A top American Telephone & Telegraph executive today blamed the current slump in computer sales on "conventional technology" that is "incapable of reaching new customers."

"Lower-priced computers aren't enough," insisted Jack M. Scanlon, group vice president for AT&T's Computer Systems Division. What is needed are more useful computers, he said.

Speaking to a group of corporate data-processing executives at the Hammer Forum here in Boston, Scanlon argued that the market for the current generation of personal computers is quickly becoming saturated because most people don't care to deal with the technology's complexity.

Moreover, he contended, existing PCs don't offer a broad enough array of useful functions to attractmost office workers, and many office personal computers aren't even being used.

"Office penetration of personal computers is roughly 22 percent," said Scanlon, whose company is a major vendor of IBM-compatible and Unix personal computers. "Penetration may ultimately be 30 percent, at best, with existing computers."

Scanlon said that a new generation of lower-cost computers is needed to reignite business demand. He forecast the emergence of a "$1,500 to $1,800 desk set" by "early in the late 1980s" that will spark a new round of sales of personal computers and boost the entire computer industry.

The machine's new usefulness would rest in its communications orientation that would meld the personal computer and the telephone more effectively into a single unit, he said.

He argued that the lower price and the communications ability of the machine, which would make such services as electronic mail more cost effective, should assure its success. He declined to say whether such a device is under development at AT&T.

"The real question is 'What's wrong here?' " said Scanlon, referring to the computer industry's current woes. "Apparently, [existing personal computers] ain't what's needed today."