Penril Corp., the Rockville-based high-technology company, yesterday announced that it has acquired Alltest Inc., a test equipment maker, for $3 million in cash.

Alltest, located in the Chicago suburb of Palatine, Ill., will become a division of a Penril subsidiary, Triplett Electrical Instrument Corp., Penril President Kenneth M. Miller said. Rod Georgiu, Alltest's founder, will continue to manage the business.

Triplett makes a broad line of test instruments, including telephone testers sold to American Telephone & Telegraph Co., while Alltest specializes in microprocessor-controlled test instruments used to service and diagnose troubles in autos.

Georgiu said that the proliferation of electronically controlled systems in the automotive market provides Alltest "an opportunity for excellent growth." Alltest reported sales of about $6 million for its fiscal 1984, ended Sept. 30, and pretax earnings of $1 million.

Stanley Lanzet, an analyst with Drexel Burnham Lambert Inc., said the acquisition "will be positive in terms of per-share earnings and fit in with what he's Miller's got already. It won't dilute per-share earnings because he's very careful about that."

However, David L. Beeghly, an analyst with Offutt & Taylor Inc., said, "I don't think the electronic test equipment area is so great" because sales tend to be hurt during recessions. "They also were looking for something an acquisition in the data communications area. That would be a plus," he said.

The acquisition of Alltest is Penril's 15th since 1973. The firm is looking to buy another company in the high-technology electronics area by the end of this year, Miller said.

Penril, incorporated in 1968, has added a number of electronics products to its original line of modems -- devices that let computers communicate over phone lines. It owns Epicure Products Inc. and Concord Systems Inc., which make stereo speakers and stereo-radio receivers and produces a variety of communications and scientific test instruments.

In 1983, it opened a highly automated data communications plant in Gaithersburg. The firm now employs 1,260, including 13 at its corporate headquarters in Rockville, and has plants in Connecticut, Massachusetts, New York, Ohio, California, Mexico, Switzerland and Japan as well as Gaithersburg.

Analysts say the firm's two previous acquisitions -- Concord, acquired in January 1984, and Triplett, bought in October 1983 -- helped push profits and revenue to record highs in fiscal 1984, ended July 31.

For that year, the firm reported profits of $2.47 million ($1.18 a share), up 71 percent from $1.45 million (72 cents) in fiscal 1983. Revenue was $65.7 million, up 70 percent from $38.72 million the previous year.

Penril recently reported record revenue of $40.18 million and earnings of $1.57 million (73 cents) for the first half of fiscal 1985, compared with revenue of $26.63 million and earnings of $1.06 million (52 cents) in the same period a year earlier. Earnings for fiscal 1985 are expected to be between $1.40 and $1.50 a share, analysts said.

Penril, traded as "PNL" on the American Stock Exchange, closed yesterday at 12 7/8, down 1/8.