The CAN bus can only function if there are at least two CAN controllers on the bus. One to send and one to acknowledge. If a CAN controller attempts to transmit frames with no other nodes on the bus then the frames will not be acknowledged, which is an error condition. If you are starting out with CAN development then typically the other CAN controller is a PC to CAN bus interface with monitoring software running on the PC showing messages.
Almost all CAN controllers have an internal loopback mode which allows a CAN controller to receive what it is transmitting without the messages appearing on the bus. This is useful for power-on self-test, developing routines for using the CAN controller, etc.
"FAQ contribution from Embedded Systems Academy, experts in CAN bus. For more information visit www.esacademy.com"