An OBD2 Bluetooth interface allows you to connect your smartphone or laptop to your car’s onboard diagnostics system. This connection enables you to read diagnostic trouble codes (DTCs), monitor vehicle performance, and access other valuable data. This article delves into the pseudo-code that governs how an OBD2 Bluetooth interface communicates with a vehicle’s system, specifically focusing on requesting and interpreting trouble codes.
Decoding the RequestTroubleCodes Function
The provided pseudo-code outlines a function called RequestTroubleCodes, responsible for retrieving DTCs from the vehicle. Let’s break down its key components:
Initialization and Protocol Selection
The function begins by initializing variables to store the bytes representing the trouble codes. It then sends a “03” command via the WriteTCP function, initiating the request for DTCs. The code differentiates between communication protocols: Non-CAN (e.g., ISO 9141-2, KWP2000) and CAN (Controller Area Network). The ConnectProtocol variable determines which protocol to use.
Non-CAN Protocol Handling
For Non-CAN protocols, the code enters a loop that reads data byte-by-byte using ReadTCP until all expected bytes for three error codes are received. Each while(!DataRdyTCP()); statement ensures that the code waits for data to be available before reading. This sequential reading retrieves the specific bytes representing each DTC. Notice the code also reads and discards “space” characters, likely delimiters between data bytes.
CAN Protocol Handling
CAN protocol handling is more complex due to its variable-length data packets. The code first checks if the requested count of DTCs is less than 3. If so, it follows a similar byte-by-byte reading process as the Non-CAN protocol. However, if the count is 3 or more, it anticipates a different data format. The code searches for a colon (‘:’) as a delimiter, indicating the start of a new DTC. It reads and discards characters until a colon is encountered. Subsequently, it reads the DTC data, again separated by spaces, until the next colon or the end of the data stream.
Error Code Storage and Final Steps
After successfully reading the DTC bytes, the code stores them in variables (errb0, errb1, etc.) for later processing. Finally, the function waits for an “OK>” response from the vehicle, signifying the completion of the data transmission. This ensures that the request was successfully processed and all data was received.
Understanding ErrorCodePrefix
The ErrorCodePrefix function translates the first byte (b0) of a DTC into a human-readable format. It determines the type of code (P for Powertrain, C for Chassis, B for Body, U for Network) and the specific system within that category (0, 1, 2, or 3). This mapping allows for easier interpretation of the DTCs.
Determining the Protocol: GetProtocol
The GetProtocol function sends an “atdp” command to the vehicle, querying the communication protocol. It analyzes the response string, searching for the sequence “157,” which indicates the use of the CAN protocol. If “157” is found, it returns 1; otherwise, it returns 0, signifying a Non-CAN protocol. This automatic protocol detection ensures compatibility with various vehicle models.
Conclusion
This pseudo-code provides insights into the low-level communication between an OBD2 Bluetooth interface and a vehicle’s diagnostic system. Understanding these underlying processes is crucial for developing and troubleshooting diagnostic applications. The code showcases the complexities of handling different communication protocols and parsing variable-length data packets. By decoding the data retrieved through these functions, users gain valuable information about their vehicle’s health and performance.
