The letters actually refer to the connector shell size and are really independing of the number of pins. For example, a DB shell could have fewer than 25 pins. Some have two or even three heavy pins (one for power, 2 for return) and some signal pins. The reason the the 9-pin is designated DE is because it was developed after A, B, C, and D, sizes.
As we unveil EE Times’ 2015 Silicon 60 list, journalist & Silicon 60 researcher Peter Clarke hosts a conversation on startups in the electronics industry. Panelists Dan Armbrust (investment firm Silicon Catalyst), Andrew Kau (venture capital firm Walden International), and Stan Boland (successful serial entrepreneur, former CEO of Neul, Icera) join in the live debate.