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Line 8: * USARTs in synchronous mode transmits data in [[Frame (networking) \|frames]]. In synchronous operation, characters must be provided on time until a frame is complete; if the controlling processor does not do so, this is an ''"underrun error''," and transmission of the frame is aborted. * USARTs operating as synchronous devices used either character-oriented or bit-oriented mode. In character (STR and BSC) modes, the device relied on particular characters to define frame boundaries; in bit (HDLC and SDLC) modes earlier devices either relied on physical-layer signals, while later devices took over the physical-layer recognition of bit patterns. * A synchronous line is never silent; when the modem is transmitting, data is flowing. When the physical layer indicates that data is flowing, a USART will send a steady stream of padding, either characters or bits as appropriate to the device orand protocol. == Devices ==

Universal synchronous and asynchronous receiver-transmitter: Difference between revisions