|UART usually stands for Universal
Asynchronous Receiver / Transmitter and is a chip located on
a serial card, it allows connection with modems and other devices.
Most card manufacturers integrate UART into other chips which
can also control parallel port, games port, floppy or hard disk
drives and are typically surface mount devices. One of such
chips is presented in 8250 line, which includes 16450, 16550,
16650, & 16750 UARTS, this line is usually used in PCs.
|The compatibility chips
16550 chip is compatible with 8250 & 16450 chips. The only two differences are pins 24 & 29. 8250 Chip Pin 24 has chip selection which works as chip activity indicator. Pin 29 is not used in 8250/16450 UARTs. These pinouts have different purpose in 16550 chip. Their purpose is: Transmit Ready and Receive Ready. This pinouts allow DMA (Direct Memory Access) usage. These Pins have two different modes of operation. Mode 0 supports single transfer DMA where as Mode 1 supports Multi-transfer DMA.
|The UART requires a Clock to run
|For this purpose there's a crystal
on the serial card usually designed for 1.8432 MHZ or 18.432
MHZ frequency. The crystal is connected to XIN-XOUT pinouts
of the UART chip, this pinouts help set CPU clock speed. These
Clock are needed to use the Programmable Baud Rate Generator
which directly interfaces into the transmit timing circuits
but not directly into the receiver timing circuits. For this
an external connection must be made from pin 15 (BaudOut) to
pin 9 (Receiver clock in.) Note that the clock signal will be
at Baudrate * 16