To improve instruction in computer engineering and computer science, instructors must better understand how their students learn. Unfortunately, little is known about how students learn the fundamental concepts in computing. To investigate student conceptions and misconceptions about digital logic concepts, the authors conducted a qualitative interview-based study. In the interviews, students verbalized their thinking while they solved digital logic problems. The interviews were analyzed to identify students' misconceptions. This paper presents five classes of students' misconceptions and methodological weaknesses concerning medium-scale integrated (MSI) circuits. These misconceptions were used to create a multiple-choice conceptual assessment, called a concept inventory, that is being used to test for the prevalence of the misconceptions discovered through the interviews. Although the misconceptions documented in the paper are primarily intended for the construction of the concept inventory, they can also point to potential ways to improve instruction. (Contains 7 figures.)