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------------------------------------------------------------
J1850 VPW transmission from GM P01 Powertrain Control Module
------------------------------------------------------------
Captured by pman92 - 2 May 2020
This is a capture of data from a General Motors "P01" Powertrain Control
Module. The control module was wired on the bench in a minimal setup
with nothing else connected. An ignition switch was included to provide
ignition power to the PCM. The following information was then obtained
from the module with PCMHammer 012 and an X-Pro VT interface (see
https://github.com/LegacyNsfw/PcmHacks/wiki/PCM-Hammer and
https://obdxpro.com/x-pro-vt/).
VIN: 6H8WHY19F2L857286
OS ID: 12202088
Calibration ID: 92113609
Hardware ID: 9386530
Serial Number: 2EB2TZJT2070
Broad Cast Code: DFNN
MEC: 0
An Arduino was connected to the data line via a basic interface circuit
that used a voltage comparator to monitor the J1850 VPW bus voltage
(high or low). The Arduino was programmed using pin change interrupts to
measure the time difference between changes of bus state (active or
passive). The Arduino would watch for SOF signals and then count the
following bus state/time changes to calculate the data within the packet.
It would calculate and confirm the checksum matched to ensure the data
was OK. It would then transmit the complete received packet back to the
PC via USB and was displayed on the Arduino serial monitor. The returned
data included a timestamp. If a checksum didn't match, an error message
would be returned instead of the relavent data packet.
Logic analyzer setup
--------------------
The capture was taken with a Saleae Logic 8 Channel clone at 16MHz for
50M samples. The ignition switch on the bench wiring was turned on and
the PCM started transmitting data.
Probe VPW
-------------
1 data
Capture verification
--------------------
1. Arduino checksum calculation and confirmation
J1850 VPW packets contain a checksum. If a bad or incorrect checksum was
found, an error message would be transmitted back to the PC by the Arduino
instead of that data packet/frame. No error messages were recorded during
the capturing of this data, so the Arduino checksum calculations were all
correct, and it considered the data intact.
2. Visual inspection of first data packet in Pulseview
The first data packet (+616.8ms to +622.1ms) was visually inspected in
PulseView and the bit values written down by hand. They were then split
into groups of 8 bit values, which were converted each to a single hex
value. The single hex values of the first data packet were then confirmed
to match the hex values that the Arduino had returned as the first packet.
3. Timing between first and last data
There is a total of 33 packets visible. PulseView shows the first packet
ending at 622ms, and the last packet ends at 3059ms, meaning 2437ms between
the "end of the first" and "end of the last" packets in the capture. The
Arduino also returned a time stamp of when the packet was processed and
sent to the PC. Between the first and 33rd packet, the difference in
Arduino time stamps was 2438ms. The difference of only 1ms confirms the
PulseView capture and Arduino are looking at the same range of data.
NOTE that there are glitches within the capture. There is a series of
short glitches/pulses around 506-507ms within the capture. I'm unsure of
the cause, but this was around the time when the ignition switch on the
bench was turned on. Maybe it was noise caused by the switch or maybe it
was noise coming from the PCM when it powered up. Either way this could
be a good example to make sure the decoder can ignore/skip bad data or
glitches.
Data
----
The Arduino returned the following data packets, written in hex, from
first to last within the PulseView capture:
68 13 10 11 00 46
68 EA 10 0A 01 AE
88 15 10 01 C8
88 1B 10 10 00 00 46
8A EA 10 20 8A 00 10
A9 CE 10 07 69
A8 F3 10 11 02 2B
C8 3B 10 3C 04 48
68 EA 10 0A 01 AE
88 15 10 01 C8
8A EA 10 20 8A 00 10
A9 CE 10 07 69
49 92 10 01 BE
49 92 10 01 BE
8A EA 10 20 82 00 4A
8A EA 10 20 82 00 4A
68 49 10 10 0B CF
68 EA 10 0A 01 AE
88 15 10 01 C8
8A EA 10 20 8A 00 10
A9 CE 10 07 69
E9 2A 10 3C EE
49 92 10 01 BE
E9 2A 10 3C EE
8A EA 10 20 82 00 4A
68 EA 10 0A 01 AE
88 15 10 01 C8
8A EA 10 20 8A 00 10
A9 CE 10 07 69
E8 FF 10 03 B3
49 92 10 01 BE
E9 2A 10 3C EE
8A EA 10 20 82 00 4A
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