How are Dragon Ball and Dr. Slump connected?

Dr. Slump, Toriyama's first hit, ran in Weekly Shonen Jump from January 1980 to August 1984. Dragon Ball began about three months after it ended. Successors by the same hand usually stay in separate worlds; these two do not. In the General Blue arc, manga chapter 81, "Chased to Penguin Village!" (collected in volume 7, running through chapter 83), Goku follows General Blue's trail straight into Penguin Village.

The village's residents appear in person, in their end-of-Dr.-Slump state: Senbei married to Midori, Turbo, Obotchaman. And Arale Norimaki, the robot girl hero of the earlier series, simply takes the fight over, headbutting General Blue away and outclassing Goku so completely that the visit becomes one of his reasons to train harder. The crossover is written into the official chronology too: the Daizenshuu 7 guide dates Arale's creation to Age 745 in Dragon Ball's own calendar.

The door has stayed open. Arale returned in Dragon Ball Super episode 69 (2016), and Penguin Village's cast has appeared as playable fighters in Dragon Ball games, including Budokai Tenkaichi 3. Two series, one map: Toriyama just never bothered to wall them off.