Understanding how to access the Controller Area Network (CAN bus) through an OBD2 port is crucial for effective vehicle diagnostics and data analysis. However, the way manufacturers implement this access can vary significantly, leading to different experiences when trying to retrieve data. This article explores the common methods employed by vehicle manufacturers to provide CAN bus access via the OBD-II diagnostic port.
One key factor is the utilization of “Vendor Option” pins within the OBD-II port. Manufacturers have the flexibility to connect these pins directly to either the Medium Speed CAN Bus or the Low Speed CAN Bus. This direct connection allows for real-time monitoring of message traffic on these specific bus lines. In such configurations, accessing data is straightforward as the OBD-II port essentially becomes a direct tap into the CAN bus network.
However, a gateway architecture is frequently employed, introducing a layer of complexity. A prevalent approach, Option A, involves connecting the OBD-II port directly to the High Speed CAN bus. In this setup, the port can readily monitor traffic along the High Speed CAN bus. To access data residing on the Medium Speed CAN bus, a diagnostic Remote Frame, adhering to CAN Specification 2.0, must be transmitted on the High Speed bus. This Remote Frame is specifically addressed to a gateway node, typically the Body Control Module (BCM) or a similar control unit. Upon receiving this request, the gateway node then generates a secondary Remote Frame on the Medium Speed CAN bus, targeting the desired node on that bus. The target node responds by transmitting the requested data back onto the Medium Speed CAN bus. The gateway node captures this data and, when the High Speed bus is available, transmits it back across the High Speed CAN bus, making it accessible via the OBD-II port.
Alternatively, Option B presents a less common but distinct method. In this configuration, the OBD-II port might be connected to a dedicated gateway. This gateway operates selectively, only delivering data from a specific CAN bus upon receiving a diagnostic request through the OBD-II port. Without an explicit diagnostic request, no discernible traffic will be observed at the OBD-II port. Therefore, initiating communication and data retrieval necessitates sending a diagnostic remote frame request message to prompt the gateway to provide data from a particular node on a specific CAN bus.
In conclusion, accessing CAN bus data through Multiple Obd2 Ports is not a uniform process. It is manufacturer-dependent, varying from direct pin connections for specific CAN buses to gateway-mediated access, especially for accessing Medium and Low Speed CAN buses when the OBD-II port is primarily connected to the High Speed CAN bus. Understanding these different implementation approaches is essential for effectively utilizing OBD-II ports for comprehensive vehicle diagnostics and data acquisition.
