ASCII stupid question, get a stupid ANSI

It's A Honda · 12-11-2006, 10:36 AM

In OBD2, does the vehicle and the interface talk to each other in binary or ascii ?

I was just wondering since it seems like the interface and PC usually talk to each other in ASCII. A typical command/request sent to the interface and the response back from are like those shown in the attached pic.

As you can see the command "0111" is actually 4 ascii bytes, instead of two binary bytes and the response is 11 ascii bytes which could possibly be sent as 3 or 4 byte binary. But thats PC<->Interface. What happens between Vehicle<->Interface ?

PPE · 12-11-2006, 10:50 AM

The OBD-II communication between the vehicle and the OBD-II interface is always binary communication. The ELM interface is the only one that I have seen so far that uses an ASCII protocol to talk to the PC. It just basically converts the ASCII to binary before sending the message on the OBD-II bus. And when it receives an OBD-II message, it converts it to ASCII and sends it up to the PC.

-Brian
www.palmerperformance.com

It's A Honda · 12-11-2006, 03:22 PM

Thanks Brian. Strange why elm would do such a thing ? Seems like a waste of valuable processing time and bandwidth - especially when running at barely 4Mhz!

digimotojoel · 12-11-2006, 03:34 PM

Odds are had they not done it, they would not have been one of the more popular chips out there. It made it possible to use the interface without any retail software at all (through windows hyperterminal) and it also made it pretty easy to understand when new developers wanted to start writing apps against it. It makes far less sense that they built the ELM327 this way, but you want your product to be backwards compatible as well.