The Status of CAN Tx and Rx Lines When No CAN Transceiver IC Is Connected – KBA90988

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Question: Will you see any traffic on the CAN Tx and Rx lines of PSoC® 3 and PSoC 5LP, if there is no external CAN transceiver IC on the board?

 

Answer:

You will not see any traffic on the CAN Tx and Rx lines of PSoC® 3 and PSoC 5LP, if there is no external CAN transceiver IC on the board. In CAN protocol, after transmitting each bit into the bus, every node checks if the bus state is the same as the bit it transmitted. If they are the same, the node continues with the transmission of the next bit; if they are different, the node stops transmitting and starts to receive the message on the bus. So, if you do not connect the transceiver IC, the node will not see the same bit that it has transmitted on the Rx pin since there is no loop back. So it will stop transmitting.

For development purposes Cypress offers the CY8CKIT-017, which has the transceiver IC and the necessary termination resistors. Figure 1 shows the physical connections between the PSoC 3/5 DVK and 017 kit for CAN communication.

Figure 1. Connections between the PSoC 3/5 DVK and 017 Kit for CAN Communication

If you are making a custom board, you can use any of the CAN transceiver ICs, such as the TJA1050 or SN65HVD1040. Note that some transceivers have a standby (STB) pin, which keeps the device in receive mode only to save power. Do not assert this pin for normal CAN communication.