UART chips control the actual flow of information over serial ports (the COM ports on your computer). All PC's have them, Macs, PC's and Unix boxes. Oddly, on old PC's (IBM XT model compatibles with 80386 processors and older), there is a bug in the BIOS that sets the wrong interrupt and memory access location for the serial ports.

MODEL
BUFFER SIZE
SPEED MANUFACTURER NOTES
8250 None < 9600 bps National Semiconductor Buggy - assigns interrupts incorrectly
8250A None < 9600 bps National Semiconductor Fixes 8250 problem, but most BIOS were built to work around this, so the combination of the 'fixed' chip and 'workaround' BIOS caused new problems.
8250B None < 9600 bps National Semiconductor Final fix in 8250 series. Detects BIOS
that fixes the interrupt problem
16450 1 byte (8 bits) 9600 bps National Semiconductor Has same interrupt fix problem as 8250A.
16550 128 bits
(16 characters)
300 - 115,200 bps National Semiconductor Larger buffer, Direct Memory Access (DMA) function
16550A 128 bits
(16 characters)
300 - 230,000 bps National Semiconductor  
16550AN 128 bits
(16 characters)
300 - 230,000 bps National Semiconductor  
16650D 128 bits
(16 characters)
300 - 230,000 bps Other  
16750 128 bits
(16 characters)
300 - 460,000 bps Other  
16850 128 bits
(16 characters)
300 - 920,000 bps Other  

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