Serial Port Basics (continued)
Data Rates
The baud rate for a serial connection is the number of bits that are transmitted per second. It is specified in bits/second or baud. For example, a 9600 baud serial link transfers 9600 bits of data per second.
The EIA RS-232C standard permits data rates up to 19200 bps and cable lengths up to 400 meters (but not both).
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Data rates and distances for RS-232 communications.
Although the specification only defines rates up to 19200 bps, communication using data rates as high as 230400 bps and a short (less than 2 meter) cable are common. Standard modems communicate with computers at up to 115200 bps.
As you may have guessed, the use of high baud rates requires more capable computer hardware. At high baud rates, a computer must process as many as 23000 characters per second. The constant attention a computer must pay to its serial port makes this problematic particularly in multitasking environments such as Microsoft Windows 3.1/95/98/NT/2000.
Data Rates and the UART
Computer hardware designers solve this problem by allowing the computer to respond to characters less frequently. A Universal Asynchronous Receiver/Transmitter (UART), the component responsible for communicating via RS-232, may contain several bytes of memory called a FIFO (first-in, first-out memory).
The original IBM PC (and many of its successors) used the 8250 UART, which contained no FIFO. That is, a computer with 8250 (or 16450) UART's must respond to every incoming character.
Newer PC's incorporate the 16550 UART or a variant. The 16550 incorporates a 16-byte FIFO and is mandatory for communications at speeds above 9600 bps and is important for error-free communications at lower speeds as well.
You can find out which type of UART's your computer uses by using the MSD.EXE tool provided with DOS and Windows or by looking in the Windows 95/98/Me/2000 device manager or the Windows NT 4.0 Control Panel.
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