ELM vs PocketLogger

Erorus · 05-09-2005, 02:36 PM

I have an ISO OBD scanner from scantool.net.. but I recently heard about a different scan tool, for Palm OS, called PocketLogger, which can scan a lot faster. (4-6 samples per second to up to 20 samples per second with my car.)

I don't have a Palm and I want to run it on the car PC.. so what's the deal with this tool? Why can't the ELM scan that fast? What's the PocketLogger's trick?

shotgunefx · 05-09-2005, 03:59 PM

Quote: Originally Posted by Erorus

I have an ISO OBD scanner from scantool.net.. but I recently heard about a different scan tool, for Palm OS, called PocketLogger, which can scan a lot faster. (4-6 samples per second to up to 20 samples per second with my car.)

I don't have a Palm and I want to run it on the car PC.. so what's the deal with this tool? Why can't the ELM scan that fast? What's the PocketLogger's trick?

I don't see how it can. I might be way off, but I thought the scan rate to a point depending on the protocol. Our ISO cars being slow as ***** and VPW and the other offering close to 20.
That being said, you can I think up the rate on the ELM based scantool. How well it works varies. I haven't gotten around to trying it yet. I'm putting in the pc this week so I'll try tweaking it and report the results.

shotgunefx · 05-09-2005, 04:07 PM

Found something interesting in the elmscan forum.

Quote:

If you use ELM322 and ELM320 based interfaces with ScanTool.net software v1.06, you will get a maximum refresh rate of about 4 samples per second. ELM323 v2.0 gives you about 2.5 samples per second.

The data rate for J1850 VPW and ISO9141-2 is 10.4kbps, and four times as high for J1850 PWM (44kbps). The maximum message length is limited to 12 bytes (for both requests and responses), which means that theoretically, you can get up to 10400 / ((12+12)*8) = 54 samples per second!

However, there are two things that must be considered.

First, you're not supposed to monopolize the bus, because it is used for communication between the ECUs. In practice, you won't be able to get 54Hz even with the fastest microcontroller, because diagnostic messages have the lowest priority, and will be blocked out by messages with higher priorities.

Second, SAE J1978 requires the scan tool to wait 300ms between sending requests, to (a) allow more important communication to take place, and (b) allow the ECU to have enough time to process the information. This requirement effectively limits the refresh rate to 3 samples/second.

So ELM32x-based scan tools working at 2.4Hz or 4Hz already approach, or even surpass the maximum allowed refresh rate. Higher data rates are possible, but are not exactly legal.

Not sure how correct this is, but it does make sense if it is.

Erorus · 05-09-2005, 04:12 PM

Thanks for the info and clarification.. makes sense to me, too. Ah well.

shotgunefx · 05-09-2005, 05:18 PM

Quote: Originally Posted by Erorus

Thanks for the info and clarification.. makes sense to me, too. Ah well.

I'm working on a digital dash, at first I'm using OBDII, but I'm going to use a pic to read the tach and speedo directly later. Just 4 pulses per rotation. Pretty easy. It's a big, big help if you get the service/electrical manuals for the car. I found most of them on ebay pretty cheap.

Just a thought.

turbo2ltr · 05-09-2005, 05:33 PM

Some rules are made to be broken.

Regards,
Mike Montalvo
digital tuning, inc.
www.pocketlogger.com

Erorus · 05-09-2005, 10:18 PM

Want to break the rules for those of us with PCs?

shotgunefx · 05-10-2005, 01:50 AM

Quote: Originally Posted by Erorus

Thanks for the info and clarification.. makes sense to me, too. Ah well.

Have you checked out the AT commands for the ELM323? Some hints in there that might help you speed it up or get more info. In particular the AT MA command. I'll have to give that one a try.

Erorus · 05-10-2005, 08:19 AM

Yeah I saw that but was having a hard time parsing it out. Plus idunno how much speed it would add, maybe 1-1.5 more samples a sec at max.

shotgunefx · 05-10-2005, 04:24 PM

Quote: Originally Posted by Erorus

Yeah I saw that but was having a hard time parsing it out. Plus idunno how much speed it would add, maybe 1-1.5 more samples a sec at max.

I think the elm is just a PIC, isn't it? Too bad the 300ms delay isn't configurable.