While the CAN driver is in active mode a TXD DTO circuit prevents the local node from blocking network communication in event of a hardware or software failure where TXD is held dominant longer than the time out period t_{TXD_DTO}. The TXD DTO circuit is triggered by a falling edge on TXD. If no rising edge is seen before the time out constant of the circuit, t_{TXD_DTO}, expires the CAN driver is disabled releasing the bus lines to the recessive level. This keeps the bus free for communication between other nodes on the network. The CAN driver is re-activated on the next dominant to recessive transition on the TXD terminal, thus clearing the dominant time out. The high-speed receiver and RXD terminal will reflect what is on the CAN bus during a TXD DTO fault. The TS terminal in driven low during a TXD DTO fault.

Figure 9-2 Timing Diagram for TXD DTO

The minimum dominant TXD time allowed by the TXD DTO circuit limits the minimum possible transmitted data rate of the device. The CAN protocol allows a maximum of eleven successive dominant bits (on TXD) for the worst case, where five successive dominant bits are followed immediately by an error frame. The minimum transmitted data rate may be calculated using the minimum t_{TXD_DTO} time and the maximum number of successive dominant bits (11 bits).