UNC Resources lost $2.5 million (20 cents per share) for this year's second quarter, one-third the net loss of $6.4 million (57 cents) in the same period last year.

Other area firms reporting included Syscon Corp., Computer Data Systems Inc. and Richmond, Fredericksburg & Potomac Railroad Co.

UNC Resources also posted a net loss of $1.1 million (9 cents) for the first half, compared with $2.9 million (26 cents) loss for the first six months of 1982.

Officials of the Falls Church-based company said the market for machine tools and offshore oil boats remains slow and losses from these divisions outweighed profits from its sales of aerospace components and minerals contract drilling services.

Syscon Corp., which develops and markets computer software systems and other computer products, posted increased earnings for both the second quarter and first six months of 1983.

Syscon said second-quarter profits were $789,000 (29 cents), up from $631,000 (24 cents), while first-half earnings climbed to $1.5 million (55 cents), compared with $1.1 million (40 cents) last year.

Computer Data Systems says net earnings nearly doubled from last year, rising to $2.1 million for the fiscal year ending June 30, 1983, compared with $1.2 million (42 cents). Computer Data Systems, based in Bethesda, is a consulting and data processing services company.

Richmond, Fredericksburg & Potomac Railroad profits rose modestly during the second quarter to $4.6 million ($12.68), compared with $4 million ($11.22) for the same period last year. For the first six months, the company earned $7.9 million ($21.82), up slightly up from last year's $7.8 million ($21.61).