What they do:
Vehicle Interface Modules (VIMs) provide an interface between a vehicle's onboard diagnostics link (e.g. OBD II) and an offboard computing device such as a PC or a Scan Tester. They provide the necessary communications interface between the offboard device and the vehicle to enable monitoring of data available from various subsystems that communicate via the diagnostic link.
Why you need them:
The communications protocols supported by vehicle data link connectors (DLC) are not compatible with various standard computing interfaces such as those typically provided by PCs today. The protocols used are designed for communications between multiple onboard vehicle electronic control units (ECUs) that support a variety of different vehicle applications (e.g., for powertrain, chassis, safety or comfort applications).
The primary function of vehicle interface modules is to enable communications between offboard devices and ECUs via the multiple communications protocols used on vehicles today. These communications protocols are typically based on both industry standards and OEM proprietary definitions which the VIMs are designed to accommodate.
In addition to these basic communications requirements, the vehicle interface modules support dedicated functions such as ECU reprogramming, and unique retrieval of data from the vehicle's "black box" event recording devices.