A Boston Transportation Depart­ment manager who stands accused of having online discussions about sexual activity with young boys and keeping a cache of illicit images pleaded not guilty Monday to child pornography charges, officials said.

James Mansfield, 47, of Charlestown was ordered held on $20,000 cash bail at his arraign­ment in Charlestown District Court on charges that he possessed more than 100 ­illegal images on his laptop computer and a flash drive, prosecutors said.