Are there master and slave modules on the CAN bus?

The nature of CAN bus communications allows all modules to transmit and receive data on the bus. Any module can transmit data, which all the rest of the modules receive permitting both peer-to-peer and broadcast data transmissions.

In CANopen, the CAN bus normally includes one module acting as a network master which starts the bus communications, but a dedicated master module is not needed. In J1939, the master and slave concept is not used.