host system knows if there is at least one character in the transmit buffer or shift register; "ready for next character(s)" may also be signaled with an interrupt. Application Transmitting and receiving UARTs must be set for the same bit speed, character length, parity, and stop bits for proper operation. The receiving UART may detect some mismatched settings and set a "framing error" flag bit for the host system; in exceptional cases, the receiving UART will produce an erratic stream of mutilated characters and transfer them to the host system. Typical serial ports used with personal computers connected to modems use eight data bits, no parity, and one stop bit; for this configuration, the number of ASCII characters per second equals the bit rate divided by 10. Some very low-cost home computers or embedded systems dispense with a UART and use the CPU to sample the state of an input port or directly manipulate an output port for data transmission. While very CPU-intensive (since the CPU timing is critical), the UART chip can thus be omitted, saving money and space. The technique is known as bit-banging. History Some early telegraph schemes used variable-length pulses (as in Morse code) and rotating clockwork mechanisms to transmit alphabetic characters. The first serial communication devices (with fixed-length pulses) were rotating mechanical switches (commutators). Various character codes using 5, 6, 7, or 8 data bits became common in teleprinters and later as computer peripherals. The teletypewriter made an excellent general-purpose I/O device for a small computer. Gordon Bell of DEC designed the first UART, occupying an entire circuit board called a line unit, for the PDP series of computers beginning with the PDP-1. According to Bell, the main innovation of the UART was its use of sampling to convert the signal into the digital domain, allowing more reliable timing than previous circuits that used analog timing devices with manually adjusted potentiometers. To reduce the cost of wiring, backplane and other components, these computers also pioneered flow control using XON and XOFF characters rather than hardware wires. DEC condensed the line unit design into an early single-chip UART for their own use. Western Digital developed this into the first widely available single-chip UART, the WD1402A, around 1971. This was an early example of a medium-scale integrated circuit. Another popular chip was the SCN2651 from the Signetics 2650 family. An example of an early 1980s UART was