One OBD II requirement is that the vehicle contains a standard serial data communications link to a scan tool. Communication protocols use changing voltage signals to communicate with the scan tool and other modules. The difference between communications protocols is the digital voltage signals generated by the ECM.
At this time, there are five communications protocols used on OBD II systems, two defined by the Society of Automotive Engineers (SAE) and three defined by the International Standards Organization (ISO):
The scope pattern below was captured from a 2005 INFINITI. The CAN "C" Hi signal transmits data from modules other than the powertrain modules through pins 6 & 14. The ISO 9141 "K" line communicates serial data from the ECM to the scan tool through DLC pin 7.
Note: Because of the three-year phase-in, both CAN "C" and ISO 9141 protocols can be on the same vehicle. High-Speed CAN "C" was required to transmit data through DLC pins 6 & 14, while ISO 9141 transmitted data on the "K" line through pin 7.
2. A Diagnostic Link Connector (DLC) was added to the vehicle to connect modules to the scan tool. It must be under the dash between the driver's door and the center console. Unfortunately, not all manufacturers comply with this requirement, so sometimes they will be outside this area.
OBD II requires that four pins be standardized on all 1996 and newer vehicles:
Pin 2 represents Global OBD II communications called SAE J1850. It is a bi-directional line, so the scan tool can send a request to the PCM and receive responses from the PCM.
Pin 7 represents the ISO 9141 or 14230 communications line. It is a bi-directional line, so the scan tool can send a request to the ECM and receive responses from the ECM.