Request: Scan speed improvement trick

Aol, there's a small trick for those of us who use an ELM v1.3 compatible OBD reader (such as the current OBDkey) by which you could improve the scan rate a fair bit.

When you request a pid, instead of sending e.g. "05" you can send "05 1". I've tried this in my car and when I send "05" I get response times from 120 to 160ms (so about 6-8 fps). When I send "05 1" response times drop to 80 to 110 (ie 9 to 12 fps).

Given how little change this should require in the code, I really hope you could implement this.

The technical reason behind it for those interested:
When the OBD interface requests data from the car it doesn't know how many message it will receive as answer from the difference systems in the car or when it will happen. Hence it has a timer included which it waits for to run out. Only when during that time no new message comes in, it returns them to the software (ie RaceChrono). New ELM versions allow to specifiy how many messages to expect so it can skip one wait cycle.

So when you send 05:
<elm sends CAN request>...<wait>...<CAN response 1>...<wait>...<elm sends CAN response to RC>

And when you send 05 1:
<elm sends CAN request>...<wait>...<CAN response 1><elm sends CAN response to RC>



  • aolaol
    edited November 2010
    I did not know this, thank you for pointing out! I'm busy with my house building currently so there will be no new versions short term, but I'll definitely look at this when I'm back working on RC, which is probably spring time.

    Do you know if OBDkey supports this? I think it's the most popular reader. Which reader did you test it on?
  • I tested it with an OBDkey. After sending ATZ it reports itself as version 1.30, which is the minimum required to support this feature. I'm not sure since when they sell this one, but older versions don't support it.
  • Is there a slight chance I could get you to make a special quick&dirty PC or WM2003 built that just always adds the " 1" to PID requests, no matter what the dongle, and release it as "for those who dare" to see what the real world performance difference would be? Or is your version building overhead so big that it's completely out of the question?

    Btw, I was cruising through Finnland, starting from Helsinki, on a trip to the North Cape. I think the only place with even more speed cameras was London city. :D
Sign In or Register to comment.