OBDII Checksum?

**chunkyks** · 09-29-2010, 10:40 PM

I received this email today, and I'm wondering if anyone would be able to clarify for me what the deal with the checksums is, as it would be output by an ELM327:

I have been using the OBDsim with great success when evaluating different OBD software packages. But when I came across the TouchScan program I realized that the OBDsim didn't output the checksum when the headers was enabled (ATH1 command; the ELM327 do in fact output the check sum), as I was missing this byte the TouchScan program reported an error and was unable to connect to the OBDsim.

How is a checksum calculated? I cannot see enough information in the specs to fully calculate it. How does it change for each of the protocols?

Thanks,
Gary (-;

**Curiosity** · 09-30-2010, 11:28 AM

The ELM327 does all the header and checksum work internally so you don't have to. That's kind of the main reason for it, along with dealing with different protocols.

The H1 command enables displaying all the extra data that comes from the ECU. The last byte is the checksum. That value is calculated from the data payload (excluding the header) by adding all the bytes up. It's just 8 bits, so it rolls over each time it goes over 255. It's pretty simple stuff.