Serial communications is sufficiently complex that special chips have been designed to do the work of forming and timing the strings of bits that comprise serial data. Such a chip is called a universal asynchronous receiver transmitter or UART. Without UARTs, communications programming would be very complicated. The IBM family of microcomputers use the Intel 8250 UART. The Intel 8250 is a programmable Universal Asynchronous Receiver/Transmitter packaged in a 40-pin integrated circuit. The UART is used to convert parallel data to serial format on the transmit side (data going to a modem for example) and to convert serial data to a parallel format on the receive side (coming from a modem). The serial format, in order of transmission and reception, is a start bit, followed by five to eight data bits, a parity bit (if programmed) and one or two stop bits. The maximum recommended data rate is 54K bits per second. It can transmit characters at the same time as it is receiving characters (full duplex). The transmission speed of the UART is also programmable. Newer UARTs are now used in serial ports in PCs : 16450 and 16550 capable of speeds up to 115,000 bps. They are more efficient than the 8250 but are compatible with it from a programming point of view. Internal registers enable the user to program various types of interrupts, modem controls, and character formats. The user can read the status of the UART at any time, monitoring transmission conditions, interrupts and modem status.