Acumen SDR2-USB manual More asynchronous serial parameters, Data rates and the Uart

Serial Port Basics SDR2-USB Configuration Guide rev 1.0

The EIA RS-232C standard permits data rates up to 19200 bps and cable lengths up to 400 meters (but not both).

data rate	maximum distance
(bps)	(meters)	(feet)
19200	15	45
9600	25	76
4800	50	152
2400	100	304
1200	200	608
600	400	1216

Table A.1. 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 (<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.

A.2.1 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 control panel.

A.3 More asynchronous serial parameters

In most cases, the data rate in bytes/second can be approximated by dividing the baud rate (in bits/second) by 10. If a byte consists of 8 bits, why divide by 10?

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