When diving into DIY car diagnostics or creating your own vehicle interface using Arduino, understanding OBD2 protocols is crucial. Specifically, the SAE J1850 PWM (Pulse Width Modulation) protocol is one you might encounter, particularly in older Ford vehicles. Let’s break down what this protocol is and how it relates to your Arduino projects.
OBD2, or On-Board Diagnostics II, is a standardized system used in vehicles to monitor and report on various parameters. Different communication protocols are used within OBD2, and J1850 PWM is one of them. This protocol, common in many Ford models and some GM vehicles of a certain era, utilizes a variable pulse width to transmit data. Unlike some other OBD2 protocols, J1850 PWM operates on a two-wire system and is characterized by its specific voltage levels and timing.
One of the key concerns when interfacing with OBD2 protocols, especially with microcontrollers like Arduino, is voltage compatibility. The J1850 PWM protocol, while part of the OBD2 standard, has its own voltage specifications. It’s important to note that the signal voltage levels for J1850 PWM may differ from the typical 5V logic levels that Arduino operates on.
According to specifications, J1850 PWM high signal voltage level can reach up to +7V or even +8V maximum, while the Arduino typically operates at 5V. This difference in voltage levels is a valid concern. Directly connecting an Arduino to a J1850 PWM line without proper voltage level shifting could potentially damage the Arduino.
While some serial interfaces might tolerate slightly higher voltages, consistently exposing your Arduino to 7V or 8V could lead to issues. Therefore, when working with J1850 PWM and Arduino, it’s advisable to use a voltage level converter or a specifically designed OBD2 interface module that can handle the voltage differences and safely translate the J1850 PWM signals to 5V logic for your Arduino.
Beyond J1850 PWM, other OBD2 protocols like VPW (Variable Pulse Width), ISO 9141-2, and ISO 14230 also have their own voltage characteristics. VPW, similar to PWM, also has higher voltage levels. ISO protocols can go even higher, reaching up to 13.5V. In contrast, CAN (Controller Area Network), another common OBD2 protocol, operates at lower voltage levels, making it generally more directly compatible with 5V microcontrollers in terms of voltage.
For DIY projects aiming to support multiple vehicles and OBD2 protocols, considering these voltage differences is essential. A versatile OBD2 interface should ideally be able to handle various protocols and their respective voltage levels safely. For a DIY scangauge project using Arduino that you plan to share with others, ensuring compatibility and safety across different OBD2 protocols, including J1850 PWM, is a crucial aspect to address. This might involve incorporating voltage level shifting circuits or utilizing specialized OBD2 interface chips that are designed to handle the voltage ranges of different protocols.
In conclusion, when working with Arduino and OBD2 J1850 PWM, be mindful of the voltage levels. Using appropriate interface circuitry to manage voltage differences is vital for protecting your Arduino and ensuring reliable communication with the vehicle’s diagnostic system. Understanding these nuances of OBD2 protocols is key to successful DIY automotive electronics projects.
