Openness for open innovation is becoming increasingly important in the field of embedded systems (ES). ES are traditionally designed for a particular purpose, however most ES firms are beginning to open up their systems. This allows third parties to enhance them with additional software, or hardware components. It is not enough to just provide access to a system though, it also requires that it be modularized accordingly. A specific challenge when modularizing for open innovation, is the consideration of ES characteristics such as safety, security and or real-time requirements. In this multiple case study, our goal is to explore how ES producers can technically open their systems for open innovation. To address this question, this paper first systematizes the different forms of openness in ES. Secondly, it uses modularity theory to analyze the implications of the specific requirements of ES on the encountered forms of openness. It turns out that, different types of openness require differing levels of effort, with regard to modularization. Overall, the study shows that, in order to guarantee the proper functioning of their systems, ES firms tend to partition their systems in accordance to the specific requirements of ES.