wrote test code.
I tried both test code in both device.
On working one, I saw messages on CAN Tx and also see messages with in a suppress way which I thought because of no termination resistor at the end.
On the not-working one, I also saw messages at CAN TX and there is no any move at CAN H or CAN L.
My question is my test was is true or still I could miss something or this test does not imply anyhting ?
HAL_Delay(100);
if(HAL_FDCAN_AddMessageToTxBuffer(&hfdcan1, &TxHeader, TxData_Node1_To_Node2, FDCAN_TX_BUFFER0 ) != HAL_OK)
{
Error_Handler();
}
/* Send Tx buffer message */
if(HAL_FDCAN_EnableTxBufferRequest(&hfdcan1, FDCAN_TX_BUFFER0) != HAL_OK)
{
Error_Handler();
}
while(HAL_FDCAN_IsTxBufferMessagePending(&hfdcan1, FDCAN_TX_BUFFER0) == 1);
/* Polling for reception complete on buffer index 0 */
while(HAL_FDCAN_IsRxBufferMessageAvailable(&hfdcan1, FDCAN_RX_BUFFER0) == 0);
/* Retrieve message from Rx buffer 0. Rec msg from Node 2 */
if(HAL_FDCAN_GetRxMessage(&hfdcan1, FDCAN_RX_BUFFER0, &RxHeader, RxData_From_Node2) != HAL_OK)
{
Error_Handler();
}
hfdcan1.Instance = FDCAN1;
hfdcan1.Init.FrameFormat = FDCAN_FRAME_CLASSIC;
hfdcan1.Init.Mode = FDCAN_MODE_EXTERNAL_LOOPBACK;
hfdcan1.Init.AutoRetransmission = DISABLE;
hfdcan1.Init.TransmitPause = DISABLE;
hfdcan1.Init.ProtocolException = DISABLE;
hfdcan1.Init.NominalPrescaler = 1;
hfdcan1.Init.NominalSyncJumpWidth = 13;
hfdcan1.Init.NominalTimeSeg1 = 86;
hfdcan1.Init.NominalTimeSeg2 = 13;
hfdcan1.Init.DataPrescaler = 2;
hfdcan1.Init.DataSyncJumpWidth = 12;
hfdcan1.Init.DataTimeSeg1 = 12;
hfdcan1.Init.DataTimeSeg2 = 12;
hfdcan1.Init.MessageRAMOffset = 0;
hfdcan1.Init.StdFiltersNbr = 1;
hfdcan1.Init.ExtFiltersNbr = 0 ;
hfdcan1.Init.RxFifo0ElmtsNbr = 0;
hfdcan1.Init.RxFifo0ElmtSize = FDCAN_DATA_BYTES_8;
hfdcan1.Init.RxFifo1ElmtsNbr = 0;
hfdcan1.Init.RxFifo1ElmtSize = FDCAN_DATA_BYTES_8;
hfdcan1.Init.RxBuffersNbr = 1;
hfdcan1.Init.RxBufferSize = FDCAN_DATA_BYTES_8;
hfdcan1.Init.TxEventsNbr = 0;
hfdcan1.Init.TxBuffersNbr = 1;
hfdcan1.Init.TxFifoQueueElmtsNbr = 32;
hfdcan1.Init.TxFifoQueueMode = FDCAN_TX_FIFO_OPERATION;
hfdcan1.Init.TxElmtSize = FDCAN_DATA_BYTES_8;
if (HAL_FDCAN_Init(&hfdcan1) != HAL_OK)
{
Error_Handler();
}