[Rxtx] Latency Woes

[Jim Redman]

I'm no expert on this, but isn't the underlying ODB-II CAN bus or 
something similar.  They always seem to require a dongle that converts 
to something.

I know you can get serial, bluetooth, USB, etc. converters.  I've seen 
serial dongles that claim multiple baud rates, so I suspect that 
whatever's providing the conversion may significantly affect the 
performance.

A big second on the required timing comment.  I suspect some intelligent 
consideration of what values change quickly (RPM, Speed, maybe things 
like fuel pressure) and read values like fuel levels, coolant temps, 
etc. much less frequently.



On 03/08/2011 06:06 PM, jfh at greenhousepc.com wrote:
> Curtis,
>
> You can't just increase the baud rate and have the device adapt to that
> rate. Embedded systems tend to have their communications parameters
> either hard coded in a memory location or else they are using a hardware
> timebase. You're welcome to try increasing the baud rate, but there is
> no guarantee it will even work.
>
> Whoever designed the ECU probably selected the baud rate for a good
> reason. My suggestion would be to look at how the sensor values are all
> laid out and go from there. Also, look at what the sensor represent and
> consider the physical properties of what is being measured. It's
> extremely unlikely that, for example, the coolant temperature is going
> to rise more than 1* per second. If that much. Some values might change
> more often, but do they really matter on a sub-second measurement basis?
>
> Part of software =engineering= is understanding the problem. Anyone can
> sling code in the hope it works.
> --
> Julie Haugh
> Senior Design Engineer
> greenHouse Computers, LLC // jfh at greenhousepc.com
> <http://greenhousepc.com> // greenHousePC on Skype
>
>
>     -------- Original Message --------
>     Subject: Re: [Rxtx] Latency Woes
>     From: Curtis Hacker <hakcenter at gmail.com <mailto:hakcenter at gmail.com>>
>     Date: Tue, March 08, 2011 5:26 pm
>     To: rxtx at qbang.org <mailto:rxtx at qbang.org>
>
>     So if I could get the baud up
>     15625, theoretical maximum latency is 1.92ms ? Basically 500
>     samples/sec ?
>     125000, theoretical maximum latency is 0.24ms ? Basically 4167
>     samples/sec ?
>
>     Keeping the protocol the same.
>
>     On 03/08/11 15:05, jfh at greenhousepc.com wrote:
>>     Curtis,
>>
>>     It's nowhere near that bad. I'd measured it once before, and I
>>     believe it's sub-millisecond.
>>     --
>>     Julie Haugh
>>     Senior Design Engineer
>>     greenHouse Computers, LLC // jfh at greenhousepc.com
>>     <http://greenhousepc.com> // greenHousePC on Skype
>>
>>
>>         -------- Original Message --------
>>         Subject: Re: [Rxtx] Latency Woes
>>         From: Curtis Hacker <hakcenter at gmail.com
>>         <mailto:hakcenter at gmail.com>>
>>         Date: Tue, March 08, 2011 3:27 pm
>>         To: rxtx at qbang.org <mailto:rxtx at qbang.org>
>>
>>         Thank you guys, cause I was really worried there. I guess
>>         another win
>>         for math today hahaha. So what is the imposed latency that
>>         rxtx has ?
>>         maybe 5-10ms ?
>>
>>         On 03/08/11 13:14, Bob Jacobsen wrote:
>>         > On Mar 8, 2011, at 12:33 PM, Curtis Hacker wrote:
>>         >> Tunerpro log : http://www.ds-map.net/tunerpro_log.csv
>>         > It's only reading one sensor per line.
>>         >
>>         > Note that e.g. Boost PSI changed in lines 6, 30 and 54; every
>>         24 lines. IAT changed in lines 16 and 40; every 24 lines but
>>         at a different phase. GramsRev in lines 23 and 47; every 24
>>         lines, at yet another phase.
>>         >
>>         > Looks like (69-1) readings in 1.30 seconds = 19msec per
>>         reading. Arithmetic still works!
>>         >
>>         > Bob
>>         > --
>>         > Bob Jacobsen, LBNL
>>         > Bob_Jacobsen at lbl.gov <mailto:Bob_Jacobsen at lbl.gov>
>>         +1-510-486-7355 fax +1-510-643-8497 AIM, Skype JacobsenRG
>>         >
>>         >
>>         >
>>         >
>>         >
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-- 
Jim Redman
(505) 662 5156 x85
http://www.ergotech.com