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UART Explained

The Universal Asynchronous Receiver/Transmitter is the device inside your computer that turns the internal parallel data stream into serial for transmission down one cable. It receives a character, generates an interrupt and stores it in a buffer till the next character comes, assuming the CPU is able to take it before then.

The original design supplied with the IBM PC was the 8250, shortly to replaced with the one for the AT, the 16450. The 8250 took 1000ns to reset after dealing with an interrupt, which wasn't a problem initially, as the PC itself took twice as long to access it. However, when faster machines came along, their access time was the same, and eventually less than the UART's reset time. The 16450 was able to reset inside 200ns, so the problem went away for a while. It is capable of transmitting or receiving up to around 9600 bits per second without framing errors, which occur when the amount of information received does not match what is indicated between the first and last bits of a frame. When transmitting at 9600 bps, you are interrupting the CPU around 1000 times a second, so with a standard UART you will lose characters if the CPU can't take up the stored characters in time. At 19200 bps, the interrupt frequency is approx 1920 times a second; if you compress your data (say with V.42), and get an effective speed of 115,200, the CPU must rescue a byte every .000087 seconds! To get higher performance, the 16550A was invented, which had a 16-byte buffer and was therefore able to store more characters until its interrupt was able to be serviced. The 16650 has a 32-bit buffer.

The amount of characters stored before an interrupt is generated is programmable, and is known as the trigger level. Windows starts at 14, which unfortunately gives you only one better byte's leeway than the 8250! As the 16550 is pin-compatible with the 16450, and is therefore a straight swap, some applications may not automatically recognize it.

Here are the maximum speeds you can expect under different UARTs:

CPU 16650 16550A 8250/16450
386SX  38400 19200
386DX  57600 19200
486SX  115200 38400
486DX 46080 115200 38400

Unfortunately, the UART isn't the whole story, being only one element in the chain of components inside and outside your computer, including modems, software and overall speed of hardware. Multi-sector disk I/O (set in your Advanced CMOS) will disable interrupts until any hard disk transfers are complete, so you could lose characters. Smartdrive delayed writes also take priority. Interrupt Latency is discussed under Performance.

This is an article from  Phil Croucher, author of  Communications and Networks. Phil has a way of explaining in "plain" English. The information is well presented and is well above A+ standard.

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Updated 07/06/04

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