Diagnostics Messages vs. Normal Message

Posted On November 19, 2009 at 9:42 pm by / No Comments / CAN BUS

When I speak to most people about the vehicle network or CAN BUS there is a common misconception that there is only Diagnostic Messages or OBD II Messages.  So what’s the difference between Diagnostic Messages (such as OBD II) and Normal Messages on a typical CAN BUS?

Simply put Diagnostic Messages are Command/Response Messages.  So if you want to get data from a controller, you have to send it a request. It will then respond to that request (hopefully).  This is done using a common diagnostic protocol.  There are only a handful that are used and they are typically specific to the OEM, however there is not much difference between OEMs on how they have implemented their flavor of Diagnostic Messages.  That said all OEMs that sell vehicles in North America support the common OBD II protocol, those in Europe support the EOBD and in China, the new China OBD (Based on EOBD).

Normal Messages are the Messages that are transmitted between controllers.  This data varies depending on the electronics systems and like the OEMs Diagnostic Protocol, this data is also proprietary.  This data does not need to be requested (is nearly 100% of cases).  This data is typically sent at a periodic rate by a controller as fast as it needs to be sent so that listening controllers get the most recent value.  If you are doing data acquisition, this is the data you want.

The goal of the this site is to help users who wish to extract both Diagnostic and Normal Messages and thier data from the vehicle’s network.

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