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

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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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Contents Acumen Instruments Corporation Page Copyright Page Table of Contents Service and Support Getting Started OverviewBefore you start Quick guide to the DataBridge SDR2-USBConfiguring your SDR2-USB via the setup file Connecting power to your SDR2-USBDeploying the DataBridge SDR2-USB Creating setup.txt CongratulationsConfiguring DataBridge SDR2-USBSetting date and time CommentsConfiguring the data port Setting date and timeSpecifying a filename and folder Configuring file creation behavior Adding time stamps to recorded filesTesting your configuration Operations Setup file readingRecord mode operation Receiving dataDevice removal while recording Power failure and improper shutdownsStopping recording Filename generationFormatting the storage media PartitionsLithium battery maintenance Resetting to factory defaultsSetup File Reference Initial Setup CommandsData Port Setup Commands Recording Behavior CommandsScheduled File Closing Commands Time Stamping CommandsOther Commands J4 Remote Control Port LocationFunctions Pinout and Mating CableForce-Record Shunt Installation Page Service Support Exception CodesService and Support SDR2-USB Configuration Guide rev Contacting Acumen Instruments Corporation Returning EquipmentExclusions WarrantyOne year warranty LimitationsSerial Port Basics Serial specificationsData rates More asynchronous serial parameters Data rates and the UartDTE and DCE Handshaking Voltage levelsSDR2-USB Configuration Guide rev Page Electrical specifications Specifications5 J7 rear power receptacle pin configuration Serial communications Mechanical Specifications6 J1 power receptacle pin configuration USB socketSpecifications SDR2-USB Configuration Guide rev
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