Serial Communications

Scan tool operation is accomplished with bi-directional communication between the tool and the PCM. OBD II technology requires a much more sophisticated PCM than did OBD I technology. The PCM diagnostic management system not only monitors systems and components that impact emissions, but also runs active tests on these systems and components. To accommodate this expansion a Class II serial data bus is used which meets SAE J1850 specifications for serial data.

"Serial Data" refers to digital information which is transferred over a single line, one bit at a time. A "Data Bus" is an electronic pathway through which digital data travels.

Class II data, which is used on OBD II vehicles, communicates at a much faster rate. The rest voltage on a Class II bus is 0 volts and is switched high (seven volts) when communications are taking place.

OBD II requirements standardize the Data Link Connector (DLC) to use 16 pins. (All vehicle manufacturers must conform to the 16 pin DLC standard.)