Doremus & Co. has named a longtime Washington public relations representative to the new post of president of Doremus Public Relations, giving that unit a separate identity. Richard H. Truitt, formerly head of Carl Byoir & Associates' Washington offices and a former Chicago Tribune business reporter, will supervise the company's corporate public relations group and its marketing and promotion division, in addition to managing its Washington office and five other regional offices.

Carl Byoir & Associates will not replace Truitt as manager of its Washington office, but will spread his responsibilities among senior executives in New York, a spokesman said. The public relations firm also announced that Philip C. Armstrong, previously deputy assistant secretary of Education for public affairs, was named an account executive for The Road Information Program, a national highway research project.

Dynatech Data Systems said that Jesse F. Lancaster is stepping down as president after 28 years with the Springfield data communications firm. Lancaster, who rose through the company's ranks as a marketing specialist, is credited with overseeing the firm's transition from a communications switching equipment manufacturer to a producer of advanced data communications systems. Claude Olier, formerly managing director of Dynatech's European data communications operations, will replace Lancaster. Lancaster will continue as a consultant to Dynatech and assist in final phases of his pet projects. Dynatech Data Systems is one of four independently operating Washington area companies owned by the Burlington, Mass.-based Dynatech Corp.