Data General Corp. today announced today that Edson D. de Castro, the 52-year-old chairman of the troubled computer maker, would resign on Dec. 31. Vice Chairman Herbert J. Richman, 56, will retire at the end of its fiscal year in September. A company statement gave no reasons for the retirements of the two executives, who helped found Data General in 1968. Ronald L. Skates, the company president and chief executive officer, praised de Castro as ``an industry legend.'' He also commended Richman for leading the company's sales and marketing efforts. Skates said Richman deserved much of the credit for Data General's rapid rise in the market for mid-range minicomputers. That market has fallen on hard times. Data General has not enjoyed a profitable year since 1985, when its employment peaked at 17,700. Since then, the company has suffered a string of losses, and employment has shrunk to around 9,500. Analysts say Data General has responded with some strong products, particularly its AViiON workstations. But the minicomputer slump has continued to act as a drag on earnings, hampering the company's repeated efforts to turn a profit. The company lost $139 million in its most recent fiscal year. Revenues were $1.22 billion, compared with $1.31 billion the year before. Neither de Castro nor Richman will seek re-election to the board of directors, but Richman will continue to serve as executive vice president for the AsiaPacific region until he retires, the company said.