Electronic Brake Control Module
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, and 2009 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The EBCM monitors the vehicle speed. The PTOM receives the status of the vehicle speed from the EBCM via serial data messages.