The '754C is a quad universal asynchronous receiver/transmitter (UART) with 64-byte FIFOs, automatic hardware/software flow control, and data rates up to 3 Mbps. It incorporates the functionality of four UARTs, each UART having its own register set and FIFOs. The four UARTs share only the data bus interface and clock source, otherwise they operate independently. Another name for the UART function is Asynchronous Communications Element (ACE), and these terms are used interchangeably. The bulk of this document describes the behavior of each ACE, with the understanding that four such devices are incorporated into the '754C. The '754C offers enhanced features. It has a transmission control register (TCR) that stores received FIFO threshold level to start/stop transmission during hardware and software flow control. With the FIFO RDY register, the software gets the status of TXRDY/RXRDY for all four ports in one access. On-chip status registers provide the user with error indications, operational status, and modem interface control. System interrupts may be tailored to meet user requirements. An internal loopback capability allows onboard diagnostics.