The "CanWarnComm" status warning indicates that CAN bus problems have been detected, but communication is proceeding. This warning corresponds to the CAN controller chip on the NI-CAN card entering what is known as "CAN Error Passive state". This can happen for a number of reasons, but from what you are describing, it sounds like when you power your external device OFF, there is no longer a second CAN device on the network to acknowledge (or "ACK")messages. Remember that a CAN message must be ACK'd by another device on the bus, or the transmitter will attempt to re-transmit. After a number of re-transmissions, the device enters the "Error Passive" state as a mechanism to confine errors.
So, the warning you are seeing is expected behavior when the NI-CAN card i
s left as the only active device on the bus.
