Diaclone was a toyline by Takara Toys launched in 1980. It consisted of transforming vehicles and robots piloted by miniature, magnet-shoed figures spun off from the prior Microman toy line that were in turn called an Inch-Man.
The toys in the 1980 line were designed by future Macross designers Shoji Kawamori and Kazutaka Miyatake (both contracted from Studio Nue), who designed the mecha and the figures respectively. Unlike Microman, which featured "full-scale" toys of its 10 centimeter tall alien cyborgs, the figures in Diaclone represented full-sized human (and enemy alien Waruder) pilots, and were in approximately 1/60th scale.
In 1982, the line later featured the "Car Robots" set of transforming robot toys. These designs were invented by Koujin Ohno with some initial designs by Kawamori and others. While the original series featured fanciful robots and vehicles, Car-Robots added the feature of the robots being able to disguise themselves as various late 20th century-era contemporary vehicles. In 1984, Hasbro licensed the Diaclone Car-Robots toyline along with the Microman "Micro Change" toyline from Takara and merged the two series of toys to create the Transformers. Most of the original Autobot vehicle-based characters came from the Car-Robots set of Diaclone robots. Other Transformers characters that came from the Diaclone line included the Dinobots, Insecticons (from the enemy Waruder toys), the Decepticon planes (originally from 2 "JetRobo" toys, produced in the colors of future Decepticons Starscream and Thundercracker) and the Constructicons, who also came from the Car-Robots set. The Constructicons came from near the end of the series, at which point Takara was starting to abandon the Inch-Man pilot figures and being limited to the 1/60th scale. The 6 TrainRobo were also produced in the same sub-line as the Constructicons, but would only become Transformers (as the Trainbots) in Japan's 1987 line.
Takara eventually discontinued the Diaclone and Microman toylines in 1985 in favor of the more successful Transformers rebranding. Hasbro company of the U.S. branded the toys and gave each one a story line, as well as supporting comic and T.V. series.
Ref: Furman, Simon (2004) and Diaclone.net