Protocol decoder:J1850 vpw

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TODO this page needs some more attention

SAE J1850 VPW is one of several signal protocols mandated by OBD2/EOBD legislation, which requires automotive vehicle manufacturers from 1996 (USA) or 2001 (Europe) to provide access to the vehicles data bus via a standard 16 pin SAE J1962 connector. The purpose originally being to allow testing and diagnostics from the "on-board diagnostic" capabilities (which were also required by the legislation), of the control module responsible for the vehicles emission control system (typically the PCM, Powertrain Control Module). However the VPW databus also serves many other purposes.

SAE J1850 came in 2 varieties:

  • PWM (Pulse Width Modulation), 41.6 kb/s, typically used by Ford.
  • VPW (Variable Pulse Width), 10.4 kb/s, typically used by General Motors.

The way the bits are represented on the bus and voltage level is different between them. This decoder is for the VPW variety.


IRC user pman92 reported on 1/5/2020 that antus created a J1850 VPW decoder in 2016, which needs migration to PD API v3 and mainlining.


pman92 has created a branch of sigrokdecode and commited the 2016 version of the PD, as well as commited an attempted change to v3 API: https://github.com/pman92/libsigrokdecode/tree/master/decoders/vpw

There is also a branch of sigrok-dumps with example captures containing VPW data packets: https://github.com/pman92/sigrok-dumps/tree/master/vpw/

Discussion about the decoder can be found here: https://pcmhacking.net/forums/viewtopic.php?f=3&t=4761

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