The bar-code scanner at the supermarket checkout line, the automatic teller at the bank. Devices like these, automating many of the billions of daily transactions that add up to the world economy, have helped build NCR Corp. into an international operation with sales of $6 billion a year. Now more than a century old, it is rich in cash, rolling out a new product line and weathering the nation's slide toward recession without major disruption.

So would it want to be taken over?

Emphatically no, said Chairman Charles E. Exley Jr. of the Dayton, Ohio-based company. "We want to be left alone," Exley said yesterday, a day after American Telephone & Telegraph Co. went public with an unsolicited $6 billion buyout offer. "We have not been interested in being acquired by AT&T or anyone else."

AT&T's offer, priced at $90 per share, is good until tomorrow afternoon. An executive at the huge telecommunications company suggested yesterday that protests by Exley and his board are a pro forma maneuver to bid the price up. But if NCR's directors do not budge, AT&T said it will consider going over the board's head to NCR's shareholders -- in other words, the company once known as "Ma Bell" will consider a hostile takeover.

If it does, AT&T would be breaking with its own normally friendly mode of acquiring companies and also bucking Wall Street -- hostile bids have gone out of style in the last year. Investors yesterday seemed to be betting that AT&T would get its way. In heavy trading, NCR shares shot up $24.75 to close at $81.50. AT&T's shares, meanwhile, fell $2 to close at $30.12 1/2.

What AT&T wants is one of the great icons of American business history, a company that more than a century ago forever changed what happens when people step up to a store counter, money in hand.

Its first owner, John H. Patterson, an eccentric entrepreneur, introduced the first commercially successful cash register in 1884 and set out to convince the nation's shopkeepers that they needed the costly machines. (Promises that theft by clerks at the counter would decrease were a big selling point.)

Patterson was known for drilling executives in horsemanship and for firing them without warning (one such victim, Thomas J. Watson, went on to build International Business Machines Corp.). He also was hailed as an industrial visionary, building, in the age of the sweatshop, well lit, modern factories with gardens outside.

National Cash Register Co., as the company was known until 1974, exercised the same sway over its market that IBM and Bell Telephone Co. exercised in theirs. It eventually drew antitrust indictments from Washington.

Today NCR remains on the scene with greatly reduced market power and different technology but a solid, blue-chip reputation. "They have made the transformation from the mechanical age to the electronic age," said Sanjiv Hingorani, vice president at securities firm Salomon Brothers Inc.

It calls itself a general computer company and it ranks fifth in that category. It makes mainframe computers and personal computers. But its core business retains its roots from the last century, in the processing of everyday economic transactions.

It makes electronic cash registers, stand-alone models as well as advanced networks that tie several registers together with phone lines and computers and can track the daily sales of a department store or drug store chain. It makes automatic teller machines and large-scale networks that run the transactions of banks.

It has also gone international, putting plants in such places as Japan and Scotland. About 50 percent of its sales and 55 percent of its operating profit in 1989 came from overseas; its subsidiary NCR Japan is traded on the Tokyo Stock Exchange.

Despite its smash-hit first product, in recent years NCR has been known for following other companies' leads. Now it is trying to change. This fall it announced a seven-member family of computers. It has stressed systems that use so-called industry-standard software such as Unix and MS-DOS, rather than its own, proprietary system that can be used to lock customers into its products. It is working on new technology, through a tie-in with a small California company called Teradata Corp., that works on information with thousands of processors simultaneously.

The year 1989 was a rough one for NCR. Its revenue fell 1 percent and its profit 6 percent. But 1990 has been something of a turnaround, with third-quarter figures showing revenue up 9 percent but net income still down.

AT&T entered the scene two weeks ago. Desperate to set straight the computer business on which it has lost an estimated $2 billion or more since the breakup of the Bell system in 1984, AT&T executives reportedly think that the purchase of NCR would give them, in one stroke, new management for their computer business and a major slice of the world market. "Our proposal is based upon keeping NCR running exactly what they're doing, with the management they have," said AT&T group executive Robert Kavner. " ... We have a deep respect for them."

NCR would be able to draw on the resources of AT&T Bell Laboratories, one of the nation's premier research facilities, Kavner said. It would benefit from expertise that AT&T, the country's largest long-distance telephone company, has in building and operating computer networks.

But NCR's Exley maintained that, despite more than two weeks of talks with AT&T, he was never seriously interested in becoming part of AT&T, saying it would throw his company off balance. NCR shareholders, he suggested, will make more money building on NCR's future than trying to correct AT&T's past.