When diving into the world of OBD2 diagnostics and performance monitoring, you might encounter terms that seem a bit cryptic at first. One such term is ATST, especially when followed by FF – often seen as ATSTFF. As an automotive enthusiast or someone keen on understanding their vehicle’s data, grasping what ATSTFF means and how it impacts your OBD2 scan tool’s performance is crucial.
In essence, ATST stands for Adapter Set Timeout. This command dictates the maximum duration your OBD2 adapter waits for responses from your car’s electronic control units (ECUs) after sending a diagnostic request. Think of it as a listening period. After your scan tool requests information (like engine RPM or coolant temperature), the ATST setting determines how long the adapter will listen on the car’s communication network (the bus) for the ECUs to respond.
By default, OBD2 adapters, particularly those built around the popular ELM327 chip, have a preset timeout value. This default setting is designed to work in a broad range of vehicles. However, sometimes, tweaking this timeout can significantly improve the efficiency and speed of data retrieval.
Delving Deeper into ATST and Timeout
To understand why adjusting ATST matters, let’s consider a scenario. When your scan tool sends a request for a specific Parameter ID (PID), like engine RPM, it’s common for multiple ECUs in your car to potentially respond. Your engine control unit (ECU) is likely to respond, but other modules might also provide related data. The OBD2 adapter needs to know how long to wait to capture all relevant responses before sending the collected data back to your scan tool or application (like a performance monitoring app).
Imagine asking a question in a room full of people. ATST is like setting a time limit for how long you’ll wait to hear all the answers before moving on. If the timeout is too short, you might miss some responses. If it’s too long, you might be waiting unnecessarily, slowing down the overall process.
The ATST command, as defined by ELM327 documentation, allows you to adjust this timeout in increments of 4 milliseconds. The standard setting for the ST timer is 32, which translates to approximately 200 milliseconds. Interestingly, setting ATST to 00 doesn’t mean zero timeout. Instead, ATST00 cleverly resets the timer back to its default value. This reset can be a useful troubleshooting step if you’ve been experimenting with timeout settings and want to return to a known good configuration.
Adaptive Timing vs. Optimized Timing: The Role of ATSTFF
Now, let’s bring ATSTFF into the picture. The “FF” in ATSTFF represents the hexadecimal value for 255, which is the maximum possible value for the ST timer setting. Setting ATSTFF essentially tells the adapter to use the maximum timeout period. Why would you want to do this? This is where the concept of “Optimized Timing” comes in, often contrasted with “Adaptive Timing.”
Adaptive Timing: Letting the Adapter Decide
In Adaptive Timing mode, the OBD2 adapter itself dynamically adjusts the timeout based on observed response times from the vehicle. When Adaptive Timing is enabled, the ATST setting, even if you set it manually, acts as the maximum allowed timeout. The adapter might use a shorter timeout if it determines that responses are consistently coming back quickly. In most cases, especially when using Adaptive Timing, it’s recommended to leave the ATST setting at its default value, allowing the adapter’s internal algorithms to manage the timeout optimization.
Optimized Timing: Taking Control with ATSTFF
Optimized Timing, on the other hand, puts the timeout optimization control in the hands of the application or scan tool software, like the LapTimer app mentioned earlier. When “Optimized Timing” is activated, the software typically commands the adapter to use ATSTFF, setting the timeout to the maximum.
Here’s how Optimized Timing with ATSTFF works:
- Maximum Timeout (ATSTFF): The application initially sets the adapter to ATSTFF, maximizing the listening time for the very first PID request.
- Response Mapping: For this initial request, the application waits and records the number of responses received from different ECUs. It learns how many ECUs typically respond to a particular PID request (e.g., RPM might be reported by one ECU, while coolant temperature by two).
- Targeted Requests: For subsequent requests for the same PID, the application now knows the expected number of responses. It sends a request to the adapter, instructing it to return data immediately once the expected number of responses has been received.
This “Optimized” approach, using ATSTFF initially for negotiation and then adapting based on the expected number of responses, can significantly reduce unnecessary waiting time and potentially lead to higher data update rates. By not relying on a fixed timeout, and instead waiting only for the expected responses, the system becomes more efficient.
Downsides and Considerations
While “Optimized Timing” with ATSTFF can be advantageous, it’s not universally perfect. The original article correctly points out that this approach isn’t well-suited for every car’s communication bus, especially older ones. Some older vehicle networks might not handle these rapid, targeted requests as efficiently, potentially leading to missed data or communication errors.
Therefore, the best approach often involves experimentation. If your scan tool software or app provides options for “Adaptive” and “Optimized” timing, try benchmarking the data update rates in both modes. See which setting yields faster and more reliable data for your specific vehicle.
Furthermore, you might encounter a “Reply Timeout” setting in your scan tool software. This setting is often related to ATST and can influence the overall communication behavior. Experimenting with slightly lower Reply Timeout values (like 0.2 or 0.3 seconds instead of a default of 1.02 seconds) might be beneficial, especially with “Optimized Timing,” to avoid unnecessary delays while still ensuring responses are captured. However, be cautious not to set it too low, as you might risk missing responses, particularly during the initial negotiation phase with ATSTFF.
The Reality of Bus Speed
Finally, it’s important to acknowledge the limitations of the vehicle’s communication bus itself. As the original author mentions, older ISO 14230 (Keyword Protocol 2000) buses are inherently slower. Even with optimized timeout settings, you might be limited to a data update rate of only 2-3 Hz (Hertz), which translates to roughly 10-15 PIDs per second. This is a constraint of the underlying communication technology, not necessarily the timeout settings.
Conclusion
Understanding ATSTFF and OBD2 timeout settings is a valuable step in maximizing the performance of your scan tool and getting the most out of your vehicle’s diagnostic data. By grasping the difference between “Adaptive Timing” and “Optimized Timing,” and experimenting with these settings in conjunction with your scan tool software, you can fine-tune your OBD2 communication for optimal speed and reliability. While ATSTFF and “Optimized Timing” can offer potential improvements, remember to consider your vehicle’s age and communication bus type, and always test and benchmark to find the best configuration for your specific needs.
