Thread: Speed on ODBII reading -> output

Readout speed depends on
a) the protocol used
b) how many ecu's respond to that protocol
Obviously a Iso 9141-2 protocol running at 10.400 baud is a lot slower than a Can protocol running at 500K baud.
Also the timeouts between messages are greater when using Iso protocols.
For instance it is very normal for a car to wait 25 - 30 mili seconds after receiving the checksum of a command, before sending the answer. The second ecu will even wait a little bit longer, say about 30 - 35 mili seconds.
Then the scantool has to wait for 55 miliseconds after receiving the checksum of the last message from the last ecu, before it can send another command.
These delays will slow down the transfer rate significantly.
Bmw's normally have 1 ecu for the engine, except some 12 cylinders, who have 3 ecu's for the engine, which all respond. When you have an automatic gearbox, that ecu will also respond.

Thank you for a detailed answer!

Then it's clear for me, that I do not need any ODBII devices/programs... Atleast not for the moment

Originally Posted by Vitaliy

What would you say be an acceptable refresh rate (in samples per second)?

With some tweaking, it is possible to get reasonably fast (~10 samples/sec), even on ISO.

Vitaliy

10 samples / sec sound fast enough, but I realy do not have a clue

For me, it should be equal to stock, so that I could mount a LCD infront of stock dashgauges and use my custom screen...

This way I could go with a big RPM and tiny speed while driving active, and vice versa while driving normal.
And active road assistance while driving to a GPS location.

10 samples per second for RPM is too slow, at least in my opinion.
It'll do for 3rd and higher gears, but not in 1st and 2nd, there you will hit the limiter.
Besides, 10 samples is when you only read 1 (ONE) item at a time.
When you need both Rpm and Vehicle speed you will get 5 to 6 samples a second.

wonder of what rate the stock system would report... if even possible to compare them

Refresh rate on stock systems depends on the protocol used and the baudrate.
Exanple 1: Bmw diesel at 250 baud. Internal there are 2 ecu's. So, first ecu 1 asks ecu 2 some things. Then ecu 1 responds with a 40 mili delay between each character.
Then the scanner has to echo each character. At 250 baud 1 character takes 40 mili.
Reading codes and some data (8 items) takes up to 10 seconds.
Example 2: Opel Iso-81 and Iso-82 protocols at 9600 baud.
Scanner requests data and ecu repeatedly sends everything at full speed.
Packets are 40 - 80 bytes, which gives you 20 - 10 packets every second.
But hey, stock system means only 1 ecu responds, not maximum 8 as specified in OBD2 and EOBD (Sae J1979).