Penril Corp. of Rockville is negotiating to acquire Syntech Corp., its neighbor and rival in the manufacturing of computer communication gear.

Penril President Kenneth M. Miller refused to confirm reports from Syntech employes that he is negotiating to acquire the firm. "Our policy is not to comment on negotiations until we have an agreement," he said.

Syntech is now privately owned. About half its stock is in the hands of Sentry Insurance Co. of Stevens Point, Wis., and another large block is owned by The Illinois Co., a Chicago investment firm.

A group of Syntech employes yesterday wrote the owners urging them not to sell the firm and disclosed some details of the talks to The Washington Post.

Penril and Sentry officials are to meet Monday, the dissident employes said. Miller indicated it would be another week or two before he could discuss the matter.

Penril reportedly has offered to pay about $1.1 million for Syntech, an amount equal to its owners' present equity in the business.

If the talks are successful, Syntech will become Penril's eighth acquisition in eight years. Purchasing additional lines of business has built the Randolph Road firm into a $33-million operation since it was founded in 1968; Penril reported profits of $1.7 million last year.

Last year Penril bought a product line called Variac from GenRad Corp. in Massachusetts and moved it to a Penril plant in Connecticut. It also acquired and shifted to Rockville United Technology's production of modems, devices which connect computers.

Syntech's business also is the production of modems, employes said. While the two firms' product lines overlap, Syntech has two products not currently offered by Penril, which did about $12 million worth of business in data communications last year.

Syntech had sales of $4.3 million last year and reportedly made more than $500,000 profit before taxes. The company's management was shaken up several months ago and its president, Graham Holland, was fired. Holland now works for Penril as vice president and general manager of its data communications division.