What limits the refresh rate for OBD-II readers ?

**It's A Honda** · 12-04-2006, 05:31 PM

are you referring to this post ?

wow. is ASCII conversion really that slow ?!
y'know, in the last few days after I received the OBDPro and started getting involved, I have many times wondered - what is really the need for an 'interface' (or rather a 'middle layer') like ELM ? Why not communicate straight from the serial port to the obd port ? If at all any 'interface' is needed for difference in line levels(voltage) it can be scaled up or down, that should be all there needs to be in an interface, no processing. A covered dip switch can serve as a protocol selector if automatic detection is not feasible or to keep things simple. I know I am oversimplifying it but thats how I feel it should be.

Having said that, looking forward to the "BINARY" version of OBDPro soon !!

**digimotojoel** · 12-04-2006, 06:17 PM

I was referring to this post: http://www.mp3car.com/vbulletin/show...2&postcount=18

When it comes down to it an interface is needed simply to keep you from frying your computer. Outside of that each interface is developed differently in how they allow the other end to communicate. ELM was an awesome product years ago when this technology was brand new, when no one understand how to use it and there were no software developers out there writing applications against it. It just hasn't kept up with evolving technologies and has slipped behind in design.

edit: On a side note god_of_cpu stated that StreetDeck pulls around 2.5 times a second against the ELM interface. You can read the post here

**It's A Honda** · 12-04-2006, 06:44 PM

yes I saw that post too in the same thread.
best I have been able to get from OBDPro using wOBD was ~6.5Hz using a timeout of 32ms.
that was at 38.4k. I did not play around much with it at 9.6k so dont have a comparison but I can check for you.

**digimotojoel** · 12-04-2006, 06:57 PM

I have been meaning to purchase one of these units for some time, so I'll probably snag one and see what it can do myself, but if you do get to run some tests anytime soon, please do post results.

**michbound** · 12-04-2006, 07:15 PM

Originally Posted by It's A Honda

Assuming the serial port is set at 38400, it sounds like the greatest limiting factor then is the timeout.
After playing around a bit with the timeout values I found that the '01 MDX ecu responds perfectly down to 32ms and the '99 Accord likes to be at 40ms but works most of the time at 32ms as well. So theoretically you should be able to get a sample rate of about 20-25Hz including the round trip and timeout. Does that mean the interface is the bottleneck ? Unless I am missing something...

Well,

If you were able to get a response at 32 ms then there is'nt a whole lot of overhead beyond that from the interface, try running a simple script and timing the responses, any software in the mix might have overheads that are unknowns so the best option is to go raw to the interface and time it, that will give you your raw response time

Paul

**It's A Honda** · 12-04-2006, 09:08 PM

I do intend on doing that hopefully using hyperterminal unless you have some other suggestions ?

joel, just checked - no difference whatsoever running 9.6k v/s 38.4k. But this is against ISO 9141-2 ecu, so for ecus/protocols with higher baud rate it might be a different ball game altogether.