Lotus and Dataspeed reached a definitive accord.

The two firms reached agreement on their previously announced plans that call for Lotus Development, the Cambridge, Mass., personal-computer software firm, to acquire San Mateo, Calif.-based Dataspeed for $6.5 million in cash. It is an effort to broaden the product line of Lotus, creator of the landmark 1-2-3 software program. Dataspeed makes radio receivers that display stock quotations on computer screens, a service that could be incorporated into Lotus software. Dataspeed has been losing money, but analysts said its tax-loss carry-forwards make the acquisition virtually free to Lotus.