By J.D. BIERSDORFER
Published: January 17, 2002
Q. What information is on the magnetic strip on the back of credit cards, and what is transmitted back and forth when a card is swiped?
A. The magnetic strip on the back of your A.T.M. or credit card has three tracks for storing data.
One track contains information reserved for the bank that issued the credit card, and magnetically encoded data like the primary account number, the user's name, a country code, an expiration date for the card and 79 characters of discretionary data, all mixed in with separators and other specialized computer characters.
Another track can also contain the primary account number, the country code and the card's expiration date, 40 characters of discretionary data, and separator characters.
A third track on the magnetic strip can both read and write data like an authorized spending amount or currency units and an encrypted personal identification number, but this track is not standardized among banks and the information may vary. Some banks that issue credit cards may not even use the third track.
When your card is swiped through an electronic card reader at the checkout counter in a store, the reader will usually use its built-in modem to dial the number of a company that handles credit authentication requests.
This company, often called an acquirer, checks the information from the electronic card reader for a valid card number and expiration date, the card's spending and usage limits and the store identification. After all the data checks out, the purchase is approved.
If the magnetic strip on the back of your card becomes scratched, dirty or demagnetized, the card may not work properly and you will have to get a new one.
Q. What is a B.B.S. used for?
A. B.B.S. is short for Bulletin Board System, and as the name implies, a B.B.S. is an electronic equivalent of a bulletin board. Traditionally it has been a text-based series of online menus
that computer users could connect with by modem to download software and exchange messages and public posts on a variety of topics with other users.
Dial-up numbers were available for many of the boards, and some could be entered by connecting to them through a Telnet program.
In the early days of the Internet, before the Web and its point-and-click browsers appeared in the early 1990's, B.B.S.'s and Usenet news groups were popular gathering places for online communities. Bulletin board systems can still be found around the Internet, although the ones with old-fashioned text-based software that requires the user to learn typed commands to navigate around the board are not as popular with new users as systems that use a Web-based interface. At www .thedirectory.org, the B.B.S. Corner link has a list of resources on how to find and use a B.B.S.
Q. I need something lighter, smaller and simpler than a laptop to use abroad to take notes in archives. What hand-held device would work with a folding keyboard and enable me to enter and store dozens of pages of text and move them later to a PC?
A. Plenty of portable keyboards are available for devices relying on the Palm operating system or on Microsoft's Pocket PC system, the major hand-held computer lines. A Pocket PC device comes with a miniature version of Microsoft Word. Palm-based systems have plenty of inexpensive text-editing software available (including Documents to Go by DataViz, which allows you to edit Word files) that let you store text on the hand-held and later transfer it to a PC.
The Handango online store (www.handango.com) offers a wide selection of both Palm-based and Pocket PC hand-held computers and lists specifications of the software comes with each model. It also has a broad variety of software and external keyboards in all sizes.
J. D. BIERSDORFER
Circuits invites questions about computer-based technology, by e-mail to QandA@nytimes.com. This column will answer questions of general interest, but letters cannot be answered individually.