J1850 VPW functional messages

They go by many names, P01, P59, VPW, '0411 etc . Circa 1999 to 2006. All VPW OBD2 PCMs.
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J1850 VPW functional messages

Postby pman92 » Thu Jun 18, 2020 9:28 pm

Hi guys,

I've been playing around looking at the VPW databus recently and trying to understand whats happening on it.

I've got an LS1 PCM and a PIM (which aren't linked, they are random units from random cars) connected up on the bench and I'm watching the data on the bus when I turn ignition on. There's around 70 frames sent within the first second of ignition on with just these 2 modules connected, and from then on it quietens down and there is only a couple of frames per second.

I've found this very useful website: https://www.fastfieros.com/tech/vpw_com ... otocol.htm and also found a PDF containing alot of J2190 information which all seems to make sense (at least from my brief look), but it seems to only cover PHYSICAL frames (when bit 2 of the header byte is set). I can't seem to find any information on FUNCTIONAL frames apart from the J1979 mode $01-$09 stuff.

Of the roughly 70 frames in the first second, only 2 are physically addressed. All the rest are functional.
I've noticed all the functionally addressed frames have a destination byte that is different / doesn't correspond to any physical node (obviously because its a functional address), but the source address byte is still the same physical node ID.

Is there any information on understanding all these functional frames? Is there an SAE document I'm missing? Or is it all proprietary GM stuff that isn't defined by any standard?

Thanks in advance
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Re: J1850 VPW functional messages

Postby picvrss » Thu Jun 18, 2020 10:37 pm

You'll find descriptions of the functional message format in J2178-2 Data Parameter Definitions, and J2178-4 Message Definitions for three-byte headers.

Google "J2178-2 baidu" and "J2178-4 baidu".

I needed these when I designed a manual paddle shift module for my son's VZ SV8. An example of the messages I use are shown below, and you'll recognise these on your VPW logger:

; Functionally addressed message format (hex notation)
; "HH", "LL" and "XX" are variable hex values. CRC is Checksum
;88 1B 10 10 HH LL CRC
;88 29 10 01 XX CRC
;88 3B 10 03 XX CRC
;68 EA 10 0A XX CRC

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